GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: aPpYe on August 13, 2016, 06:37:45 PM
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Maybe I SHOULD have bought a Disco in the first place! Anyway, I figured I would start a new thread, as this is going to be a different gun ... I am further modifying the 2400kt I originally converted to a bottled HPA several months ago. This will be a bit of a long read, and I hope that you can take the time to provide your thoughts.
It seems the P-Rod valve pin screws have a smaller head than the standard Disco variety, so I am once again waiting on parts. If I ever get the parts squared away, I will finally be rebuilding this gun.
The powerplant (besides gauge port) will be all Discovery once I am finished. I am not sure what my ultimate goal is at this point, and if shot counts drop too low, or accuracy not worthy with the heavier pellets I may go right back to the ~18fpe I am at now, which I am perfectly happy with. Beyond replacing the main tube, everything else is just for fun, and is a learning experience.
Originally, I had the idea of mounting a regulated bottle into a Disco via the gauge port, and having it double as the forestock. I was also enamored with the idea of having a pistol grip on a rifle. Keeping it as light as possible was also a big deal for me. Those hollowed out plastic buttstocks and pistol grip frames from Crosman eventually won me over, and I went for a 2400kt from the custom shop. In the interest of being cheap (I have a card for that), I opted to feed the original 2400kt tube with hoses and fittings. Inside the tube, everything was replaced with stock Disco parts.
Well fast forward to today, and the little mounting bracket I made on the bottom of the bottle keeps breaking the (JB)weld holding that end in place. I have been wanting something a little more solid. I finally picked up that disco tube I originally wanted and will rebuild the gun with that, giving the bottle a solid mount, right into into the gauge port.
This brings me to my questions! Currently, the gun is shooting about 1.5fpe/ci with JSB 10.34gr Heavies, totalling about 130-140 18fpe shots before velocity begins to drop. The larger plenum of the Disco tube got me thinking about how I might tune the gun differently. I am now curious about how powerful I can make this gun, while still being able to keep the 1.8k burst disk intact. In that interest, I picked up a couple tins of the 13.4gr JSB Monsters and even grabbed tin of 16.2gr JSB Beasts. Preliminary testing (only a few groups) with my peep sights and at ~25 yards shows that both pellets shoot reasonably well through my barrel...
Here is what the gun looks like now:
(http://i1066.photobucket.com/albums/u404/appye/IMG_0215.jpg) (http://s1066.photobucket.com/user/appye/media/IMG_0215.jpg.html)
I will start by listing current relevant specs:
- 24” barrel in .177
- Standard 10” (end to end) pressure tube
- Disco valve with ~.043” spring, pinned in place by three retaining screws from a PRod
- Disco hammer spring (~.034”)
- Disco hammer (well, it came with the 2400kt, but I am pretty sure they are the same)
- Brass (I think?) extended probe from MAC1
- Polished hammer with some of the “sharp parts” rounded off so as to reduce galling.
- RSterne's first-gen SSG, made with 3/16 steel rod.
- Challenger RVA with degasser threads drilled out to allow for SSG
- 13ci ninja bottle, regulated to ~1200psi. It feeds the tube through the end, via fittings and a hose.
Here are changes I will definitely be making:
- Replace 2400kt tube with Disco tube. The gauge port allows for solid mounting of the bottle, and the tube has a much larger plenum. To this end, I am using “bottle top adapter” from MAC1 a PRod gauge block because it is wider and will have better flow than the Disco block. I already have high pressure rated fitment to get a 90º angle, and extend it into the block. The bottle will be mounted with the “top” oriented toward the back of the gun.
- Replace PRod valve retaining screws with disco screws, as the existing holes in the Disco tube are larger, requiring a larger head diameter.
And finally, my questions:
- What kinds of flow mods will provide a performance (either power or efficiency) in .177? There seems to be a consensus that stock port sizes are already at the maximum for .177 and any enlargement of ports, etc will actually cause a LOSS in performance. Thoughts?
- I remember someone mentioning in another thread an “oblong port” … does this apply to .177, and if so, what kind of benefits will I see from doing this? How would I do it?
- I guess I will not know until I try, but is it feasible with ~1500 psi (upper limits of a 1.8k burst disk) to get 13.43gr .177 pellets up to 915fps in a 24" barrel? This would be 25fpe, the lofty goal I am looking at right now.
- If I want to go beyond the capabilities of my 1.8k burst disk and install a 3k, is the Disco tube safe at 3000psi?
- Why is it that the transfer port on most of these Crosman/Benjamin CO2/HPA/PCP guns is only sealed at the tube/valve end and not at the breach/barrel end? I see that the PRod schematic shows that there are two seals. I am thinking of doing away with the metal transfer port for this reason. I have some of that clear plastic tubing that I have seen others mention, and it looks to be about the same diameter as the metal port, and I would think it would seal better.
- I am thinking of completely removing the valve poppet return spring and allowing the air pressure itself to close the valve. The idea is that there will be less work for the hammer spring to do, thereby allowing me to decrease preload and further ease cocking effort. I realize that filling the gun from empty may prove problematic, but the logic seems sound. Thoughts on this? Does the spring even matter? I would think 1500 psi of air is already pushing much harder on the poppet than the spring ever would. What about lighter springs? That .045” spring is stiff!
- I was also thinking about reducing the poppet diameter to something closer to the groove shown in the picture below. The idea is that there would be less turbulence and better flow into the area behind the poppet, and up into the exhaust port ... Is this a good idea?
(http://i1066.photobucket.com/albums/u404/appye/Poppet.jpg) (http://s1066.photobucket.com/user/appye/media/Poppet.jpg.html)
...Any other non-pressure related tried and true methods of increasing performance in .177?
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I know that you can order special regulators for the Ninja bottles, I get up to 900fps with 1800psi in .22 with a stock (pinned valve) on a 2240
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What kinds of flow mods will provide a performance (either power or efficiency) in .177? There seems to be a consensus that stock port sizes are already at the maximum for .177 and any enlargement of ports, etc will actually cause a LOSS in performance. Thoughts?
In general, a free-flowing system will give you the opportunity to achieve both high energy and high efficiency. It allows you to use a short valve dwell (oversimplified: a light hammer strike) for economy of air usage while also producing high energy. These two things are achieved in tandem because the base of the pellet is struck with a brief high-pressure pulse to get it started accelerating and the valve is allowed to close, meanwhile the air continues to expand to accelerate the pellet down the barrel.
So contrasting that with a system that does not flow as well, one must use a longer valve dwell to achieve the same energy. That of course means the system uses more air, and that extra air gets expelled from the muzzle, meaning a louder report.
So how much porting is necessary? There's not much point in going over the equivalent of the bore diameter. The generally accepted guideline is to use about 75% of bore diameter for pellet shooters and full bore porting for bullet shooters or super heavy pellets. To my way of thinking, a 10gr in .177 is heavy and a 16gr is super heavy. So the 13.4gr you are looking at using is around the tipping point.
As a practical matter, you don't really have to worry about negatively affecting efficiency by having "too much porting". But to qualify that statement, imagine you had a really large valve throat--the region between the poppet seat and the valve exhaust port. That represents a volume of air that becomes pressurized during the shot cycle, so if it's really large, it wastes air. I said it's not much of a concern in practice because the way most valves are designed don't lend themselves to be modified in such a way that you could produce that scenario.
I am thinking of completely removing the valve poppet return spring and allowing the air pressure itself to close the valve. The idea is that there will be less work for the hammer spring to do, thereby allowing me to decrease preload and further ease cocking effort. I realize that filling the gun from empty may prove problematic, but the logic seems sound. Thoughts on this? Does the spring even matter? I would think 1500 psi of air is already pushing much harder on the poppet than the spring ever would. What about lighter springs? That .045” spring is stiff!
I think you have that exactly right. You can get away with not having the poppet spring in a regulated system. In an unregulated rifle, doing so will tend to produce a shorter usable shot string. The downside, as you stated, is getting it filled from zero. There's a good chance you wouldn't be able to do it with a hand pump but with a tank you should be fine.
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If you try and reduce the diameter of that Disco poppet, you will end up with three pieces.... It is an "overmoulded" design, with the plastic moulded around a flanged metal stem that from the side is "T" shaped.... IIRC, the diameter of the flange is 3/8", so if you reduce the OD very much the plastic ends up in two pieces (front and back), and you will be buying a new poppet....
You can, however, get the poppet from an MRod, and bush the stem hole in the Disco valve down to 1/8" (is it currently 5/32").... The MRod poppet is much smaller in OD (0.335"), and made of a harder material.... You get more flow around it, both between the ID of the valve body and the poppet and through the throat (because of the smaller stem).... and it is easier to open at the same air pressure.... For the bushing you can use a short piece of K&S brass hobby tubing, glued in place....
Bob
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Patrick, here's a worksheet I use when I'm blueprinting a PCP's porting and planning what types of flow modifications to make.
(https://www.gatewaytoairguns.org/GTA/MGalleryItem.php?id=5368)
I take my calipers and gauge pins to the various parts and load in the IDs and ODs into the "Before" table. Then I put in my caliber at the top and look down the last column to see where the restrictions are. Take note it gives the area as a percentage of the caliber, so for example if you wanted to follow the "75% of bore diameter" rule of thumb for a pellet shooter, the corresponding area you want to see in that column would be 56% because of the squaring term.
From that, it's easy to identify which parts need to be modified, but perhaps just as importantly, which ones don't. I take the parts in my hand and look at how I can modify each one (and by how much) to alleviate the restriction, and I plug those numbers into the "After" table. It makes it easy to play with the numbers and see the result, particularly for flow paths that involve two parts. A good example is your question about reducing the poppet OD. In some cases it's necessary to also bore out the ID of the valve to get enough flow. This little spreadsheet makes it easy to see the net result of doing both mods.
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Patrick, here's a worksheet I use when I'm blueprinting a PCP's porting and planning what types of flow modifications to make.
(http://i779.photobucket.com/albums/yy80/jmneal1/files/porting%20spreadsheet_zpsv0xoensz.gif)
I take my calipers and gauge pins to the various parts and load in the IDs and ODs into the "Before" table. Then I put in my caliber at the top and look down the last column to see where the restrictions are. Take note it gives the area as a percentage of the caliber, so for example if you wanted to follow the "75% of bore diameter" rule of thumb for a pellet shooter, the corresponding area you want to see in that column would be 56% because of the squaring term.
From that, it's easy to identify which parts need to be modified, but perhaps just as importantly, which ones don't. I take the parts in my hand and look at how I can modify each one (and by how much) to alleviate the restriction, and I plug those numbers into the "After" table. It makes it easy to play with the numbers and see the result, particularly for flow paths that involve two parts. A good example is your question about reducing the poppet OD. In some cases it's necessary to also bore out the ID of the valve to get enough flow. This little spreadsheet makes it easy to see the net result of doing both mods.
That is awesome nervoustrigger. Do you have a chart for any other calibers?
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Do you have a chart for any other calibers?
You can just key in your caliber at the top under "caliber for comparison".
By the way, the gray cells are all calculated values. The only numbers you need to supply are for the white cells.
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Hey thanks for taking the time to write up your replies!
I know that you can order special regulators for the Ninja bottles, I get up to 900fps with 1800psi in .22 with a stock (pinned valve) on a 2240
I think I can shim pretty close to that with the reg I have. For now, I am going to be seeing what I can get with ~1500psi though. That being said, I MAY at some point go with higher pressures and higher FPE, but I do want to keep a decent shot count ...
There's not much point in going over the equivalent of the bore diameter. The generally accepted guideline is to use about 75% of bore diameter for pellet shooters and full bore porting for bullet shooters or super heavy pellets. To my way of thinking, a 10gr in .177 is heavy and a 16gr is super heavy. So the 13.4gr you are looking at using is around the tipping point.
As a practical matter, you don't really have to worry about negatively affecting efficiency by having "too much porting".
Hmm. I do seem to remember something about too much porting causing an issue where there is too much of a pressure drop in areas between the poppet and the pellet. In my mind it was analogous (as a ridiculously extreme example) to having say, a sealed 50 gallon drum in between the poppet and the pellet. Once air is released, it enters the 50 gallon drum, where it does increase overall pressure, and the pressure does need to escape through the barrel but the change is so nominal at this point that it has far less effect on the pellet, likely just escaping between the rifling and the skirt! I know the example is crazy, but I find insane examples like this to help with my own understanding of things. Does this seem right to you? I know that if we were working with hydraulics, it would make no difference, but since air can compress ...
The idea is that this is much more prevalent in .177 than in the larger calibers. I'll see if I can find some quotes somewhere... I want to be sure of what I am doing before I start getting things on my drill press.
I think you have that exactly right. You can get away with not having the poppet spring in a regulated system. In an unregulated rifle, doing so will tend to produce a shorter usable shot string. The downside, as you stated, is getting it filled from zero. There's a good chance you wouldn't be able to do it with a hand pump but with a tank you should be fine.
That makes sense to me. The return spring has a lot to do with the "balanced" valve thing in an unregulated gun then? It flattens the string? I have zero experience with unregulated PCP's, as this is the only one I have.
So what level of effect do you think this would achieve? I seem to remember that there are hundreds of pounds on the poppet when the tube is pressurized... I am using a hand pump, but I am first feeding it with air from my shop compressor. I might not be remembering this right, but I think may have tried it before with some success. I was looking for performance gains at the time and I wasn't really thinking about cocking effort. I ended up putting the spring back because I figured I did not know enough about valve tuning and that Crosman knew what they were doing. I will try and see what happens. I might even be able to get a couple of hundred psi in there directly with the shop compressor before switching over to the hand pump. I am keeping my fingers crossed anyway. If that does not work, then I can at least go with a weaker spring.
If you try and reduce the diameter of that Disco poppet, you will end up with three pieces....
Lol ... I was seriously eyeing that thing! I figured I would check the forum before chucking it up! Thanks for saving my poppet...
You can, however, get the poppet from an MRod, and bush the stem hole in the Disco valve down to 1/8" (is it currently 5/32").... For the bushing you can use a short piece of K&S brass hobby tubing, glued in place....
Awesome! That is absolutely happening! It is a bummer that I have to wait until Monday before I can call Crosman. Is this K&S hobby tubing typically available at hobby stores? Hardware stores? Do I glue it in place with JB Weld or some such?
Patrick, here's a worksheet I use when I'm blueprinting a PCP's porting and planning what types of flow modifications to make.
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From that, it's easy to identify which parts need to be modified, but perhaps just as importantly, which ones don't.
That's really cool! I will definitely be measuring things up and posting the results here. The only thing is I do have some digital calipers, but I do not have gauge pins, unfortunately. I can still fit the calipers into some tight spots and get at least a rough measurement...
That is awesome nervoustrigger. Do you have a chart for any other calibers?
... Are those numbers arbitrary? A caliber size of .217 seems odd.
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Hmm. I do seem to remember something about too much porting causing an issue where there is too much of a pressure drop in areas between the poppet and the pellet. In my mind it was analogous (as a ridiculously extreme example) to having say, a sealed 50 gallon drum in between the poppet and the pellet. Once air is released, it enters the 50 gallon drum, where it does increase overall pressure, and the pressure does need to escape through the barrel but the change is so nominal at this point that it has far less effect on the pellet, likely just escaping between the rifling and the skirt! I know the example is crazy, but I find insane examples like this to help with my own understanding of things. Does this seem right to you? I know that if we were working with hydraulics, it would make no difference, but since air can compress ...
The idea is that this is much more prevalent in .177 than in the larger calibers. I'll see if I can find some quotes somewhere... I want to be sure of what I am doing before I start getting things on my drill press.
Yeah, we're on the same page. That's precisely what I was trying to convey where I talked about the volume of the valve throat...the 50 gallon drum in your analogy. It is something to bear in mind but not so much of a concern in practice because of the way the vast majority of valves are designed. And if you calculate the cross-sectional area of the various passages using something like my spreadsheet, you won't go overkill in one area or another when you're opening things up.
That makes sense to me. The return spring has a lot to do with the "balanced" valve thing in an unregulated gun then? It flattens the string?
Yep, you got it :D
So what level of effect do you think this would achieve?
Frankly I don't see any appreciable benefit. I've run the numbers a couple of times and the force of the air pressure holding the valve closed is so much greater than that of the valve spring, that it does not affect how hard it is to cock in a meaningful way.
... Are those numbers arbitrary? A caliber size of .217 seems odd.
5.5mm = .2165in
Not so odd after all ;D
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Do you have a chart for any other calibers?
You can just key in your caliber at the top under "caliber for comparison".
By the way, the gray cells are all calculated values. The only numbers you need to supply are for the white cells.
Awesome! I didnt know it changed valves if you entered different numbers! I thought you just made a bunch of charts.
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Love this mod
You can, however, get the poppet from an MRod, and bush the stem hole in the Disco valve down to 1/8" (is it currently 5/32").... The MRod poppet is much smaller in OD (0.335"), and made of a harder material.... You get more flow around it, both between the ID of the valve body and the poppet and through the throat (because of the smaller stem).... and it is easier to open at the same air pressure.... For the bushing you can use a short piece of K&S brass hobby tubing, glued in place....
you can make your hammer lighter too helps with efficiency and shot cycle
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Love this mod
You can, however, get the poppet from an MRod, and bush the stem hole in the Disco valve down to 1/8" (is it currently 5/32").... The MRod poppet is much smaller in OD (0.335"), and made of a harder material.... You get more flow around it, both between the ID of the valve body and the poppet and through the throat (because of the smaller stem).... and it is easier to open at the same air pressure.... For the bushing you can use a short piece of K&S brass hobby tubing, glued in place....
you can make your hammer lighter too helps with efficiency and shot cycle
Thanks. I will have to try that if o dont throw my maximus off a bridge first.
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These guys know their HPA conversions.
Do think you could get a no-return spring valve to seal from zero...because you have that on/off on the HPA bottle. Cock the strikerm fill the bottle with it to "off", turn it to on, and hit the valve all at once with 1200psi...it will seal.
Reduced valve seal diameter really can increase power, more so as you've got the barrel length.
Was running co2 on this one for the summer, but HPA in winter...but never pushed it to the max.
(http://i157.photobucket.com/albums/t50/ribbonstone/co2/DSCF0002-6_zps2fcc7e34.jpg) (http://s157.photobucket.com/user/ribbonstone/media/co2/DSCF0002-6_zps2fcc7e34.jpg.html)
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Patrick, here's a worksheet I use when I'm blueprinting a PCP's porting and planning what types of flow modifications to make.
Okay, so I have taken some measurements (I lucked out and some hex keys actually worked pretty well in this case as gauge pins. Nice snug fits!) and here are my numbers:
(http://i1066.photobucket.com/albums/u404/appye/Initial%20Thoughts.png) (http://s1066.photobucket.com/user/appye/media/Initial%20Thoughts.png.html)
The poppet stem od was tricky, as I had two different diameters to deal with. I went with the larger one (the smaller one was .143) because I figured that as the valve is open, the larger diameter is largely what I see when peering down the exhaust port. Here is what it looks like:
(http://i1066.photobucket.com/albums/u404/appye/IMG_0669.jpg) (http://s1066.photobucket.com/user/appye/media/IMG_0669.jpg.html)
The probe had two different diameters as well. The small one was .070. I went with the large one here as well. Why is it shaped like this anyway?
(http://i1066.photobucket.com/albums/u404/appye/IMG_0667.jpg) (http://s1066.photobucket.com/user/appye/media/IMG_0667.jpg.html)
The exhaust port seems to be slightly oblong. I measured sideways at .150 and lengthwise at .161. I just put in an average there.
It appears that I am already pretty large on my porting for .177. The MRod poppet seems like it would be overkill, seeing as how I am already at 224% there ... I guess it would help with turbulence... What about the transfer and barrel ports? Other thoughts?
You once told me that getting the 16.2gr Beasts would be impossible to get to 900fps with ~1500 psi ... What about the 13.43gr monsters? Has anyone successfully run 1600psi with a 1.8k burst disc?
Frankly I don't see any appreciable benefit. I've run the numbers a couple of times and the force of the air pressure holding the valve closed is so much greater than that of the valve spring, that it does not affect how hard it is to cock in a meaningful way.
I was wondering the same. What about consistency or other things...?
5.5mm = .2165in
Not so odd after all ;D
Huh... I had forgotten that .22 airgun is not the same as .22lr! Why is this, anyway?
Thanks. I will have to try that if o dont throw my maximus off a bridge first.
I'll be waiting at the bottom to catch it!
...because you have that on/off on the HPA bottle. Cock the strikerm fill the bottle with it to "off", turn it to on, and hit the valve all at once with 1200psi...it will seal.
Reduced valve seal diameter really can increase power, more so as you've got the barrel length.
Well, I am actually doing away with the on/off on this one, due to the way the bottle will be mounted...
As far as reducing poppet diameter, do you still think so after seeing the numbers above?
And that is a frankengun for sure! I am seeing 2240, PRod, and aftermarket mods there!
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Smaller diameter poppet seal is held closed by less force, so it takes less force to open it to the same dwell...which gives hop to more air flow without excessive cocking effort. So yes, I would go with that mod first. Never used an M-Rod valve stem, as when I made the pictured carbine, the M-Rod was pretty much the new-guy on the airgun-campus. Made my own Delrin poppet, but considering the effort to do that, if I had to do it today, would use an M-Rod poppet and just bush the stem to fit.
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Smaller diameter poppet seal is held closed by less force, so it takes less force to open it to the same dwell...which gives hop to more air flow without excessive cocking effort. So yes, I would go with that mod first.
Good enough for me!
I also had another idea. What if I were to fill in the 90 degree angles on the poppet with some epoxy or something to improve flow? Basically make it more cone shaped than tophat shaped... The theory is that it would decrease turbulence as well as decrease the surface area for the air to push on, making the valve even easier to push open.
I'm feeling like I am speaking with some real big leaguers here! We've got nervoustrigger, ribbonstone, and rsterne! If we can get timmymac and Tom Gaylord, we'll have a real murderer's row!
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Still have the same friends who made that version my "rat sniper". Still have a giant three level deck around their swimming pool. patio, and BBQ area and give lots of parties/gatherings. Food gets dropped...under the deck is like "rat nirvana". So they are use to me staying out later than most and sniping a few or the critters as they scamper about in the Tikki-torch light...so long as I "bag'em and trahcan'em".
Made that one in 5mm, and eventually set it to 14-14.5foot pounds (call it 13.4gr. @ just under 700fps) which worked out great for the rat-task.
Oldest son of that pair (who I took my turns at diapers and babysitting) fell in love with it. told him as soon as he got a way to fill an HPA bottle, I'd give it to him. He did/I did.
(OK...I kept the laminate grips and he got the black plastic girips that match the fore end.)
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The poppet stem od was tricky, as I had two different diameters to deal with. I went with the larger one (the smaller one was .143) because I figured that as the valve is open, the larger diameter is largely what I see when peering down the exhaust port.
Agreed, the thicker portion of the valve stem is the proper dimension to use. It's what affects flow through both the valve throat and the exhaust port.
The probe had two different diameters as well. The small one was .070. I went with the large one here as well. Why is it shaped like this anyway?
I think this one would be better represented as an in-between dimension. I don't know why they have the bulb on the end. I would remove it (and have done it).
It appears that I am already pretty large on my porting for .177. The MRod poppet seems like it would be overkill, seeing as how I am already at 224% there ... I guess it would help with turbulence... What about the transfer and barrel ports? Other thoughts?
You once told me that getting the 16.2gr Beasts would be impossible to get to 900fps with ~1500 psi ... What about the 13.43gr monsters? Has anyone successfully run 1600psi with a 1.8k burst disc?
I ran some simple calculations and, assuming you stay with a round barrel port of 0.130, you will need a setpoint around 1800psi to get the 13.4gr Monsters up to 900fps. I'll call that a semi-educated guesstimate, nothing more :)
Larger porting changes the picture significantly but as you stated, the barrel port needs to be a slot so as to prevent chambering issues. For example, a 0.130" wide slot that is 0.168" long equates to 74% of the bore area, and would probably allow you to reach your goal with 1400psi. A matching transfer port would need to be fashioned as well, and preferably blend the exhaust port to it.
[regarding removing the valve spring] What about consistency or other things...?
No other benefits that I can think of. Springs are very consistent so I don't think it would improve ES or anything of that nature. While the spring produces some turbulence as air rushes by, that portion of the system is so generously large that it won't have a meaningful effect.
Huh... I had forgotten that .22 airgun is not the same as .22lr! Why is this, anyway?
I always assumed it was a classic standard vs. metric thing. 0.22 LR versus 5.5mm airgun, and the 5.5mm was so close to .22 that it was just rounded up for marketing purposes. But I really don't know.
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During lunch today, I will be heading up to a hobby store to look for the brass hobby tubing, then I will be ordering the Disco valve screws and the Marauder poppet. Anyway, on to my responses...
(OK...I kept the laminate grips and he got the black plastic girips that match the fore end.)
Heh ... I think the gun looks better all black anyway! The laminate grips aren't bad looking, but I would not like it mismatched like that. I don't like my guns looking pretty anyway! Polishing stocks, stressing about dings, not for me!
I think this one would be better represented as an in-between dimension. I don't know why they have the bulb on the end. I would remove it (and have done it).
I am thinking the same ... perhaps it is bulbed to make the air hit the skirt better somewhow? I can see how it does restrict things though ...
Larger porting changes the picture significantly but as you stated, the barrel port needs to be a slot so as to prevent chambering issues. For example, a 0.130" wide slot that is 0.168" long equates to 74% of the bore area, and would probably allow you to reach your goal with 1400psi. A matching transfer port would need to be fashioned as well, and preferably blend the exhaust port to it.
1400psi! Message received. Larger porting = better breathing = higher efficiency/performance. I'm excited now!!!
Forgive me for being dense, but is the general idea to get as close to 100% as possible to achieve maximum efficiency? At some point it's gotta be too much, ie. my 50 gallon drum analogy, right? ... Let's assume I want max efficiency/performance at 30fpe with 16.2gr. What numbers would you suggest I strive for in the 'After' section? What about 25fpe with the 13.43gr? Assuming I port everything for max-uber power, how would that affect things if I were to dial it back down to 18fpe and 10.34gr pellets?
... Do I need to be a math master to be able to do these calculations? I can add, subtract, multiply and divide. It has been over twenty years since high school algebra ...
Also, I was thinking of using some of that rubber/plastic tubing stuff I have seen people using for a transfer port. The ID is already very similar to the ID of the existing port, and there is plenty of meat on it for enlarging things, and the OD is maybe just a few thousanths smaller than the largest OD of the transfer port... I would think it would seal better than metal to metal contact anyway, assuming I don't make the walls too thin.
... Aaaand can these things be done with some careful dremeling and/or a drill press, clamps, maybe some jigging, and lots of patience? I do have occasional access to a machining lathe of some kind, but I have no skills on it, and I don't know how much skill the guys at that shop have or how much they are willing to help... It is also nearly two hours away ...
EDIT: All right! $11.48 later and my parts are in the mail!
You can, however, get the poppet from an MRod, and bush the stem hole in the Disco valve down to 1/8" (is it currently 5/32").... The MRod poppet is much smaller in OD (0.335"), and made of a harder material.... You get more flow around it, both between the ID of the valve body and the poppet and through the throat (because of the smaller stem).... and it is easier to open at the same air pressure.... For the bushing you can use a short piece of K&S brass hobby tubing, glued in place....
Is JBWeld good enough to glue it into place? Crazy glue? Also, I hope copper is okay, as they did not have brass. I picked up a length of tube (pictured below) from a hobby store. If the measurements you gave me (5/32" and 1/8") and the tube dimensions (5/32x.014) are exact, that's a good fit!
I see it like this:
("Tube OD" minus "Twice the wall thickness" minus "MRod stem diameter") divided by 2 equals "MRod stem to Tube ID clearance"
or
(.15625 - .028 - .125) / 2 = .001625"
Cool!
(http://i1066.photobucket.com/albums/u404/appye/IMG_0676.jpg) (http://s1066.photobucket.com/user/appye/media/IMG_0676.jpg.html)
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Uh oh ... crickets. I am thinking some of my questions can be somewhat loaded ... I apologize for that.
Anyway, I am thinking I want to shoot for 100% porting, unless any of you think that would be a bad idea. Jason, you mentioned an oblong barrel port of .130" x .168" would achieve 75% bore area, so I am thinking I will have to go with an even longer (and maybe ever so slightly wider) port to achieve 100% ... As I understand your use of the word "blend," I need to make sure that the transfer port matches both the barrel port and the exhaust port, seeing as how the exhaust port is already .150" x .161" ... They are going to be different shapes, as a .150" barrel port will be too wide for a .177" pellet, creating potential loading issues (right?). I am thinking that I still want to keep the shapes as close to one another as possible though, so maybe instead of just going for a round exhaust port and blending that to an oblong barrel port, I am going to want to go with as oblong of an exhaust port as I can while still maintaining the equivalent of .177" x .177"
It was also mentioned that there is no real point in making things over 100%, so I am thinking there is no good reason to open up the valve throat any further, since I will already be at 466% there with the MRod valve stem... This is logical, right?
... Unless someone tells me this is not a good idea, I will likely fire up the dremel tonight! Now to actually figure out the calculations... There's gotta be a website for that!
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Sorry brother, just been super busy. I will reply in more detail tonight but for now I can say you have a good grasp of the situation. :)
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Lol... Don't feel pressured. I realize I tend to ask a lot of questions that may have many different answers depending on perspective.
And it isn't like you guys are getting paid, though maybe dinner of these forums SHOULD be paying people like you, rsterne, ribbonstone, etc... Advice from you guys is what keeps readership of sites like this going. Most of us are just lurking or asking questions, hoping an expert or two might take the time to chime in...
I look at some of the write-ups you guys do, just to advance the hobby, and I am astounded by the level of knowledge being shared and the time it must take to convey it.
I don't mean to gush, but airgun forums are really friendly! Try asking questions (especially newbie questions) on computer forums (I'm in IT by profession) and check out the snarky remarks generated there...
EDIT
Here's a pretty standard IT forum response:
http://lmgtfy.com/?q=airgun+performance+mods (http://lmgtfy.com/?q=airgun+performance+mods)
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So I started with the first bit of machining ... I turned the probe down to .025" on my drill press using a file and some 500grit sandpaper:
(http://i1066.photobucket.com/albums/u404/appye/skinny%20probe.jpg)
Brass is soft! It filed down quicker than I thought it would. Anyway, I got it down to .025" and I hope it is gonna be sturdy enough to push pellets into the rifling without breaking. If it breaks it breaks, and I will get a new one.
I am also hoping it isn't gonna poke deformities into the backs of some of the softer pellets as it pushes them into the bore. The small .025" diameter works out to only .000625" (whoops, this is an ellipse ... Okay let me check with google ...) worth of probe pushing on the end of the pellet now! The 13.4's go in easy enough, but the 16.2's are tight, and may be problematic. I guess I won't know until I try...
Also, thanks to your chart I learned that it is impossible to reach 100% bore area without some sort of two piece retractable probe that pushes the pellet into the bore and then retracts to leave the chamber completely free of any obstruction. Maybe that's a good idea! Anyway, .025" gives me 98% according to the chart...
Another thought is that for optimal efficiency, areas from the throat to the pellet should be the same percentage all the way through to the pellet. They should at least be progressively smaller, without any larger pockets following smaller ones, else we run into the 50 gallon drum analogy. So to optimize efficiency, I need to make sure that I am at least 98% all the way back through the exhaust port. Do I have this right?
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Anyway, I am thinking I want to shoot for 100% porting, unless any of you think that would be a bad idea. Jason, you mentioned an oblong barrel port of .130" x .168" would achieve 75% bore area, so I am thinking I will have to go with an even longer (and maybe ever so slightly wider) port to achieve 100%.
For 100% of the bore area, a 0.130" wide slot would need to be 0.218" long overall (0.088" CTC).
I added a box at the top of the spreadsheet for calculating the area of an oblong port so you can play with the numbers. Copy attached.
As I understand your use of the word "blend," I need to make sure that the transfer port matches both the barrel port and the exhaust port, seeing as how the exhaust port is already .150" x .161" ... They are going to be different shapes, as a .150" barrel port will be too wide for a .177" pellet, creating potential loading issues (right?).
You're right on the money!
I turned the probe down to .025" on my drill press using a file and some 500grit sandpaper. ... If it breaks it breaks, and I will get a new one.
Yeah, I wouldn't have taken it down that far. A 1/16" probe (0.063") is plenty small as to not be a restriction in any practical sense but if it breaks, replacing it isn't a big deal. Or you can drill it out and glue in the shank of a drill bit.
It was also mentioned that there is no real point in making things over 100%, so I am thinking there is no good reason to open up the valve throat any further, since I will already be at 466% there with the MRod valve stem... This is logical, right?
Yes. Having a "cross-section overage" upstream of the poppet is just fine, and indeed preferable due to inevitable turbulence effects. But as you go on to postulate, downstream...not so much.
Another thought is that for optimal efficiency, areas from the throat to the pellet should be the same percentage all the way through to the pellet. They should at least be progressively smaller, without any larger pockets following smaller ones, else we run into the 50 gallon drum analogy. So to optimize efficiency, I need to make sure that I am at least 98% all the way back through the exhaust port. Do I have this right?
I don't think I could have said it better. I would simply suggest not to fret over a few percentage points here and there but yes, if anything, you want passages to get progressively smaller rather than the reverse. In an ideal world, there would be a straight-line path from the valve throat to the base of the pellet...a polished tube of a consistent diameter, for example. Perhaps next best would be a gently curved tube. If you think along those terms, it may help guide the manner in which you modify parts.
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Nice spreadsheet, Jason, I do the same calculations, just one at a time.... been doing it so long I pretty much know where to look for the obstructions now.... Since most of the ports are circular, you only need to worry about them where there is something in the way, like the valve stem or the bolt probe, otherwise I just take the smallest diameter (greatest restriction) and work from there.... The limiting factor is usually the barrel port, and if that is 75% of the barrel diameter, I just make sure the transfer port and valve exhaust port are the same size and the transitions as smooth as possible.... Then I calculate the valve throat area like Jason does in his spreadsheet (hole area minus stem area) and make sure that is at least as large as the rest of the porting.... I have found that making that area slightly larger usually doesn't help the maximum power, but does help the efficiency slightly, probably because of the smaller area during the opening and closing parts of the shot cycle.... Even there, I don't increase the area more than about 10%.... so on a 75% build, I would use a throat that is the equivalent of 80% of the caliber.... The bolt probe (and I never use flow-through, I find them too restrictive for most of my builds) I don't worry about too much on "normal" builds (ie ones with 75% diameter, circular barrel ports), I just make sure it is LESS than half the caliber.... With the common calibers, I use 5/64" for .177, 3/32" for .22, 7/64" for .25 cal, 1/8" for .30 cal, and 9/64" for .35 cal.... Those work out to 90% of the caliber diameter, and when everything else is 75%, I don't see any point in making it smaller, it just gets too fragile, and can damage the base of the pellet on loading.... On the inlet side of the poppet seat, however, I won't accept any less than a full 100% of the caliber, and 200% is better.... I want to make sure that the air can get to the valve seat a LOT easier than it can get out....
The exception to the above, of course, is when I try for bore-size porting to get the absolute last bit of power from a given caliber.... ie I want 100% everywhere.... In that case, again my choices are simple.... I make the transfer and exhaust ports the same diameter as the caliber.... and the valve throat area large enough that it's area is about 10% larger (equivalent diameter about 105% of caliber).... The trick is the barrel port.... You can't just drill it out to caliber size, or the pellet will fall in during loading and get severely damaged.... The solution is to make the port oblong, keeping the width at 75% and making the length about 120% of caliber.... so that the area works out to what the bore is.... The bolt, in this case, I make retractable, using a "J-slot" for the bolt handle (or locking lug).... The nose of the bolt is flat, and long enough to load the base of the pellet just past the barrel port.... Then there is a second slot for the bolt handle, which after turning it to the locking position, you use to retract the entire bolt so that the nose of the bolt is flush with the back of the barrel port.... Voila, no restriction....
While we are on this subject, I will pass on my observations about pellets vs. bullets, and their affect on how big you need to make the ports.... Although this may change now that pellet manufacturers are starting to make really heavy pellets (for the caliber, ie high SD).... with most of the pellets used today I find NO point in going past 75% of the bore diameter for the barrel, transfer, and exhaust ports.... and equivalent areas elsewhere.... You just don't need the extra flow.... Bullets are different.... They are heavier (higher SD), so they can utilize the extra flow possible with bore-size porting.... So, for me, its pretty simple.... When I build a gun to shoot pellets, I use a round, 75% barrel port, and work back from there and use a probed bolt of the sizes I quoted above.... When I build a flat-out bullet shooter, I use transfer and exhaust ports that are caliber diameter, an oblong barrel port of equivalent area (and with bullets, you can go 80% of width, because they are longer and less prone to damage on loading), and a retractable bolt.... I make sure the valve throat is slightly larger area than the rest of the porting, and that the inlet side of the valve is about twice the area....
Bob
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thanks for sharing there bob.. very informative and useful info... i love this tech savy stuff.. cheers
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Five minutes after I got home yesterday afternoon, the power went out! So no dremeling done yet ... Tonight, however ... !!!!
Okay, responses:
I added a box at the top of the spreadsheet for calculating the area of an oblong port so you can play with the numbers. Copy attached.
Cool! I had found the "area of an ellipse" formulas on the internet, but I was having trouble building them into the spreadsheet. Very cool! Of course, I am thinking they have to be taken only as an estimate, because I am likely not going to be making the perfect ellipses (ie. the "sharp" ends could be more rounded or wider) the calculations intimate, especially with a dremel.
Also, what are the calculations on the bottom-right for? I am imagining they have something to do with how you are figuring I would need 1800psi with existing porting and 1400psi with better breathing for the 13.4gr's at 900fps? These guys here:
(http://i1066.photobucket.com/albums/u404/appye/Capture.png)
Or you can drill it out and glue in the shank of a drill bit.
...Is epoxy good? Or is there something more preferable? I was thinking of just turning down a finishing nail for the actual piece.
I don't see any point in making it smaller, it just gets too fragile, and can damage the base of the pellet on loading....
That’s what I am worried about … I may just cut it off, drill it out and go with a section of finishing nail or something… I guess I could use a drill bit shank as Jason suggests... I might have a broken 1/16 laying around somewhere. I dunno. I guess want to see how the pellets look after being forced into the bore with it… If I remember right (I am at work and the gun is in pieces at the moment) the 10gr pellets go in with minimal resistance, the 13gr go in about the same, and the 16gr pellets do provide noticeably more resistance, maybe enough to mar the back.
I calculate the valve throat area like Jason does in his spreadsheet (hole area minus stem area) and make sure that is at least as large as the rest of the porting.... I have found that making that area slightly larger usually doesn't help the maximum power, but does help the efficiency slightly, probably because of the smaller area during the opening and closing parts of the shot cycle....
...
...when I try for bore-size porting to get the absolute last bit of power from a given caliber.... ie I want 100% everywhere.... In that case, again my choices are simple.... I make the transfer and exhaust ports the same diameter as the caliber.... and the valve throat area large enough that it's area is about 10% larger (equivalent diameter about 105% of caliber)....
That’s interesting. I would have thought that anything above 100% would hurt efficiency. Perhaps turbulence also plays a part here … The 107% I am at now sounds about right then, assuming I am going for 100% everywhere else. What do you think? All I have are drill bits to enlarge things and the next size up from .222 (what I measured it at currently) is 15/64” which is .234. This would bring it to 125% “area as % of caliber” in Jason’s spreadsheet. This would be too big, right?
The bolt, in this case, I make retractable, using a "J-slot" for the bolt handle (or locking lug).... The nose of the bolt is flat, and long enough to load the base of the pellet just past the barrel port.... Then there is a second slot for the bolt handle, which after turning it to the locking position, you use to retract the entire bolt so that the nose of the bolt is flush with the back of the barrel port.... Voila, no restriction....
That sounds a lot like the two piece probe I was imagining in the post above! Do you have any pictures, or a link where you describe it further? Very interesting!
While we are on this subject, I will pass on my observations about pellets vs. bullets, and their affect on how big you need to make the ports.... Although this may change now that pellet manufacturers are starting to make really heavy pellets (for the caliber, ie high SD).... with most of the pellets used today I find NO point in going past 75% of the bore diameter for the barrel, transfer, and exhaust ports.... and equivalent areas elsewhere.... You just don't need the extra flow.... Bullets are different.... They are heavier (higher SD), so they can utilize the extra flow possible with bore-size porting.... So, for me, its pretty simple.... When I build a gun to shoot pellets, I use a round, 75% barrel port, and work back from there and use a probed bolt of the sizes I quoted above.... When I build a flat-out bullet shooter, I use transfer and exhaust ports that are caliber diameter, an oblong barrel port of equivalent area (and with bullets, you can go 80% of width, because they are longer and less prone to damage on loading), and a retractable bolt.... I make sure the valve throat is slightly larger area than the rest of the porting, and that the inlet side of the valve is about twice the area....
So does 100% porting HURT efficiency/performance/etc if I were to decide I want to go back to shooting 10.3gr pellets, as I have been these last few months? What about 8.4 gr “regular” pellets? My goal is to get 13.4gr pellets to 915fps (25fpe). These are somewhat longer, and more “bullet shaped,” than the 10.3gr pellets I am mostly shooting now. If it is possible I may even want to shoot heavier pellets.
And what is SD? I can only think of Standard Deviation, but if so, I am not following what it means for a pellet or a caliber to have an SD ...
EDIT: Ah ... Sectional Density.
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Also, what are the calculations on the bottom-right for? I am imagining they have something to do with how you are figuring I would need 1800psi with existing porting and 1400psi with better breathing for the 13.4gr's at 900fps? These guys here:
(http://i1066.photobucket.com/albums/u404/appye/Capture.png)
That's right, I was doing some extrapolation from other tunes I have done to look at your scenarios.
Is epoxy good? Or is there something more preferable? I was thinking of just turning down a finishing nail for the actual piece.
I've used both epoxy and superglue to good effect. If you bottom out the probe in the hole, you don't need much to hold it in place because there is nothing ever pulling on it. I wouldn't be surprised if Elmer's glue works well enough.
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That's right, I was doing some extrapolation from other tunes I have done to look at your scenarios.
I am going to have to look at those again. In the screenshot, is it talking about stock porting vs opened porting? I am not sure I understand it. Is "initial force (lbs)" referring to muzzle FPE? I think I am missing something. 25fpe in .177 with 1036 psi doesn't sound right.
Anyway, on to today's post. I want to make sure I have all my ducks in a row before starting any of this. I had some niggling questions ...
From what I have learned these last few days it sounds like I am probably not going to need to, but if I were to go beyond the capabilities of my 1.8k burst disk and install a 3k, is the Disco tube safe at the higher pressures, just in case my regulator fails? Here's a math question one of you may be able to answer for me: I have a reg set point of 1800psi, my 13ci bottle is “full” at 3000psi, the tube is already pressurized, and regulator fails. What will be the pressure in the system after regulator failure? Is this question even valid? Besides things like valve lock or the gun not shooting right, how would one know when a regulator fails? I could see not knowing the reg has failed and pumping the whole system up to 3000psi ...
How does the maximum porting I am planning on affect things like efficiency if I were to again detune it back to 18-20 fpe and shooting 10gr pellets, or even lower?
And now for some crazy ideas I would like to get your thoughts on:
Crazy Idea #1:
I had the idea of maybe filling in the poppet with some epoxy or maybe turning the shape into something more aerodynamic to reduce turbulence? I dunno. Has anyone tried any of these ideas before? Are these ideas even feasible? When I had the idea, I looked around the internet and saw various shapes for homemade poppets. Here are some drawings to illustrate, the grey is supposed to be epoxy:
(http://i1066.photobucket.com/albums/u404/appye/poppet.png)
- Fill in the side parts, making a more aerodynamic shape?
- An even more aerodynamic shape by bringing it all the way to a point
- Rounded?
- Cut the nose off alltogether? Maybe throw some epoxy on there?
Crazy Idea #2
I was thinking about how I could go about fabricating my transfer port. I had the idea originally of using hot glue. Bear with me here. The idea is to goop it right into the exhaust port (blocking it off, of course, only allowing a little glue just enter the port) while the valve is mounted into the tube, making a blob that I could then sandwich in there with the breach and barrel. Once the glue was hard, I would have a mold that perfectly fits into both ports. The bits that went into the ports would serve as as indicators for where to machine the actual transfer port. I would machine it with a drill bit at first, then maybe heat up a nail to go in and “polish” the sides. The final product would be the exact size I need, with nice thick walls, and a perfect seal that conforms to even very small imperfections in the metal.
I might want to first put a notch somewhere, likely on the exposed portion of the valve body itself, to ensure that I place the finished product back in exactly the same orientation. I would probably also want to coat the parts in some vegetable oil or something before putting the glue on it. I could even wrap a piece paper around the tube on either side of the holes act as a shim, ensuring that that when the gun goes together (without the paper) the final product is smashed down a little to ensure a good seal.
I thought of hot glue, because it is easily workable, and seems to have a similar suppleness and rigidity to those clear tubing lines I have read about people using to make their transfer ports. I have no idea how well the final product would stand up to thousands of shots (much less one) when running 1200-1500 psi air through it as it expands to push a pellet forward.
The next thought would be to use something more durable. I thought of melting those plastic shopping bags down and using that. I saw a walkthrough where it is melted down in vegetable oil, as the melting point is above the temperature of boiling water. This would probably be more durable.
I also found these plastic pellets online that will melt in boiling water … I have yet to see how hard and durable they are. One brand is “friendly plastic” if you want to check it out…
[/list]
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ehh ... screw the hot glue idea. I made some mock ports out of PVC and it was too hard to get the piece out without totally deforming it. Maybe the plastic bag stuff would work...
I think I was totally overthinking this thing. I have some other tubing that is more of a milky white, and sort of harder to the touch than the clear stuff, and I think that is what people are typically referring to. It has 1/4 OD x .170 ID written right on the side! It is still flexible enough to just squeeze slightly into an oblong shape, maintaining the same area. I think I could massage that out to .177 pretty easily, or even just leave it alone, giving me 92% ... Yeah, right. I got the 100% bug something fierce! Irrational as it may be.
Anyway, in looking at pictures of the oblong ports people have made online, I am seeing that people are completely machining away (lengthwise) the tapered metal I would want to keep so as to provide a seat for the tubing to seal against. I see two solutions for this. One is to make the oblong barrel port LONGER than required to account for the thickness of the tube walls, perhaps also notching the sides to allow the tube to sit flush with the inside of the barrel. The other solution would be to make the tapered part oblong as well. I believe the second solution would likely provide a better seal, but is also more difficult to do without machining skills ... Decisions Decisions.
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Seems to me that last bit about the machining skills is the most important factor in this. Hot glue, melted shopping bags, friendly plastic, epoxy... none of these is going to be good for more than a prototype part, if even that. Adhesion is the biggest issue with the epoxy on poppet stuff, which I'm not clear on what you're attempting anyway. Why not just make a more aerodynamic poppet? Out of one piece of delrin or nylon would seem the most sensible. Epoxies with which I'm familiar don't tolerate a lot of pressure along thin edges, tending to lift off, peel, crack, flake. A high grade construction adhesive might be better, the sort they use to glue cracks in bridge concrete... but you'd be paying $40+ for a massive tube of the stuff and using less than a dime worth of it. And it probably wouldn't work anyway. Seems what you really need is at least a rudimentary lathe. Even a drill mounted firmly in a vise can provide the basic spinning thingy, then a few files and a block of wood for a tool rest would let you shape plastic rod to some extent.
I've found 95 durometer polyurethane rod to make the most durable and well-sealing seals for transfer ports, Foster nipple check valves, and bolt probe seals. Very challenging to machine nearly though. Delrin is easy to machine, but sometimes too firm for seals. Still, if the surfaces are shaped correctly it makes an excellent poppet. As does nylon.
Considering your obvious enthusiasm for modding, I'd suggest firstly that you consider buying a cheap used lathe. A small bench top model of some sort. Most critical is that it run true, not wobbling or vibrating weirdly, so the bearings have to be good. After that, most things can be smoothed out with a bit of filing, polishing, common sense. A lathe would open up a world of possibilities for modding airguns. You'd love it. And you'd soon understand just how silly it is to consider glomming on bits of glue in a high pressure system.
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Seems to me that last bit about the machining skills is the most important factor in this. Hot glue, melted shopping bags, friendly plastic, epoxy... none of these is going to be good for more than a prototype part, if even that. Adhesion is the biggest issue with the epoxy on poppet stuff, which I'm not clear on what you're attempting anyway. Why not just make a more aerodynamic poppet? Out of one piece of delrin or nylon would seem the most sensible. Epoxies with which I'm familiar don't tolerate a lot of pressure along thin edges, tending to lift off, peel, crack, flake. A high grade construction adhesive might be better, the sort they use to glue cracks in bridge concrete... but you'd be paying $40+ for a massive tube of the stuff and using less than a dime worth of it. And it probably wouldn't work anyway. Seems what you really need is at least a rudimentary lathe. Even a drill mounted firmly in a vise can provide the basic spinning thingy, then a few files and a block of wood for a tool rest would let you shape plastic rod to some extent.
I've found 95 durometer polyurethane rod to make the most durable and well-sealing seals for transfer ports, Foster nipple check valves, and bolt probe seals. Very challenging to machine nearly though. Delrin is easy to machine, but sometimes too firm for seals. Still, if the surfaces are shaped correctly it makes an excellent poppet. As does nylon.
Considering your obvious enthusiasm for modding, I'd suggest firstly that you consider buying a cheap used lathe. A small bench top model of some sort. Most critical is that it run true, not wobbling or vibrating weirdly, so the bearings have to be good. After that, most things can be smoothed out with a bit of filing, polishing, common sense. A lathe would open up a world of possibilities for modding airguns. You'd love it. And you'd soon understand just how silly it is to consider glomming on bits of glue in a high pressure system.
Yeah, I have been considering the same thing. I have seen some of these "mini lathes" online for a couple hundred dollars. These tiny little 6 in 1 lathes (http://www.ebay.com/itm/Mini-Multipurpose-Machine-6-In-1-Kit-Wood-Soft-Metal-Woodworking-DIY-Tool-Lathe-/181991146540?hash=item2a5f84842c:g:QQIAAOSwgNRV8kcq) are of interest to me ...
I know it wouldn't be super precise like a real metalshop lathe, and I likely couldn't do steel with whatever bits it has, but I could use it to make things like delrin/polyurethane transfer ports, my ideas for poppets, maybe even some aluminum...
I do have a drill press ... I'm not a huge fan of spending money ...
Edit:
Okay screw it. $113.05 (with s&h) later, and one of those cheapie 6in1 lathe/mill/etc. tools is on its way. I'll be playing with the big kids now! Well, maybe not quite the all-the-way big kids ... Call it jr high!
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Hi All,
So my new lathe should arrive before the weekend. I have some 5/16" virgin PTFE (teflon) rod on the way as well.
And here I am once again with a ton of questions. ...
Reading up on this, I have seen Bob say that the oblong ports he has done are "only oblong at the boreline, and round at the transfer port. (http://www.gatewaytoairguns.org/GTA/index.php?topic=51503.msg489990#msg489990)" Why is this? I would think that would create turbulence that could otherwise be avoided if the path was oblong the whole way through...
I took a photo of my barrel port as it looks with the barrel slid all the way back into the breech. Notice how far back the barrel port sits in the hole. It is obviously not centered. Is this typical of these parts? In the past, I have either assembled it as such, or eyeballed it as center as I could before cinching down the grub screw... Theoretically, one way would seal better at the port, and the other way would seal better at the bolt. I was never really sure what way to go with it. I never really noticed any air leakage, but also never ran the "tissue test" where you see if taking a shot makes a piece of tissue draped over the breech move. Perhaps this (well, along with a transfer port seal that has definitely seen better days and a bolt o-ring with a flat spot, as I have discovered!) could be the cause of the gun's ever increasing ES, as high as 35 fps at one point ...
(http://i1066.photobucket.com/albums/u404/appye/Off_Center_Port.jpg)
Now, this (off-center barrel port) being the case, and since I am wanting to go with an oblong barrel port, should I just grind it "forwards?" Meaning, should I leave the back of the barrel port alone and do all the grinding on the muzzle end, getting things more centered? I am also not sure what I am supposed to do with the (uncut) part of the taper that leads into the actual bore, as I will have to be cutting into it on at least one end to lengthen the port. Should I attempt to maintain the taper on that end as well? Should I taper it the other way in an effort to maintain the same area on any given horizontal plain leading into the bore itself? Should I leave that end strait? Obviously I am going to want to soften the new edge(s) so as to prevent the pellet from getting damaged as it passes over the hole.
Also, I have read Bob's thread on Installing a 5/16" OD Transfer Port in a Disco (http://www.gatewaytoairguns.org/GTA/index.php?topic=81821.0) ... Filing a flat in the barrel certainly seems like an easy way of going about this, though I have a couple of questions:
- Seeing as how I have an off-center barrel port, would I want to also move the flat forward proportionately, assuming I go with my first idea above of grinding it "forwards?"
- Since the barrel is filed flat, this means that the outside walls of the transfer port are not being held in place there. How does this affect long term reliability of the seal?
Not sure what to do ...
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Haven't handled a Disco, but the basics of barrel and transfer ports are fairly similar. Here's a slightly awkwardly carved oval barrel port in onenof my QB78D barrels:
(http://www.luthier.ca/other/forum/QB78/QB78D_barrel_port.jpg)
It's off at a bit of a silly angle because Dremel bits tend to dive in asymmetrically and I was in a bit of a rush carving this one, not taking it very seriously as it was just to test the full length barrel for a few shots then going back to my 13.4" barrel which I prefer for balance and size. Pretty much identical in area to my short barrel though, measuring 0.175" 4.4mm) x 0.225" (5.6mm). I use a urethane transfer port seal with a round opening of 0.195" (4.9mm). And I suppose there's turbulence owing to the differing shapes, but the similar area of each opening ensures minimal restriction of flow. Shrinking my urethane TP seal by 20% reduces velocity considerably. Enlarging it by 10% (as big as I could manage with that material without it beginning to let air blow by through distortion on firing) did not increase velocity at all. So for this oval barrel port dimension a 0.195" port seems as big as it gets.
The oval with a width 1mm narrower than the bore means pellets don't get damaged on loading. Placement of the extended oval lines up with the TP centre below it, so when doing yours check carefully that you're not extending past your seal area. With the distortion of the urethane seal I use (a fair bit of trial and error getting the depth of the seal just right for a compressed fit) the barrel O-rings don't come into play, there is zero leakage past the TP.
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...
I am dubious. As I am seeing it (correct me if I am wrong) in my head, a round hole overlapping a narrower oblong hole is going to lose some area:
(http://i1066.photobucket.com/albums/u404/appye/roundoval.jpg)
The red ellipse represents your barrel port, the blue circle represents your seal. The purple area represents useful air flow. I would think that the flow from the transfer port is being blocked by the sides of the barrel port (left and right areas in the drawing). Since the barrel is above the transfer port, air only enters the top and bottom areas after passing through the purple area. Using the orientation of my drawing, I see a port that is .195" tall and .175" wide, with a shape that is sort of squared up at the corners.
This would not take full advantage of the .195" round port, nor the .175"x.225" oblong port. You are getting flow that would equal less than either of them.
To account for why a larger round seal did not make much difference, I would say it is because anything beyond .175" is already blocked off by the barrel (the sides), and you are only getting marginal gains at the top and bottom. When you make your seal 20% smaller (.156"), it makes more of a difference because you cut well into the top and bottom areas of the purple, as well as restricting things on the sides below .175"...
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It's actually a lot closer to this illustration:
(http://www.luthier.ca/other/forum/QB78/barrel_port_and_seal_ID.jpg)
And sure, the two shapes are not matched, so there is as I said some turbulence introduced. But getting the barrel port longer made a massive difference in velocity while using the same urethane seal. I had a bit of a dimwit moment around this, initially forgetting that my short barrel's port was elongated like this and doing a few Chrony shots with the full-length barrel then reporting (to rsterne of all people!) that my velocity had changed only a small percentage. After realising my error I carved the port just slightly wider than it had been and much longer, matching exactly the dimensions of my short barrel's port. Velocity, predictably, jumped up. Specifically, it went up about 80 to 85fps with 18.13gr JSB from the baseline of 700fps I am getting with the 13.4" long barrel. With the round ~4mm port it had previously velocity had only jumped to about 835fps with the full-length barrel. Hardly enough to justify carrying all that extra weight around. In my case 700fps is exactly what I want, as any faster with the 18gr pellets would put me over the HFT limit of 20fpe and start damaging the steel targets.
I may yet try the JSB RS pellets at 13.43gr. Just haven't had any handy lately. Efficiency would drop, probably putting my power output to around 18.5fpe (going to 11.9gr Hobby pellets gives me about that much energy with the current setup), but the velocity would go up to something like 790fps, for a significantly flatter trajectory, useful for HFT. I could keep one range card for matches, while going back to the heavy pellet for squirrels at home. Not that I'm thrilled with the idea of using different zeros for different purposes with the same rifle... so nah, I probably won't bother. Prefer to fine tune my holdover for one pellet, one velocity, and really learn that at whatever range.
Oh, and I should probably mention that the hollow probe on my QB78D also has a much enlarged port, matching that of the barrel port and aligning with it perfectly. And I've made a urethane probe seal, 95 durometer so much harder than a normal O-ring and slightly over-sized, square section not round, so it's a fairly snug fit on loading and not leaking at all. Doesn't need a lot of lubrication either as the stuff is incredibly durable. Similar hardness to skateboard wheels.
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I had never heard of HFT before reading your post. That sounds interesting! Why do you use .22 instead of .177 though? It seems you would have an inherent advantage with the smaller caliber, ie, less chance of "plating" a target with the edge of the pellet...
.22 is the more "huntable" caliber though. Myself, I find myself primarily hunting pebbles, cans, bottles and paper. That being said, I am NOT averse to shooting rats, mice, squirrels, rabbits, quail, etc. I love the feeling of eating something I have killed myself. I don't know about eating rats and mice though!
Anyway, I am already on track to go with a transfer port made completely from PTFE, which should seal itself.
On that note, it seems I was once again over thinking things regarding the barrel port taper. Since this is a .177 barrel, it looks like the taper ends long before the bore itself. Looking more closely, I am seeing that I could just file away the taper completely and still have plenty of "meat" left in the vertical transfer port before even getting close to the bore.
Also, since I have a drill press, I was going to try my hand at putting a 5/16" hole in some scrap material with a milling bit. It just seems to me that with a perfectly fitted round recess, the metal of the barrel (and the valve body) would provide reinforcement to the outside walls of the teflon, creating a stronger seal than simply filing everything flat...
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Exactly, I get more *WHACK* out of an 18gr .22" pellet on a squirrel or rat than I did when I tried .177" initially. Some brilliant shots on squirrels in the smaller calibre required a follow-up shot owing to lack of sufficient damage and clean pass-through. So I favour the bigger pellet for that, and shoot HFT casually to both bolster my skills in shooting from various positions and to have fun with other airgunners. Going to an HFT specialty rifle would be cheating, making it too easy. With the QB I was able to shoot two almost perfect rounds last month, my few misses being low by a pellet width each time on the closest targets. I can still get better in .22", so I'm aiming at perfect rounds every time while sticking to the steeper trajectory of the bigger pellet.
Your Teflon should indeed seal well. My only worry with Teflon is its memory, the tendency to take a shape and keep it. Used to use it for doublebass endpin stopper cores with rubber crutch tips on them. But the 20 to 30 pounds of a doublebass gradually squished the 1" teflon rod. It stayed usable, nice and slippery so the rubber stayed stuck to floors while letting player's basses turn freely (no squeaking during performances or recordings), but i wanted something more durable. Went to delrin. Each application has different needs... So long as you keep the seal oriented the same way and the initial furnished snug, it ought to be fine. I went to polyurethane because though more difficult to work with, it has no memory, always springing into shape on release from pressure.
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I did not know about the memory issue. I just went with ptfe because I see that Bob uses it for his transfer ports. 95 duro sounds pretty hard. How hard is it to machine compared to teflon?
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Hi Patrick, just to lend some perspective, using an elastomer for the transfer port seal is fantastic if you are taking the rifle apart and reassembling multiple times. Otherwise a more rigid material will last indefinitely if you're putting it together and leaving it.
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Exactly Jason. I found my previous seal, the one which came with my already-modded QB78D, was gradually getting more distorted through all the disassembly/reassembly cycles as I worked on various tuning elements. After probably 50 cycles it wasn't sealing very nicely, blowing a bit of tissue paper noticeably. It was made of something slightly harder than PTFE, probably a softer sort of nylon though I'm not certain. It wasn't a huge amount of air leaking but any air is more leak than I want.
Hm, how to compare Teflon and urethane... Teflon is like carving a firm, evenly moist Parmesan cheese. Only easier, as it slices incredibly smoothly. Polyurethane is more like carving a superball. In fact I'd bet many superballs are probably urethane, perhaps all. On a scale from 1 to 10 with 1 being extremely easy and predictable and 10 being so frustrating as to make one abandon a material for something less confidence-destroying, I'd put Teflon around 2 and 95 durometer polyurethane around 8. In a 1" rod (the size I picked up from an eBay seller some years ago when I was hoping to make a piston seal for my Baikal 46m - couldn't do it by the way, much too difficult for me, though I may try again some day now I have more practice) it is almost wood-like in stiffness and apparent hardness, sort of like pine. In the dimensions of a transfer port it is squishy, easy to distort, making it necessary for the tools to be extremely sharp and the lathe spin to be fairly high. Cuts extremely easily, no resistance, but tends to grab the tool. I'm not recommending it for its ease of use. I just love how it works when I finally get one made in the perfect dimensions. It'll survive any number of rebuilds.
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do you have a link to the mini lathe you bought??
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Googling "elastomer" ... (doe-eyed newbie staring at the headlights) ...
Okay, so teflon would be an example of a rigid material, and polyurethane is an elastomer then? Since it is the only plastic rod of any kind (besides hot glue sticks) that I have in my posession, I will go ahead and use that for now. If I find that I keep taking the thing apart, I may look into an elastomer.
On another note, it seems I will be turning down the nose end of the probe, being careful not to get too close to the oring slot. My understanding is that anything protruding over the barrel port is a restriction:
(http://i1066.photobucket.com/albums/u404/appye/another%20restriction.jpg)
My understanding is that the little lathe I bought should do okay turning brass or aluminum, so I may try my own take on a retracting bolt. I wonder how well Bob's "j-slot" retractable bolt seals compared to a traditional o-ring design. I was thinking of something more along the lines of having a retractable probe inside of the bolt. Of course, there's the whole "bolt handle threads are in the way" issue ...
I should be doing the machining and (hopefully) assembly on Friday, assuming everything arrives by then, and I don't chicken out. That's the big thing for me, I only have one chance to get this right!
David, here is the link:
http://www.ebay.com/itm/181991146540?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT (http://www.ebay.com/itm/181991146540?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT)
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That looks like a nifty little machine. My only concern when reading through the description is that it keeps mentioning a motor speed of 20,000rpm, with no mention of gearing that down to different speeds. The 300rpm to 1,200rpm range is practical for metal machining, but 20,000rpm could be dangerous to the user as well as being extremely difficult to work with. Let us know how it works out.
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Right. Teflon is a rigid material with very little elasticity. When forced into a particular shape, it pretty much stays that way.
Contrast that with an elastomer (elastic polymer). What the layperson would generically refer to as rubber.
Regarding the bolt probe, yes it would benefit from backing the tapered section up where it obstructs the port.
I wonder how well Bob's "j-slot" retractable bolt seals compared to a traditional o-ring design.
Hmm, well usually a retractable bolt seals in the same manner with an O-ring on its shank. Just give some thought as to how far forward the O-ring must go to seat the pellet, and where it will come to rest when withdrawn to clear the barrel port. That path needs to have parallel walls so it may be necessary to shape the chamber to accommodate the O-ring coming forward farther than usual.
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Well, the whole idea of a chamber without a probe is gonna have to wait a bit anyway, depending on what I am able to glean from this gun. So far, the 10.3gr and 13.4gr slide right into the chamber without any marks from my .025" probe. The 16.2gr pellets are a tighter fit and do exhibit a light dent where it comes in contact with the probe. I will not know how that affects things until after I fire the gun of course.
That looks like a nifty little machine. My only concern when reading through the description is that it keeps mentioning a motor speed of 20,000rpm, with no mention of gearing that down to different speeds.
I believe it was just referring to motor speed, and not shaft speed. Likely it is only one speed though. I might be able to fashion a regulator or something to slow things down. It is obviously not the most accurate machine in the world, but for a newbie, I think it is perfect for now. If I really get into this, I may pick up a real lathe and/or mill.
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looks like a cool little gizmo. versatile too. probably works just fine for small light duty stuff.... cheers
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sigh. This is disheartening. When they came in the mail a couple of weeks ago, I turned the "GAUGE ADAPTER FOR BENJAMIN MARAUDER AIR RIFLES" (as it is listed on "archery's" website) into the PRod Gauge port about 4-5 turns. It seems like it went in okay, and I have not yet inspected the threads on the gauge adapter, but now I am seeing a post in another thread by Bob saying that they are not compatible, as the adapter is straight thread and the port is npt... I hope I did not screw up gauge port. What is bothering me is that it says right on their website that it "Also fits Benjamin Discovery air rifles and Benjamin Marauder pistols, Model BP2220." ... even though on closer inspection, the adapter is nowhere to be found on the PRod EVP...
First, they sent me an MRod port instead of a PRod port, and insisted that I pay for shipping back to them (rudely, I might add ... The lady on the phone kept cutting me off and tried to tell me that this is the way it always works when ordering things from anywhere on the internet) before they would "reimburse" me (shipping was about $4.50 and their "reimbursment" was barely $2.00) for the shipping and send the correct part back, now I find they are mislabeling things on their website. I am not sure how I feel about this ...
Do you guys think I messed up the threads on my gauge port? I do remember that the threads do seem to interlock pretty well when putting two male ends against each other, so I figured everything was good ...
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Okay, so I am learning. Without a return spring, I was able to fill the gun up without any problems. Perhaps this is because I first "feed" the pump with 130psi from my shop compressor to get the first 1000 pounds into the gun, then I drop the shop compressor's regulator pressure down to about 30-50psi and fill the gun the rest of the way from there.
Anyway, I did my machining (more on this later) and got the gun together. The bottle mounts on there SOLIDLY through the gauge port and with the plastic mounting brackets I had on there. Awesome.
Of course, this wasn't going to go without any problems... I seams I had a LEAK! I am pretty sure it was at the seat...
With 1500psi in the tube, and with the bolt closed and hard against the breech, I can put my finger over the end of the barrel and feel pressure build up against my finger. When I release my finger, I get an audible release of pressure there. Either I did not adequately clean the dust opening the exhaust port, and some bits got in there ... Maybe it was when I was using hex keys to get a throat diameter, I nicked the seat somehow! I dunno. At first, I thought the poppet needed to be broken in (the new poppet lacks the groove that was worn into the original poppet by slamming into the valve seat thousands of times), so I dry fired the gun to empty. Refilling it did not avail any improvement. To make a long story short, I tried and failed (horribly!) at resurfacing the seat and the valve is probably ruined ...
On to my question: I still have the original CO2 valve that came with my 2400kt. If I can manage to get the holes centered correctly for the other two valve pins, is it safe to use the back half of this valve? Other than only having one pin hole, it seems identical at first glance, other than it being brass and not aluminum. Is it bad to have a brass back half and an aluminum front half? If not, then I am gonna have to wait another week for a new valve ... sigh.
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Hmm... It should probably be noted that I did try to streamline the poppet by turning it into a slightly more aerodynamic shape:
(http://i1066.photobucket.com/albums/u404/appye/PoppetSide.jpg)
...Perhaps the leak is going THROUGH the poppet somehow?
I made sure that I did not change the diameter of the edge where it contacts the seat ... and the face (sealing surface?) of the poppet seems undamaged and feels smooth all the way around:
(http://i1066.photobucket.com/albums/u404/appye/PoppetSeat.jpg)
Unless anyone dissuades me, tonight, I will try my hand at converting the rear side of brass CO2 valve to this gun. As I understand things, I need to get the screw holes centered just right as well as file flats on the hull to allow the screws to fully seat. Then I will have to put another piece of that hobby tubing in there to get the stem sitting pretty.
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Okay, so this will be valve number THREE I have screwed up. The first was the original disco valve (rear half) I had that would not seal at the poppet. The second was the rear half of the brass 2400kt valve that I tried tapping. The tap went right through into the valve throat. This one, while I was enlarging the port, I wasn't paying close enough attention, and drilled right into the adjacent valve pin hole.
... Being in the throat, and not in the main part of the valve, would it still seal if I were to goop the screw with lots of threadlocker? Maybe ptfe tape? How badly would it affect efficiency and/or performance, seeing as how that section of the throat would no longer be perfectly round? I would think (maybe the operative word is "HOPE") that with the screw filling in the hole just about all the way to the throat, it would be negligible. Most importantly, would it be safe to use? It would only be holding back pressure during a shot, as it is not in the HPA side...
These setbacks have been a learning experience to say the least ......
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Yeah, it's not ideal but it's more of a psychological setback than an actual one. :)
A number of folks have drilled into a valve throat while adding anchor points to secure the valve. Your are correct that some threadlocker on the screw will seal it, but even lacking it, there would not be enough loss through the threads during the very brief time the valve is open for it to really matter.
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Well, I feel better now then! :o
It seems I just might be finally putting this gun back together after these weeks. I will be sure to share every hair pulling detail and post pictures of this hackjob learning experience.
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I am finally seeing a light at the end of this tunnel! All I have left to do is the transfer port, and I have plenty of material to go through in case I mess up. I wanted to check with you guys on something though:
(http://i1066.photobucket.com/albums/u404/appye/IMG_0801.jpg) (http://i1066.photobucket.com/albums/u404/appye/IMG_0800.jpg)
Now that's COMEDY! Is it still gonna work, or will the stem end up coming loose now? Is the poppet just molded around a straight piece of stem, or is it molded around something bigger as well? I made sure I had a spare before I started hacking away at this one...
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If that's a delrin poppet, then yeah, no, that's gonna come loose as it's just press-fit onto the stem. Black comedy indeed! Ouch! By the way, a bit of 0000 steel wool polishes delrin nicely if you don't spin it too fast and melt the stuff. Just go easy, pause to let it cool, and you can get quite a decent finish, although of course a lathe and sharp tooling work better.
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well, i don't know what the material is honestly ... It is a stock m-rod poppet straight from Crosman. I am thinking that if it is just a straight shaft, then it will come loose, but if it is molded around something besides just a straight shaft ...
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So this is what a Disco poppet looks like when removing the plastic part. I am wondering if the mrod stem is "over molded" around something similar. I suppose I could destroy my existing poppet and find out, seeing as how I do have a spare, but I was hoping someone could clue me in?
(http://i1066.photobucket.com/albums/u404/appye/Mobile%20Uploads/image_1.jpeg)
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I took an Mrod stem apart before and it had groves in the stem that held the delrin on. I also did what you did to a stem before and it worked fine in my crosman 2100.
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Okay, I ALMOST have this project finished. The turned down, cone shaped poppet has been holding so far, and the tube holds pressure over a couple of days without any movement of the needle.
Once I am ready to finally TUNE this thing, seek out the knee, etc, you will be seeing a MARATHON post I have been working on that chronicles the trials and tribulations I have gone through getting this hackjob up and running properly!
So last weekend I had a leak at the transfer port where I could feel a puff of air during every shot. I also had a leak at the breech seal, the o'ring that sits at the end of the bolt, sealing it to the chamber. Well, I got the transfer port sealed up really nicely and there is no air coming out of that during the shot.
The breech seal is still leaking however. What is odd, is that I never noticed this before. While I suppose it is possible that I just wasn't aware of it, seeing as how I was moving much less air, there are a few other things that are different now:
- I installed a white polyurethane 90 durometer o'ring instead of the softer, stock black one.
- I turned the nose of the bolt down so only the probe itself sits above the transfer port. The o'ring still seems to snap right into place though, and the part it is smashed against while the bolt is closed is unmodified... That is where the actual seal takes place I would assume.
- I briefly spun the bolt itself in my drill press and lightly polished it with some 500 grit sandpaper.
My initial thinking is that this problem is two fold:
- The 90 duro o'ring, being much harder than the original o'ring, does not seal against the barrel as readily.
- Last winter, when I "polished" the leade and broke the sharp edge going into the rifling, I also worked the surface of the barrel that the o'ring sits against. The highest grit sandpaper I had at the time was 500 grit. I have since rectified this situation and I have 800, 1000, 1500, and 2000 grit sandpaper sitting next to me as I write this. I will be polishing it up tonight, and if this does not fix things, I will try it with the old o'ring...
I was thinking I may also try epoxy and some beer can material on the bolt handle, making it a little thicker, thus pushing the o'ring harder against the barrel when the bolt is closed.
Any other ideas?
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Okay, so polishing things up did not help. Neither did the softer o'ring. The puff of air is definitely coming from the breech (bolt?) seal, and not the transfer port.
I am hoping someone can help me with this. I THINK I know what is happening. The o'ring may be extruding slightly into the elongated barrel port. At least this is what the current running theory is anyway...
What I am thinking now is that I can move up to a larger o'ring, and cut the channel on the bolt back slightly to move the new o'ring backwards from where the current one is. Any input is appreciated...
Here is what things look like through the barrel port with the bolt in the cocked position:
(http://i1066.photobucket.com/albums/u404/appye/IMG_0895.jpg)
You are seeing a steel probe that is embedded in some nylon rod, which is inside of the brass bolt. The white o'ring looks sort of grey/green in the picture, and is what you see closest to the edge of the port. From straight up, I did not notice the o'ring before:
(http://i1066.photobucket.com/albums/u404/appye/IMG_0900.jpg)
I did the ball grider thing on the inside part of the port as well, so whatever little bevel there isn't going to help the situation ...
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Okay, so I put a tissue on the front part of the breech, where it meets the barrel and it popped very noticeably when I fired a pellet. I guess this means the leak is the transfer port after all. Is a transfer port made completely from polyurethane (95a) not going to work? Is the material still too soft? I opted for that in lieu of PTFE because I wanted the transfer port to stay the same shape between a bunch of rebuilds. At some point, I do plan on keeping the gun assembled for a long period of time, but for now, while I am continuing to take it apart and put it back together (sometimes several times a day), I need a transfer port that is going to survive many rebuilds...
Since my last post, the barrel port and the valve face are now filed perfectly level and polished for a good seal. This did add another ten to fifteen thousandths to the needed size of the transfer port, so I am going to have to make another. Third time's a charm I hope...
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My second attempt at fitting a 95 durometer urethane transfer port works great, with zero tissue flutter on firing a JSB 18.13gr at 700fps. Checked both sides with a single ply of tissue and nothing moved but the trigger, hammer and pellet. The first one leaked slightly. It was an easy fit, no effort to insert it into the hole in the cylinder. So I made the second seal a bit harder to snap into place and it's working great. It is also squished between barrel and valve by about 0.012".
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Cylinders would have been fantastic, and it what I originally did, using a mill. It wasn't until I got home that I realized that it milled them both oversize and the valve/tube one off center. I think the bit was bent or something. Once I got my 3/8" polyurethane rod in, I put it in and saw that the valve had a leak at the poppet, and I ended up ruining the valve trying to fix it.
With the new valve, I just filed a flat. Yesterday, I also filed the barrel itself flat (the breech still has a nice round hole, so placement should still be easy) because on closer inspection I discovered that the bit left a high spot around the edges, which I thought might be the cause of the leak.
Anyway, I will also go with somewhere between .010" to .015" oversized to make it squish in there really well.
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Okay, so total distance between the valve exhaust should be .277" ... Yeah, I filed the @$%^ out of the barrel. Both surfaces are perfectly flat as far as I can tell, and polished with 320, then 500 grit sandpaper. I have higher grits but it seemed pretty shiny and smooth with 500... I always put a film of silicon grease on each side of the transfer port before installation.
I made a transfer port that is .290, and I drilled a 1/8 hole through it just so the gun would work and I could test a shot or two. Once I have a port that does not leak I will open it up to match both the valve and the barrel ends.
I still get a puff of air... I feel it on my cheek. Perhaps facial hair helps with this! I won't bother with tissue until I am not able to feel it.
Firing a pellet with a clamp on the front of the breech does seal things better (I feel less "puff") but not completely. The breech screw was upgraded to 8-32 and all screws are nice and tight. I don't know if this means I should go shorter or taller with the transfer port, but I will make one at .300 and another at .285 and see what happens. I am also wondering if I should purchase a second barrel band so it can hold it things more tightly at the breech end...
This project is trying my patience!
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FINALLY!!!!!!!!!!!!!!!
.300 did it. At least with the 1/8 hole in there. I also moved the barrel band to the breech (though I don't know that it was necessary) and really tightened the @$%^ out of all the screws. I may be ordering a second band for the muzzle end tomorrow to avoid the changing poi issues I see people have complained about... Since it is regulated, however, I won't have the problem with changing tube pressure bending things around. I guess I can see how it behaves with the barrel "floated" like that... the barrel sure does flex around though!
Here's to hoping that opening up the transfer port doesn't cause me pain and heartache.
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Okay then! Gun is done with final assembly (for now!) and I shot a five shot string with an arbitrary spring/ssg setting (basically turned in just before coil bind and a lot of preload). It produced a 12fps spread with a maximum of 929fps using jsb 16.2gr. Tomorrow I will actually tune the thing, and calculate efficiency...
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Very nice, Patrick, I'm glad to hear you whipped it into shape. I've been following but haven't had much to contribute. Keeping my fingers crossed that the heavy .177 pellets group well. A 30fpe .177 would be interesting for some distance shooting.
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You and me both! I have probably 100 of the jsb 16.2gr, so probably enough to tune and maybe get a few groups in. Depending on how well they do, my next ammo order will include some more Jsb 16.2 gr, and some h&n 15gr to try, and I guess I'm gonna have to see how piledrivers work out. They're a bit pricey, but still considerably cheaper than .22lr... The 15gr are priced really well ...
I do know that the jsb 10.3gr produced reasonably consistent sub 1" groups at 55 yards, so maybe I gained a little efficiency with those. I can definitely shoot them faster! I also have two tins of 13.4gr jsb's to try...
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Okay, I still haven't had a chance to actually tune it yet. A brake job, the wrong set of pads, and another trip to the auto parts store put a stop to that. Oh well.
I did play with it for a couple of minutes though (in the dark) and found something kind of interesting. More testing required I guess. I backed the stop nuts for the SSG way back to the end of the rod, well out of the way of any hammer/spring travel. This effectively disables the SSG, no?
All shots were fired using 16.2gr JSB Beasts.
Now, If I am remembering this correctly, starting with the RVA (the one that comes on a Challenger) turned out 1.25 turns (the maximum preload possible, 1 turn out will not cock, as it hits coil bind), I took a few shots that were all in the mid 930's ... I then backed the RVA way out (perhaps 9 turns? I did not pay close attention) and took three shots. 978, 977, 978. The power went UP! That's also awesome consistency, but of course, each shot likely used A LOT of air, and I would not want to tune the gun this way. I took a quick shot this morning before work, backing it out further to exactly 12 turns and found it was at 925, so it looks like the knee is in there somewhere.
This leads me to believe that this particular spring (2261-009, from the Disco EVP) somehow hits harder with something less than full preload. Or something else is going on ... Perhaps I am imagining things? Can someone shed some light on this?
New brakes are on the car, and barring any disasters. I will verify these results tonight, and hopefully tune it ...
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Okay.? I guess I was imagining things... Maybe I grabbed summer 13.4's without realizing it ... Or alien hairless elves from the planet mergatroid... Anyway, I was unable to reproduce 978 with any setting on the RVA with the ssg nuts removed. The plateau is around 940 with these pellets, so I will shoot for 915-925 or so with as little preload add possible while maintaining a small gap. I am pretty close now...
I also ordered one tin each of JSB 16.2gr, H&N 15gr, Eunjin 16.1gr, and some 21gr piledrivers. I am going at least one of these will group well. I have read anecdotal reviews on all of these that go either way, so I guess I will have to find out for myself..
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All right! I still haven't had a chance to tune for efficiency because I don't have enough jsb 16.2gr pellets left to run a string... More on the way. That being said, the 16.2gr JSB Beasts have DEFINITELY replaced the 10.34gr as my top pellet! After cleaning the barrel and extending my probe a little further, I am CONSISTENTLY getting .5" (and even below!) 5 shot groups at about 40 yards when shooting at 900-910fps, scoped and bag rested. Normally, my long range here at home is more like 55 yards, but a large burn pile sitting in the way says otherwise. But just look at these groups!
(http://i1066.photobucket.com/albums/u404/appye/Mobile%20Uploads/image_3.jpeg)(http://i1066.photobucket.com/albums/u404/appye/Mobile%20Uploads/image_2.jpeg)
The 13.4gr Monsters seem to group well too, but I didn't get the chance to really test them out, and find their optimal velocity. That will be tomorrow. The Beasts are obviously a tighter fit as they load into the barrel, which may account for the better accuracy.
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Dang! It's been nearly six years since I posted on this!
Anyway, over the last few days I have been machining (milling machine and lathe versus by hand with rotary tools) on a new bolt, valve body and barrel. I am going with a Maximus barrel, hopefully it shoots as straight as the old one! Besides assembly and testing all I really have to do now is to mill an oblong barrel port and blend a (95a Polyurethane) transfer port. The porting should theoretically start at 97.75% at the exhaust port, tapering down to 97.5 at the barrel port, and finally 97.13% behind the pellet, allowing for the (.030") pin probe. I had ideas about going to 100% by machining a retractable pin into the bolt (or going with Bob's "j-breech" idea) but I just want to shoot this thing. If I remember right, I was somewhere around 1.7 fpe/ci before, so perhaps that'll go up a little with the more precise machining and the longer barrel. I will tune it to 30-35fpe with JSB Beasts. I'll probably try some of the .177 slugs I see on the market as well. Grouping on paper is the most important factor to me, really.
The gun has been sitting in pieces for years now, so once I have it reasonably tuned and shooting straight without leaks, I plan on leaving it that way for some time. Famous last words, I know.
QUESTIONS!
I would like to fit an air stripper ... Does anyone know of one that fits a .177 Maximus barrel? If not, perhaps there is one that will fit with some light machining? Suggestions welcome!
I originally outfitted this guy with an original iteration of Bob's SSG on a Challenger RVA, and I will likely use that. I did purchase a TSS (P-Rod version?) some years ago that I may tinker with, but I don't like that I can't adjust both springs. I've never even used it. I'd also have to drill and tap the holes in it so I could mount it to this Discovery tube as well. I want to be able to drastically adjust power levels like I could before. This way, i can shoot at loud, full power here at my house (rural, in the mountains), and lower it down to some super quiet back yard friendly level when visiting family in L.A., and everything in between. If I remember right, the gun barely made any noise when shooting at ~8fpe and was still precise and fun to shoot at ~30 yards at my folks place. At lower power levels, I can easily shoot through an entire tin of pellets on one fill. With the current regulator setting (1500 psi or so if I remember right), I was able to adjust from 30 fpe all the way down to not even opening the valve when releasing the striker. I guess the question I have here, is more or less as follows: The SSG was a brilliant idea that was pretty groundbreaking in terms of vastly increasing the efficiency of our guns. Have there been any other improvements like this in the last five or so years that I might not be aware of?
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so ya want to add a stripper... need the 1/2 unf adapter from crosman... then ya gotta go to the red light district at Hastsanville...
https://www.airgundepot.com/hatsan-air-stripper-ha90100.html (https://www.airgundepot.com/hatsan-air-stripper-ha90100.html)
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Hi Patrick, good to see you around again.
An SSG is still a favored approach for eliminating air-wasting hammer bounce.
Another one you may not yet be familiar with is a “Cobra” valve. It uses an internal chamber as an air spring to manipulate valve dwell. The general strategy being to tune for a snappy shot cycle (low dwell). See Scott’s excellent writeup for more info:
https://www.gatewaytoairguns.org/GTA/index.php?topic=139401 (https://www.gatewaytoairguns.org/GTA/index.php?topic=139401)
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so ya want to add a stripper... need the 1/2 unf adapter from crosman... then ya gotta go to the red light district at Hastsanville...
https://www.airgundepot.com/hatsan-air-stripper-ha90100.html (https://www.airgundepot.com/hatsan-air-stripper-ha90100.html)
I don't know if a thread adapter would work ... Here is a picture of my barrels, the Maximus (the one I plan on using) on the left, followed by the barrel (same as discovery?) and muzzle brake thing that came with my 2400kt on the right. None of these are threaded.
(https://lh3.googleusercontent.com/pw/AM-JKLUnKDO_E_71auEchm37OQxWJYBnHW3rlxjvcnz5v3zfBH10_upI_ouNEBCQTdXiZyM7lzN0AxbeuGLSwV4mVp_oKjSv5hIOYKbhQPE0ty38ozURo1CDi8BUsiMsD7xb3uIFhSv90zta9d3Tlb0BevJEQw=w1328-h1020-no?authuser=0)
Hi Patrick, good to see you around again.
An SSG is still a favored approach for eliminating air-wasting hammer bounce.
Another one you may not yet be familiar with is a “Cobra” valve. It uses an internal chamber as an air spring to manipulate valve dwell. The general strategy being to tune for a snappy shot cycle (low dwell). See Scott’s excellent writeup for more info:
https://www.gatewaytoairguns.org/GTA/index.php?topic=139401 (https://www.gatewaytoairguns.org/GTA/index.php?topic=139401)
Thanks! Good to be around again. I definitely caught the bug again! By the way, have people broken the "2 fpe/ci barrier" yet? That was the goal we were shooting for when I was last active.
I'll check this valve out! That said, while perusing some of my old posts, I see that I mentioned an "art/sykes" valve a few times. It seems I have forgotten what this is, and I can't seem to find any information on what it is or what it does.
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See no pic... the adapter is slip/glue on but I drill and tap and use a grub screw...
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Whoops. I added the pic.
Okay, so perhaps I can drill the threads out of the air stripper itself then, mounting it directly to the barrel? Then drill and tap a grub screw. Would this work? The knurled end of the barrel is about .440" at the end. The rest of the barrel is more like .435" or so.
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My Maximus barrels are the same as the disco barrel... the adapter still works I drill and tap so the grub screw engages on the flat...
The other Knurled barrel I think is from a Maximus Euro or Hunter... which came with the adapter... I would have to dig to find the part #... but have to go very soon... will look tonight if hasn't been posted by then...
take a look at pic of fill port and muzzle..
https://www.airgundepot.com/benjamin-maximus-hunter-22-cal.html#gallery-3 (https://www.airgundepot.com/benjamin-maximus-hunter-22-cal.html#gallery-3)
can see it on the EU fpe version also
about 2 min in on vid...
https://www.youtube.com/watch?v=QwfOSG3m6wc (https://www.youtube.com/watch?v=QwfOSG3m6wc)
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Hm. I found these:
- 6-2250XL-001 - Adapter, Silencer, UK
- GBMP-033 - Dust cover
These would allow me to attach the +atsan air stripper? Does the adapter just press onto the barrel?
I didn't realize I got a EU version of the barrel. That big ol' adapter is kinda weird looking. Maybe I will just buy the stripper and see if it can be made to work before getting the adapters ...
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Hm. I found these:
- 6-2250XL-001 - Adapter, Silencer, UK
- GBMP-033 - Dust cover
These would allow me to attach the +atsan air stripper? Does the adapter just press onto the barrel?
I didn't realize I got a EU version of the barrel. That big ol' adapter is kinda weird looking. Maybe I will just buy the stripper and see if it can be made to work before getting the adapters ...
The Hatsan stripper is threaded 1/2-20, hence the adapter.
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The Hatsan stripper is threaded 1/2-20, hence the adapter.
Fantastic! This means I can thread the end of the barrel and attach the stripper directly, much sexier than using the funky adapter thing!
... I wonder. Is it possible for a device to be a stripper AND a moderator? If not, the potential accuracy gains of a stripper is more important to me. Plus ... You know ... stripper at the end of my long ... barrel ... *cough* *ahem* ... 'scuse me ... ba-dum bum.
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A chap named UK Neil does make a stripper-moderator, but he is in the UK.
I have one of his "pickles" in Asia.
Speaking of Hatsan strippers, I have 2 of them.
Was not satisfied with the finish, so I had the housing powdercoated.
I also had different cones made for me.
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Heh heh ... you'll likely hate me for this then. I generally like my things to look old and beat up. Sort of a mad max style! Really, I just don't care what my stuff looks like so long as it performs. The stripper is gonna look like a shiny polished thing on the end of a rather yucky looking gun, that's for sure! I'll post pics when it's all put together!
I'm about to finalize the purchase of the stripper, and will likely do some more machining tomorrow. The most difficult part will be the transfer port I am sure. I have six inches of 3/8" 95a polyurethane to play with for that. This cobra valve thing does have me intrigued though. I will be researching that a bit tonight.
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Yeah, I like my stuff without blemish, lol!
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One of my Hatsan strippers:
https://www.gatewaytoairguns.org/GTA/index.php?topic=200979.0#msg156348063 (https://www.gatewaytoairguns.org/GTA/index.php?topic=200979.0#msg156348063)
The cone is stainless in lieu of the brass OEM.
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UK Neil:
https://www.gatewaytoairguns.org/GTA/index.php?topic=196943.20#msg156339791 (https://www.gatewaytoairguns.org/GTA/index.php?topic=196943.20#msg156339791)
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Like you, I am fond of Crosmans/Benjamins:
https://www.gatewaytoairguns.org/GTA/index.php?topic=200035.0#msg156335744 (https://www.gatewaytoairguns.org/GTA/index.php?topic=200035.0#msg156335744)
https://www.gatewaytoairguns.org/GTA/index.php?topic=199666.0#msg156330300 (https://www.gatewaytoairguns.org/GTA/index.php?topic=199666.0#msg156330300)
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Lol. I wouldn't say I am partial to Crosman/Benjamin so much as that is what I have. My total gun collection is as follows:
AIRGUNS, in order of how much I like them ...
- Daisy Red Ryders - A regular one and a girly pink one. They're both really imprecise and not all that fun to shoot.
- Steroided Sheridan silver streak - This thing ... not worthy. It can't shoot any pellet accurately, and the barrel keeps coming separated from the tube. It is what I give credit to for initially getting me into airguns though. At least as an adult. I had one when I was a teenager and never had any problems, but this one ... yeesh.
- CO2 powered Dirty Harry's revolver looking something or other .177 pellet gun. My dad gave this to me a year or so ago when he got one for himself as well. It's fully capable of maybe hitting a can at 20 yards sometimes. It's absolutely ridiculous!
- A break-barrel something or other that my brother gave me when he got divorced. I've fired it a handful of times. I don't really like break barrels.
- Bug-a-Salt - it shoots table salt. I removed the safety because it caused problems. My wife and I shoot each other in the @@@ with it sometimes. It does kill flies as advertised.
- Crosman 1377 - Mostly stock. Several days after I moved into my house, I found this thing full of rust and spider webs in an abandoned camper trailer a mile or so out in the woods! It sat in my garage for a year or two before I did anything with it. I eventually took it apart, replaced a single o-ring, cleaned and lubed it and put it back together. It's still really precise and functions perfectly but looks like it's been through a war. I did add a rifle stock to it and polished the lead and the crown. I used a hand drill to make front and rear aperture sites for it and epoxied a spirit level to it. It's actually pretty fun to shoot! I'll probably add the 24" 2400kt barrel to it once I am done with the frankengun.
- Daisy Avanti 499 - a grown-up's red ryder, more or less! Single shot muzzle loader. This thing is fun, and super precise at short distances. It mostly shoots regular daisy zinc coated bb's because they're cheap (some fraction of a cent per round) and available at any any box store. With these, it'll smack around bottle caps at 10 yards, and hit cans at 25 yards all day long. I haven't really tried to see what it is capable of beyond that. At about $0.006 per round, it's even more precise with the precision ground bb's, busting aspirin at 10 yards! I love this little thing!
- Frank! The gun discussed in this thread. It'll be tuned to 30-35 fpe (.177!) or so, regulated, 2 stage trigger, ssg, stripper, ported, polished, yada yada yada ... It's gonna be awesome! This gun is my ugly baby.
POWDER GUNS, also in order of how much I like them ...
- 12 Gauge shotgun of some kind, again from my brother and his divorce. I've still yet to actually fire it.
- An old single-shot remington .22 something or other, once again from my brother when he got divorced. Once I cleaned it up and serviced the barrel, it's plenty accurate and fun to shoot.
- A CZ-452 Ultra-Lux .22 - first firearm I ever purchased. It hasn't seen much use since I got into airguns and it's still all pretty and shiny. It's a bit cumbersome to carry around all day, but it sure is fun to shoot! It's the only gun I shoot with a scope.
- A Henry lever action .22 I got from a buddy for building his computer some years back ... I love lever actions. It's my favorite, besides the Frankengun, it's super light and I can carry it all day. It's actually still quite pretty!
We do have a a couple of squirt-guns that I am aware of ...
Years ago I made a blow dart gun out of a length of copper tubing. The darts were essentially music wire with a small cone made of paper and scotch tape at one end. Small finishing nails weren't so bad either, so long as they were light ... I actually got pretty good with it!
I've also got a folding wrist-rocket and a slingshot. I don't care much for slingshots but do enjoy the wrist-rocket. I've even gone so far as drawing elevation notches on one of the bands. I shot at cans from 25 yards or so.
We also have plenty of tissue paper and I am sure I can scrounge up a straw or two if I look around.
I've been know to throw rocks from time to time ...
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UK Neil:
https://www.gatewaytoairguns.org/GTA/index.php?topic=196943.20#msg156339791 (https://www.gatewaytoairguns.org/GTA/index.php?topic=196943.20#msg156339791)
That long stainless thing on the end of the 1377 is the stripper/moderator thing? I looked up UK Neil, and I could only find old forum entries about him. I couldn't find any kind of detailed info on them.
I went ahead and ordered a Hatsan stripper anyway. I don't think I am going to care about quiet shooting all that much anyway.
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UK Neil:
https://www.gatewaytoairguns.org/GTA/index.php?topic=196943.20#msg156339791 (https://www.gatewaytoairguns.org/GTA/index.php?topic=196943.20#msg156339791)
That long stainless thing on the end of the 1377 is the stripper/moderator thing? I looked up UK Neil, and I could only find old forum entries about him. I couldn't find any kind of detailed info on them.
I went ahead and ordered a Hatsan stripper anyway. I don't think I am going to care about quiet shooting all that much anyway.
No, it's one of my custom UK-made moderators aside from the ones from Matt Winser.
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This is how it looks like, the Hatsan AS, with an adapter (24" 22xx Crosman barrel):
(https://i.imgur.com/6Kgg3K1.jpg)
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(https://i.imgur.com/3Wb6x60.jpg)
(https://i.imgur.com/Cc3niMv.jpg)
(https://i.imgur.com/JIOzyZf.jpg)
IIRIC, I had 3 or 4 close-tolerance cones (very cheap in Asia) made.
The OEM .22 orifice is 6.5mm or more, as I recall, so I had some made in 6.0mm and 5.0mm.
The cone is adjustable, btw, and is locked by a set srew.
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Sigh. I guess I overtightened my collet nut and created some excess runout ... Or something. Now my .1325x.2095" barrel port is closer .140x.2095" ... Instead of 97.5% at the barrel port, it is more like 106.37% according to Jason's spreadsheet. Now I'm just mad ... >:(
(https://lh3.googleusercontent.com/8ztdb-fF25eUV1rjIAxXFvIplwjo7u7OK2nMr8udVqUN_cRxa_NGY96tltLuRZ2RoWd8it1cL3vxLw1m4X1rL71MMcgpAWvD1uxNcQj436RLyn2bYgnx_564jXGSgl1nGrvm8je9E3eVr003ulSfF_kth6pcNZC5Dy1onpzpwG1NGCTxOYaDzVXNzo75yOEsLMm97sSot1alF_fUfsLH0Ui7-exVKCudC25MA9Ggp-SgIPWe1mITIY-Qi6XY8yvYqOlbzCCJWxb-pzb8BVttUfrIqpIqQnvYfIuGfyP4Qd-r8b21h5Oi4qr4PTTD0H2gdL5JUe5sbQ_cNaWQKIm5f9X8UhNLyC_utJFve6Y8qJdq_1GeoWnxymzgZWR6-Qrvwh6Z8UUOV-NSqNZbNVdR9OzvteLEZrhSKowORBEMXg3rg8wQr-vePh2sjwRS5MRkRyPmz7XVeIKE9AKQmPDAtxxbKeuRpxmjT7QJbnChn7T57MyxZw1YEXxJaP8JpVbQhS8Sizr83noN9JlOpHWa9oissa_EUBq1TeU4mAJIvSSGlHKD9T9AwT8vwb3Swj8TR7w_3tpYwcYGMSpAzfUDpeUQMrOnj37cl4bUYcMtzGl5oWPnwqe2SGMJtsLzTdCpSOSUg6f3elAlo49nUT3AfJ-T5iN3xbz1ZdhupmQmk1uyCDkJ4kmQ2bNPp51KjcVSE4mSZz_y-8BfT1ZiY3sA6xXk2cEQ6gvwggJvqlCkGgsZ6DeldwAP_HRGL-NQSV0=w881-h1020-no?authuser=0)
It is what it is ... :-\ The metal there is thin, probably 1/16" or so. Maybe if I stay as close as I can to .1325x.2095" on the barrel side of the transfer port I'll be okay. I realize I'm not building a rocket ship, but there goes my perfect build ... Walking away from this for a few minutes ...
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Okay. Finally back from vacation and cooled down quite a bit. I hope it won't make TOO much difference having the oversized barrel port. I'll be working on the transfer port tomorrow, the idea is to make the oblong end a tad smaller than the barrel port. More to come.
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Heh ... Okay so I finally got the air stripper in. I don't know how I ended up getting it in my head that 1/2-20 could be cut to the end of this barrel. It's too small. Oh well. I have a lathe, so I guess I can try machining the end of the barrel down a bit so I can thread it. The idea is to tap the inside of a 1/2-20 bolt (probably with 3/8-xx, whatever taps and dies I have on hand) then cut it down to make an adapter. Maybe even a helical insert could work, hmmm.
As I understand it, optimal distance from the cone to the end of the barrel is generally 1-2 calibers. I do not want to go with an adapter as suggested above, as this would drastically increase that distance, and potentially introduce other forms of turbulence I would rather not deal with.
One step forward, two steps back I guess ... This job WILL get done!
... So! I plan on mounting the front (aperture) sight on the stripper itself. Seeing as how it will be threaded on, how do people who have done this before go about clocking it perfectly? I can use a grub screw or even a thin lock nut to lock it down once I have it in the right position, but what about finding that perfect initial spot that sits perfectly on top, aligned with the rear site? If it is not perfect, I imagine point of impact would move left and right as the target distance varies ...
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Post a pic when you're done.
In all honesty, the stripper has neither worsen nor improved my accuracy.
My AG friends in Asia were into it as well, and we've used it intensively during our 20 to 100-meter comps.
We eventually got rid of the attachments almost simultaneously.