GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Bullfrog on February 08, 2014, 07:37:09 PM
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My Mrod's custom valve has a .18 exhaust port but a stock transfer port and barrel port. My gun gives me 8 shots at or over 70fpe at or near a 1.0 efficiency. However, when I turn the power down to 50fpe my efficiency becomes .8 so I waste air when the power is down.
What would you expect to happen if my transfer and barrel ports were made to be .18 like my exhaust port? Specifically, what would you expect it to do for my air usage and numbers?
Edited to add: I should state that if I stay with heavy pellets then my efficiency will go above 1.0 when I turn the power down, its only when I try to use "normal" pellets like Kings that my efficiency gets out of whack which suggests to me my valve is staying open to long. Unfortunately I can't fix it with any adjustment I try.
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Generally if the valves throat/poppet/body passage sizes are opened up the flow potential and power can be much greater.
Tho if the transfer tube between valve body and /or barrel port is smaller, it going to act like a throttle and choke the power down.
* No different than a factory valve with passage restriction screw blocking the exit path to some degree.
Can't speak of first hand .25 cal knowledge .... only state that in my own .22 modified gun, it runs an unrestricted .165" passage size throat to barrel.
Think as you reduce spring preload reducing power, the spring rate still remains and the Dwell of valve is eating air.
* Have you tried to Keep the power tune setting an DE-STROKE hammer travel to drop the power ?
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I assume you are talking about a .25 cal.... I would try and get the ports as close to the same size all the way through as you can (ie 0.18"), and insure that the valve throat area (throat minus stem) is at least that big as well.... something like a 1/4" throat to go with the 1/8" stem.... What usually happens when you increase the breathing on a PCP is that the power at a given pressure goes up, allowing you to get the same power at a lower pressure or more power at your previous sweet spot pressure.... If you make no changes to the hammer strike, generally the bell curve becomes a bit "peakier" (ie higher and narrower) at a lower peak pressure.... and the efficiency generally goes up because you get more power without using a lot more air.... If you back the gun down to the previous power level, generally the efficiency is a bit higher.... The only down side, really, is you may get a shorter shot string for the same ES....
Bob
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Yes, I'm sorry I didn't specify, I am referring to my .25 Mrod.
Motorhead, I've been running the gun set with a shorter hammer stroke. 7 turns clockwise was giving me a 3300psi fill. However, I discovered 2 days ago that I wasn't running my spring preload at full tension like I thought so I turned it another 3/4 turn CW for max preload. I then turned my hammer 1 turn CW for a total of 8 turns. Weather has been bad here most of the weekend so I haven't run a chrony on my new numbers. All I can tell is that I'm getting a lot more shots. I got 11 shots from 3200 to 2700 yesterday with the new settings. Unless I drastically lowered my fill pressure with the CW turn (which I would think was balanced out by increasing the pre-load), I would predict my power numbers are probably pretty close to what they were originally. Note that earlier this week I re-installed my Gaska air tank extension so that's one reason for the increased shot count but it alone shouldn't give me that drastic of a shot count increase. I figured with my previous air usage the Gaska extension would give me 2-3 more shots, unless my math was off.
rsterne, I think your advice is sound. It hit me yesterday that the reason for my mysterious decreasing efficiency with decreasing power might be linked to the fact that I simply have miss-matched ports but I wanted advice from the experienced airgun smiths in case I was thinking about it wrong. Before I pull the barrel off to drill it out I'm going to run my numbers. If I can (or already have) increased my efficiency beyond 1.0 at 70fpe I probably need to leave well enough alone for a while until I absolutely can't help myself. I find that is what most of my air gun tinkering is, an addiction to making the gun as "best" as possible, whether best means power, efficiency, quiet, ect. I just can't leave well enough alone! If I can get 1 and a half clips all shots at 70fpe I should probably be thankful and stop. But by golly what if I can make it a clip at 80fpe? Arrrghhh my tinkering addiction will kill me ;-).
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Yes, I'm sorry I didn't specify, I am referring to my .25 Mrod.
Motorhead, I've been running the gun set with a shorter hammer stroke. 7 turns clockwise was giving me a 3300psi fill. However, I discovered 2 days ago that I wasn't running my spring preload at full tension like I thought so I turned it another 3/4 turn CW for max preload. I then turned my hammer 1 turn CW for a total of 8 turns. Weather has been bad here most of the weekend so I haven't run a chrony on my new numbers. All I can tell is that I'm getting a lot more shots. I got 11 shots from 3200 to 2700 yesterday with the new settings. Unless I drastically lowered my fill pressure with the CW turn (which I would think was balanced out by increasing the pre-load), I would predict my power numbers are probably pretty close to what they were originally. Note that earlier this week I re-installed my Gaska air tank extension so that's one reason for the increased shot count but it alone shouldn't give me that drastic of a shot count increase. I figured with my previous air usage the Gaska extension would give me 2-3 more shots, unless my math was off.
rsterne, I think your advice is sound. It hit me yesterday that the reason for my mysterious decreasing efficiency with decreasing power might be linked to the fact that I simply have miss-matched ports but I wanted advice from the experienced airgun smiths in case I was thinking about it wrong. Before I pull the barrel off to drill it out I'm going to run my numbers. If I can (or already have) increased my efficiency beyond 1.0 at 70fpe I probably need to leave well enough alone for a while until I absolutely can't help myself. I find that is what most of my air gun tinkering is, an addiction to making the gun as "best" as possible, whether best means power, efficiency, quiet, ect. I just can't leave well enough alone! If I can get 1 and a half clips all shots at 70fpe I should probably be thankful and stop. But by golly what if I can make it a clip at 80fpe? Arrrghhh my tinkering addiction will kill me ;-).
sounds like you have the NHRA stock eliminator engine builder curse, the things that haunt us in our dreams
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I will be ordering mine this Friday and I'm already researching to make it a beast, Im a engine builder and do to the economy I closed, but this fits into my current buget and gets my blood flowing. Just hope the predictions I've heard on here that PA will offer a 10% off plus free shipping on valentines day. ???
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sounds like you have the NHRA stock eliminator engine builder curse, the things that haunt us in our dreams
The other side to it is that I've learned that power isn't everything. There comes a threshold where the killing power of the gun is enough and anything more is redundant. My deal is that I'm stuck with a high-power gun unless I figure out how to restore efficiency on low power settings. If I could ever do that then I can have an all-purpose gun... 70fpe for the big jobs and lots of shots at 40fpe for my backpacking trips and squirrel hunting.
As I reason it out to myself it makes sense that I should try to get my gun shooting as efficiently as possible even at low power and with light pellets to make the gun versatile. Yes, that is my justification. Why would I want a gun that only shoots 43.2 Eunjins or bullets efficiently? Surely my gun should be able to do it all. Therefore when I ruin this otherwise awesome 70fpe tune I can tell myself its for a greater purpose in the end. ;)
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sounds like you have the NHRA stock eliminator engine builder curse, the things that haunt us in our dreams
The other side to it is that I've learned that power isn't everything. There comes a threshold where the eliminating power of the gun is enough and anything more is redundant. My deal is that I'm stuck with a high-power gun unless I figure out how to restore efficiency on low power settings. If I could ever do that then I can have an all-purpose gun... 70fpe for the big jobs and lots of shots at 40fpe for my backpacking trips and squirrel hunting.
As I reason it out to myself it makes sense that I should try to get my gun shooting as efficiently as possible even at low power and with light pellets to make the gun versatile. Yes, that is my justification. Why would I want a gun that only shoots 43.2 Eunjins or bullets efficiently? Surely my gun should be able to do it all. Therefore when I ruin this otherwise awesome 70fpe tune I can tell myself its for a greater purpose in the end. ;)
I cant wait ot see that tune, I did locate a used A2A complete valve assembly farely reasonble, just wondering that is built into the valve already or if its been hacked and thats why hes getting rid of it
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It seems to me that with all the adjustments on an MRod, it shouldn't be difficult to achieve what you want.... I would start by cranking in the velocity adjuster (port restrictor) and leaving the hammer settings alone (assuming you have the pressure range where you want it).... I have a similar adjuster on my Disco Double, and it certainly does the trick there.... less power and way more shots.... Generally, with lighter pellets, you need less hammer spring preload for the same pressure range, of course....
Bob
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I've got my gun depressurized to change out the quick-connect fitting that's been giving me trouble and I'm down for the count until my new Hill pump gets in this week, therefore this would probably be a good time for me to give in to my cravings and drill the barrel port out.
To recap, my valve exhaust port is .18 but my transfer port and my barrel port are stock. I have a .18 transfer port that came with my valve. I just need to get the barrel port drilled out.
How should I do this? In my mind its simply a matter of taking the barrel off the gun, putting it in a vice, finding the right sized drill bit, at drilling out the port with a hand drill. Right or wrong? What material does my drill bit need to be? Also, how do I keep the shavings out of my barrel?
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You can't keep the shaving out of the barrel.... Drill one size at a time, using numbered drills if you have them, and being careful not to hit the top side of the bore.... Then carefully deburr the port on the inside with a needle file.... then clean the barrel before reassembly....
Bob
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Use a VERY sharp general HS steel drill bit and feed it slowly so it cuts and does not create a burr.
In reality it will burr and your going to need to dress hole from bore side to remove it and make sure no sharp edge is there to shave lead from a chambered pellet.
* This best done with a small stone in the dremel tool, Breech o-ring removed. Blow out barrel when done, shove a few patches threw and your good to go.
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and being careful not to hit the top side of the bore....
Bob
For those of us just beginning to try some machining, do you mind painting the picture a little more? Once you have the barrel in a vice, and you have the right drill bit on a hand drill - what does it mean "not to hit the top side of the bore" - how do you go about doing that?
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If you push on the drill too hard and it "bites" just as it breaks through into the bore, it can "pull in" and hit the top side of the bore.... I'm just saying to be careful....
Bob
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I like to put a wooden dowel rod in the breech end of the barrel as a safety so I can't drill too far. I prefer to use a drill press if it's available.
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OK - got it, understand now - you mean don't hit the opposite diameter of the bore-circle to where you're drilling from. Thanks for the tips...
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Bullfrog,
One you make the ports the same ID your valve will breath much better and theoretically this should help with opening the valve faster and increasing your efficiency.
I would also open up the intake and the gauge port to deliver the most air to the valve. This will also help with closing the valve faster.
This are pictures of a valve I just finished of a friend.
Gauge block (http://farm4.staticflickr.com/3692/12574310874_5089dc180b.jpg)
Valve:
(http://farm4.staticflickr.com/3831/12572379963_ba5bb30e1f_n.jpg)
(http://farm4.staticflickr.com/3731/12572712644_daa7ed5a5a_n.jpg)
(http://farm4.staticflickr.com/3702/12572363123_5ec956e14a_n.jpg)
This is very similar to my valve. Which can generate 59.5fpe with 31g with a 12.5# spring at full preload.
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You say that the efficiency goes down with lighter pellets. What weight had you been shooting before you went to the 25.4gr JSB? In theory, a lighter pellet will leave the barrle more quickly. This could happen quickly enough to have the valve still barely open compared to the heavier pellet. In a nut shell, It isn't a fair comparison. I think Bob's thought on choking the gun down will result in the lower power output while maintaining or improving efficiency. Also, I would have thought that the reservoir extension would have delivered more than a couple of extra shots even at 70fpe. I've never had one so I have no clue really. I don't even know how much additional volume it gives.
Bill
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In most of my test and observations, the more you choke a gun the more the efficiency goes down. The highest efficiency comes from delivering air as fast as possible for the shortest amount of time. Chocking the air flow does the opposite for both. Although, if you reduce power on the gun then you might have to choke it some to flatten your string. The strings tent to get a higher peak.
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The bottom line is, the efficiency is tied to where the pellet is in the barrel when the valve closes.... The closer it is to the breech, the better.... the closer to the muzzle, the more air you will waste for a given power level.... When you are attempting to get maximum USEABLE power, then you want maximum flow, regardless of the pressure, so that you can close the valve sooner while getting the power you NEED.... Heavier pellets, moving slower, will see the valve close before the pellet has moved as far, for a given valve dwell, hence increasing the efficiency (and usually the FPE).... However, you can also get high efficiency by simply closing the valve sooner.... That will reduce the velocity and energy, but almost always increases the efficiency as well....
The downside to wide open porting is that the shot string gets "peakier", just like putting a hot cam in an engine.... more power but over a narrower range.... This reduces the shot count within a given ES.... If you open the gun right up, and get it producing more power than you need, and then choke up the transfer port to get back to your required power level, you will initially increase the efficiency as the FPE drops.... Eventually, yes, the efficiency will drop again, but this method "smooths out" the shot string, giving you more shots within a given ES.... An extreme example of this is the Hatsan AT44 set up for the Canadian non-PAL rules (under 500 fps).... By restricting the airflow through the valve, Hatsan change a 30 shot, 30 FPE gun to a 170 shot, 6.5 FPE gun, while maintaining the same fill and refill pressures.... and at greater efficiency.... by "choking" up the flow....
Bob
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You say that the efficiency goes down with lighter pellets. What weight had you been shooting before you went to the 25.4gr JSB? In theory, a lighter pellet will leave the barrle more quickly. This could happen quickly enough to have the valve still barely open compared to the heavier pellet. In a nut shell, It isn't a fair comparison. I think Bob's thought on choking the gun down will result in the lower power output while maintaining or improving efficiency. Also, I would have thought that the reservoir extension would have delivered more than a couple of extra shots even at 70fpe. I've never had one so I have no clue really. I don't even know how much additional volume it gives.
Bill
I shoot 43.2 grain Eunjins for max power and efficiency. If I tune them for 60fpe I can get 1.2 - 1.3 efficiency depending on how tight I have my specific tune. If I max them out at 70 fpe I can get .9-1.0 efficiency. All other pellets lighter than the Eunjins give me poorer efficiency. I regularly shoot Kings and Barracudas. Both of those will equal out at the same FPE, as opposed to the Eunjins which always give me more FPE. Generally the Cudas and Kings give me an efficiency of .8.
My idea of getting two more shots at 70fpe is just a guess on my part. I figured that if I have my gun shooting at 1.0 efficiency at 70fpe the extension is adding just enough cc space for 2 or more shots. Or to look at it even more simply, the extension is adding roughly 25% more volume to my tank, therefore I would get 25% more shots. If I can get a full clip at 70fpe, 25% more of that would be two more shots. Math isn't my strong suit, but nonetheless that's how I reasoned it.
Last week I shortened my hammer stroke to give me a 60fpe tune with Eunjins starting 3100psi. I got 21 shots with a 30fps spread starting at 3100psi. That's not bad, but it is very spikey. One clip at around 2800 will be at the top of the curve from the high 810s to the 830s. The remaining shots before and after the top of the curve hover at 800fps. More than half of my shots are between 800 and 810. So for hunting purposes I have a 20 shot tune, but for "show off" purposes I've only got 8 shots within 2%. That's no different than what I have when I set it for 70fpe it just starts at a lower fill pressure and uses less air.
Thanks for all of the info gentlemen. As I already have a spikey curve that is acceptable for 1 clip at either 60fpe or 70fpe, I don't see a disadvantage in making my ports all the same size. If it makes my curve fall outside of 2% within a clip I can always put a bstaley buffer back on it to try to flatten it out. I'll also try choking down the velocity port to see what that does for me. I'll probably need to remove the valve's velocity port screw. It is custom cut to curve with the valve when wide open instead of being blunt like the factory screw. Therefore I may put the factory screw back it in when I try choking it.
Finally, my intake is already very large and it is made so that I can take the entire head off of it if I want for maximum flow. The builder told me I would get about another 20fps if I did that. I'm not comfortable drilling the gauge block without having a backup on hand. I may order a custom one.
I'm this far into it, I might as well go for maximum air flow and not leave it half baked. I'll update with my progress. I'll probably break my gun down tomorrow. While I'm in there I'll take the valve out and take some pictures so you can have an idea as to how its built.
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Something else to add: I definitely notice better numbers and efficiency when my spring tension is jacked up to maximum. I use a 10# spring. I'm considering going to a heavier spring jacked up all the way and shortening my hammer stroke even further.
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I am right where you are trying to avoid - I have an 80+ ft lb gun on cast bullets, but it is way too much for pellets!
Kings leave the muzzle at 1140 fps with ZERO preload on the top of a 3000 psi fill. Once I've fired 5-6 pellets there's no point in shooting pellets.
So I built a 30 ft lb pellet shooter specifically for JSB Kings.
I could detune my 25 Challenger Double Tube by using an unmodified RVA, factory Prod hammer and a Prod spring and probably get 40 shots on a 40 ft lb tune...............but the bullets I cast only cost me 1.3 cents each plus the time to make them.
I can't shoot pellets that cheaply!
Why don't you build a different gun? One gun wearing two distinctly different hats is tough without changing quite a bit as far as tuning goes.............unless you have a 'seasonal tune' that makes you happy for a few months each time you re-tune your gun.
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Bullfrog,
One you make the ports the same ID your valve will breath much better and theoretically this should help with opening the valve faster and increasing your efficiency.
I would also open up the intake and the gauge port to deliver the most air to the valve. This will also help with closing the valve faster.
This are pictures of a valve I just finished of a friend.
Gauge block (http://farm4.staticflickr.com/3692/12574310874_5089dc180b.jpg)
Valve:
(http://farm4.staticflickr.com/3831/12572379963_ba5bb30e1f_n.jpg)
(http://farm4.staticflickr.com/3731/12572712644_daa7ed5a5a_n.jpg)
(http://farm4.staticflickr.com/3702/12572363123_5ec956e14a_n.jpg)
This is very similar to my valve. Which can generate 59.5fpe with 31g with a 12.5# spring at full preload.
Since were talking in engine theory now can you explain the holes you've added and enalrged and their purpose :o
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I am right where you are trying to avoid - I have an 80+ ft lb gun on cast bullets, but it is way too much for pellets!
Kings leave the muzzle at 1140 fps with ZERO preload on the top of a 3000 psi fill. Once I've fired 5-6 pellets there's no point in shooting pellets.
So I built a 30 ft lb pellet shooter specifically for JSB Kings.
I could detune my 25 Challenger Double Tube by using an unmodified RVA, factory Prod hammer and a Prod spring and probably get 40 shots on a 40 ft lb tune...............but the bullets I cast only cost me 1.3 cents each plus the time to make them.
I can't shoot pellets that cheaply!
Why don't you build a different gun? One gun wearing two distinctly different hats is tough without changing quite a bit as far as tuning goes.............unless you have a 'seasonal tune' that makes you happy for a few months each time you re-tune your gun.
I think you're right, I need 2 Mrods set up differently. Or I need to just detune this Mrod and then buy a stronger gun of another brand for when I need extra power.
Its easy to tune for summer time because the only non-bird animals I can legally hunt during that time are hogs, coons, possums, and coyotes, animals where the max fpe would be welcome (rabbits are legal too year round but I don't count them). When I'm done with this round of mods I'll be leaving the gun as is (hopefully) until fall. Its trickier in the fall because they I can also squirrels where shot count does better for me than power and too much power on the squirrels can actually be detrimental (I found this out shooting some squirrels in the shoulder this year with Polymags).
If I can find a Gen 1 gun for cheap I'll pick it up and make it a dedicated backpacking gun tuned for high shot count. Ultimately I want a backpacking setup that lets be pack out a bullpup .25 with a high shot count for head-shooting game with.
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Just a bit of perspective here. At 70 fpe, you are delivering what a .22 rimfire pistol delivers and I'm good with that. But how much game do we generally hunt that requires that kind of energy, not much i'm guessing. 60+fpe guns are leaning toward specific application airgun. Just my .02 cents worth.
The spikey curve that you are getting is the norm with enlarger port guns. You can choke it down and get it to level off a bit while maintaining the higher efficiency. What do you say BOB?
Bullfrog, Are you using the stock hammer with that 10# spring?
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I was thinking that you may be able to lighten the hammer while choking the port a bit to flatten the curve.
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Bob, I'm using the stock hammer. I've never tried a lighter or heavier hammer.
I agree the extra power isn't necessary. My problem is that by the time I had that epiphany, I had already modded my gun to the point of no return and I've been unable to detune it to be efficient on lower power levels. I've shifted my focus from power to efficiency, yet I'm seeing that the way my gun is set up that I'm most efficient on the highest power. If 50fpe and 70fpe both use about the same 65-70psi per shot, I'm not having a big advantage in turning my power down. If I can work out this efficiency issue then I can make a decision as to whether I want to keep my gun at 70+fpe or whether I want to turn it back down to 50fpe for lots more shots.
Here's the progress I've made so far, see the attatched pictures:
1. The valve intake with the end cap on.
2. The valve intake with the end cap removed next to a factory valve. I will be running it without the end cap to see what happens. A2A estimated removing the end cap would add 10-20fps.
3. The valve exhaust at .18 next to a factory valve.
4. The valve exhaust next to my barrel port that I drilled out yesterday to as close to .18 I could.
Today I'll be re-assembling the gun. If my Hill pump arrives before dark and if I'd done with all of my obligations today I'll shoot a string.
I used a wooden dowel as a guard. Thanks for that advice. I also was able to file the jaggies down with a chainsaw sharpener and blow out the dust easy enough. It really wasn't a hard operation.
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Once you get "enough" air into the valve, and that includes through the gauge port, opening up the inlets more is unlikely to add much advantage, if any.... with a 3/16" exhaust port I would think the stock valve inlet is nearly big enough, and any of your other combinations should work just fine.... This would not apply to to a regulated gun, of course, where you have a very limited volume in the plenum between the regulator and valve seat, but there you are actually just creating more volume by removing material (eg. the front of the valve) not increasing the inlet side flow rate.... I pretty much figure if the inlets are all significantly larger than the outlets, you're looking in the wrong place to increase them further.... As an example, I recently converted a Disco valve to .25 cal, and it has 0.219" porting (or equivalent) on the exhaust side straight through, and the inlet side is only 0.266", slightly bigger than the 0.25" throat.... Even so, the gun hit 57 FPE with Kings at only 2000 psi, and will do a nice string in the low 900s, still staying over 900 at only 1200 psi.... If I thought going larger on the inlet would help, I would have done so....
Bob
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I've run into an unexpected problem. My stock transfer port orings now have an outside diameter that is too big for the valve's exhaust port when they are placed on the .18 transfer port. I've spent an hour trying to squeeze the stock orings as well my smallest SAE and metric orings on the transfer port into the valve's exhaust port with no success. Tomorrow after work I'll run by the local auto parts store and try to find some smaller outside diameter orings. Has anyone had this problem and find a solution?
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I just want to say this thread has been very helpful. I just opened up my disco, and will probably do a post on it this weekend after i get some numbers. After reading this thread, i felt a lot more confident and well informed, so thanks for all the help!
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I've run into an unexpected problem. My stock transfer port o-rings now have an outside diameter that is too big for the valve's exhaust port when they are placed on the .18 transfer port. I've spent an hour trying to squeeze the stock o-rings as well my smallest SAE and metric orings on the transfer port into the valve's exhaust port with no success. Tomorrow after work I'll run by the local auto parts store and try to find some smaller outside diameter orings. Has anyone had this problem and find a solution?
I've been using 1/4" OD drip line hose. I found some at home depot that had 0.280 OD and an ID of about 0.153. Using hose you don't need a machined transfer port or o-rings, you just cut it a little long and compress it. I just cut it with a razor blade type knife in one single motion, if It doesn't come out just right I try again. I'm just going to a bigger port on a different rifle and need an ID of about 0.266. I haven't found a good sized tube yet, but I expect I'll have to go to a .375 OD.
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I've run into an unexpected problem. My stock transfer port o-rings now have an outside diameter that is too big for the valve's exhaust port when they are placed on the .18 transfer port. I've spent an hour trying to squeeze the stock o-rings as well my smallest SAE and metric orings on the transfer port into the valve's exhaust port with no success. Tomorrow after work I'll run by the local auto parts store and try to find some smaller outside diameter orings. Has anyone had this problem and find a solution?
I've been using 1/4" OD drip line hose. I found some at home depot that had 0.280 OD and an ID of about 0.153. Using hose you don't need a machined transfer port or o-rings, you just cut it a little long and compress it. I just cut it with a razor blade type knife in one single motion, if It doesn't come out just right I try again. I'm just going to a bigger port on a different rifle and need an ID of about 0.266. I haven't found a good sized tube yet, but I expect I'll have to go to a .375 OD.
That's an interesting idea. By the time I read it, I had already come to a solution, but had my solution not worked I would have tried your method.
To solve my oring problem I drilled out the exhaust port on the gun's body (the port on the gun itself that the valve's exhaust port sits under) to 9/32. This was an exact fit for my factory port orings when they are flailed out on the .18 transfer port. I didn't have to make any mods to the breech port hole that the barrel port sits on top of.
I got my gun together and pumped up this morning. After working with my hammer stroke I got it adjusted to a 3200 fill and here are my initial results:
43.2 Eunjins
3200psi fill
10# hammer spring
Spring at max preload, hammer stroke at unknown setting (I had to keep turning it ever so slightly to get it set for a 3200 fill and I lost track of the turns).
3200psi
884.2
892.2
884.4
889.4
893.8
902.9
902.0
896.9
2800psi
I stopped here, not knowing if I was on the downward curve or if I was still rising, yet I was happy with that initial clip of 8 shots for a hunting tune.
My tank size is 284.5cc (215cc stock tank + my Gaska extension that is worth 69.48cc). According to Lloyd's calculator (awesome app by the way), my efficiency is 1.28 for an average of 76.5fpe.
No doubt making the ports all the same size was worth it. As ya'll expected, I gained power and saved air. I'm very pleased.
I'm going to shoot some accuracy strings now to make sure none of my mods are causing problems. I noticed when I was manually loading the Eunjins that I was getting flecks of silver on my fingers. Not sure if its just a new tin of pellets or where they might be rubbing a jaggy in the breech.
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After working with my hammer stroke I got it adjusted to a 3200 fill and here are my initial results:
It would be interesting to see how that string ended below 2800 PSI.
I noticed you filled the Marauder to 3200. Is the maximum recommended fill 3000? If so, do you have any reservations about filling 200 PSI above the maximum 3000 recommended fill? Just asking for my own education...
EDIT: By the way, nice work all hand-held without a drill press
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I am comfortable filling to 3200psi but I don't recommend it to anyone. Its a risk I'm willing to take. At this time I'm not willing to go above 3200psi for a tune. I have experimented with tunes up to 3400psi and I've tried higher than that for giggles, but I felt nervous the entire time and decided it wasn't worth it after my initial experiments. 3200 is the upper limit as to what I feel is safe based on what I've observed others in the Marauder community tune to. But again, you're taking the risk on yourself if you go above Crossman's recommendation. Part of my reasoning to explain why I am comfortable taking the risk myself is that I figure Crosman makes the gun to actually be safe within a certain range within the recommended fill pressure. Crosman's air gauges on their guns and pumps are so inaccurate that they are begging for law suits if the gun will fail at 3200 psi, where their gauges are often off within 300psi or so. Its realistic for someone to have a gun from the factory that the Crosman gun gauge says is filled to 3000psi, when in reality its 3200 or 3300psi.
So if you're asking me whether I condone YOU filling to 3200psi, I cannot. I must recommend what Crosman does. For myself, I will fill to 3200psi and not think much of it. If it hurts me, then I assumed the risk myself. Myself I'm not filling above 3200psi anymore. Even putting safety aside, there's no need to. I can produce 70fpe at less than 3000psi as it is. I still prefer 3200 because it is a little bit more efficient at the higher pressure.
Its sort of like the you-know-what barrel devices that shall not be named. I'm actually an attorney and I'm comfortable in my own ability to read case law. I have confidence for myself that the you-know-whats are legal for airguns based on the Crooker case. Therefore I'm not very secretive about my airgun you-know-whats. Yet if you were my client and your asking me what's the legally safest thing to do, I would have to say stay away. Not because the law isn't on your side, it is actually. But the law doesn't always mean anything to the government.
So it is with over filling your gun, young grasshopper. ;) Do as I say, not as I do.
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Yes, you do sound like an attorney. ;) I don't rely solely on anyone's advice. I'm pretty independent-minded myself, thanks. I just like to ask a lot of questions to see what others are doing (and understand their reasoning) - so this is helpful.
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My accuracy tests today aren't good. All of my pellets of all sizes and brands are being thrown all over with the exception of Benji Domes and Barracudas, which are giving me dime sized groups at 30 yards with the occasional flier. One trend I'm noticing is that the fliers go away on the down side of the bell curve near 2400psi. Its too overcast today for the chrony. Some time this week on a sunny day where I have not much going on I'll turn the power down to test both lower power efficiency and accuracy. If my gun is now too hot for pellets, that's ok, I can live with that. I can either try to turn it down or I can go to bullets. I need to figure it out though because I have a fear in the back of my mind that I screwed something up yet unknown when I last modded it. It could also be that I fouled by barrel again, but I gave it a medium cleaning yesterday and it didn't seem that dirty.
Previously at 70fpe (with Eunjins, about 63fpe with other pellets) my gun was dead on accurate at speeds that conventional wisdom would say are at the border of being too much for pellets. It would seem odd to me that an extra 30-40fps made the difference between dime sized groups and lemon-sized groups.
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Looks like you have now found out how fast is too fast....
Bob
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Unfortunately I now think there's something else going on besides the pellets flying to fast (at least that being the direct problem). I figured that I could turn the spring tension all the way to do get enough reduction in velocity to tighten the groups up. I shot several groups at an unknown lower velocity, some of them were tight and others were not. I finally shot 2 groups side by side, one at max preload and the second at minimum preload, both with Barracudas, which have been grouping the most consistently good. My results were terrible. Basically the same spread at max velocity and the reduced velocity.
The gun is acting like it did when the barrel was severely fouled. I've put about 750 pellets through it since the heavy cleaning I did a few weeks ago although I've ran patches through it twice since then. Although it didn't seem dirty yesterday it didn't seem dirty before either. I think my next step is to get some fresh brushes and put a good scrub on it.
If this turns out to be the problem, this itself will be instructive. Previously I thought the heavy cleaning was needed simply because the barrel never got a deep cleaning in 2 years of heavy use besides patches and mild solvent. Now I'm thinking that this barrel fouls easily when the pellets run too hot. Others have suggested it. I never discounted it, but now it may be more drastic than I imagined. If I have time tomorrow I'll give it a heavy scrub and shoot some groups.
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Have you checked to make sure you didn't lose the o-ring in the barrel?
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Transfer sleeve may not be sealing properly/consistently, during the shot.
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I have checked the breech oring, it is still there.
I would suspect that if my transfer ports aren't sealing properly I would be experiencing unstable velocities, correct? Yet Saturday when I had the gun on the chrony I had nothing but stable velocities for all of my strings.
This evening I'll patch it out to clear and then put a scrub on it to see if it knocks anything significant loose. If not, I'm back to the drawing board.
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Initial thought would be to the most invasive step that you did with the last mods: drilling the port. My hunch is that a burr has been left which is throwing off a skirt.
Load a pellet as you would with the mag (I'm assuming that you're using one based on your comments), but then disassemble enough to push the pellet through with a dowel.
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Initial thought would be to the most invasive step that you did with the last mods: drilling the port. My hunch is that a burr has been left which is throwing off a skirt.
Load a pellet as you would with the mag (I'm assuming that you're using one based on your comments), but then disassemble enough to push the pellet through with a dowel.
That's what I've been scared of. However, let me update ya'll on where I'm at. After several hours of working on the gun last night I'm only a little bit closer to getting is figured out, but I think after this afternoon I'll be able to eliminate some possibilities.
Yesterday afternoon I took the barrel off the breech and started the same cleaning regime I did 2 months ago to free it of the terrible fouling it had. The only difference is this time I used 4 fresh brushes instead of the worn brush I used then. I started with Shooter's Choice lead solvent, CLP, and SC's Quickscrub III. I then brushed out until nothing else came out. I then used a brush of JB Bore Paste and scrubbed it through good. I then spent the next 4 hours scrubbing and patching. When I finished I was still getting soot out after a brushing but the bore visibly looked shiny and I decided that surely all that must be left was light residue that kept getting scraped out. That, or my brush was actually putting it back down, as there seemed to be no end to the soot.
Around midnight I set up at 20 yards and shot 2 four shot groups with Barracudas. Beautiful sub-dime groups. I then refilled and loaded up with 4 Barracudas and 4 Eunjins and went to 30 yards. Those groups were terrible just like all the others.
So 1) I may still have a barrel problem, or 2) the big, quick, blast of air is somehow destabilizing the pellet. I'm sure there are other possibilities, but one of those two are where my suspicions lay. I need to shoot more groups at 20 and then 30 yards to see if the pattern stays consistent where 20 yards is tight and 30 is all over. That would tell us something.
As hard as I studied the barrel last night I don't see where there are any jaggies at the barrel port.
I have one solution that may help me eliminate or confirm the barrel as the problem. I have another barrel here that is brand new that I can try. If I drill out the barrel port to the same size and I'm careful to not leave any jaggies that should help me at least rule out my barrel. I really don't trust my barrel. I've road that poor barrel hard for 2 years, 10,000+ pellets, and speeds it was never meant to see. If it was a horse, it would have been shot by now to put it out of its misery.
If the new barrel performs the same (assuming no jaggies), and if a pattern emerges of tight groups up close and wide groups after a certain point, wouldn't that suggest the problem is with the movement of air itself? Perhaps destabilizing the pellet? If so, I would presume my options would be to either choke the port back down or try bullets. Thoughts?
I'm done with work obligations today. I've got plenty of daylight to work on it. I'm getting on it now.
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If the new barrel performs the same (assuming no jaggies), and if a pattern emerges of tight groups up close and wide groups after a certain point, wouldn't that suggest the problem is with the movement of air itself?
Interesting thread. We airgunners become gunsmiths and ballistic detectives even more so than PB shooters...
One possibility (of several) is that you have so much energy now that within a close distance the pellets are moving supersonic or near supersonic - and as they decelerate rapidly (which diabolos do) - they hit the trans-sonic chaotic zone and destabilise.
Another possibility is that the pellets are already destabilised by something right from the muzzle (super blast of air, burr in the loading area) - but the effects naturally become magnified and noticeable at longer range, and as the pellet slows down and loses whatever early aerodynamic stability that it had in its flight.
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I think I've found the likely culprit. The only question now is going to be figuring out how things got to this point.
I took my gun out to 20 yards with the chrony and shot 3 groups. 1 at max pretension, 1 at min pretension, and another at min pretention with different pellets. First, my numbers:
Group 1
Kodiak Match (Barracudas for all practical purposes)
Max Pre-tension. 3200psi fill
990.3
988.8
1005
1006
1032
1021
1019
Group 2
Kodiak Match
Min. Pre-tension. 3200psi fill.
709.5
696.1
686.6
679.8
695.7
691.7
710.0
Group 3
JSB Kings
Min Pre-tension. Continued from Group 2's fill
724.5
723.4
731.1
741.0
742.2
739.4
744.5
As you can see, all velocities for each group were consistent enough to test accuracy. All of the groups were atrocious. The worst was the Kings. The most interesting observation was the massive POI shift between the high and low speed groups of the Barracudas/Kodiaks. I marked the point of aim on each target.
(http://i225.photobucket.com/albums/dd84/Bullgoblin/IMG_20140225_144313_703_zpsafe54572.jpg) (http://s225.photobucket.com/user/Bullgoblin/media/IMG_20140225_144313_703_zpsafe54572.jpg.html)
So my observations from last night were flukes. My groups are off whether I'm up close or far away. Speed itself is not the issue, neither is inconsistent velocities.
t-jones gave me a good idea. Push a pellet through the barrel. I did so and found no nicks. Unfortunately, I also found barely any rifling. Bingo. I then pushed a pellet through the new barrel. The difference is night and day. Take a look:
(http://i225.photobucket.com/albums/dd84/Bullgoblin/IMG_20140225_144751_616_zpsbb9c762d.jpg) (http://s225.photobucket.com/user/Bullgoblin/media/IMG_20140225_144751_616_zpsbb9c762d.jpg.html)
Pellet on the top went through the new barrel. Pellet on the bottom went through the old. That's got to be the problem. The old barrel had a rough spot in the middle but otherwise the pellet met no resistance through the barrel. The new barrel gave the pellet no resistance until the end, where it is apparently choked or the rifling is deeper and it took some effort to push the pellet through.
Question is, how did this happen? Did my use of JB on a brush do this? Or have I just worn the barrel out? I posted a thread about barrel wear back a few months ago and it was generally agreed that an airgun barrel can't be worn out through lots of shooting. Therefore conventional wisdom would say it was the cleaning. Yet, I did have this same accuracy problem before I even did any special cleaning except run patches through it (and a brush a few times) over the past 2 years. About the middle of last year I started getting spotty accuracy. Superficial cleanings would solve for about a month and the problems would return usually worse. Finally the aggressive cleaning I did 2 months ago restored superb accuracy until this past week. Had I worn the barrel to almost nothing in December and the deep cleaning just prolong its life? Or was it severely fouled in December and my aggressive cleaning remove the fouling and then the rifling too? I can't say. I'm not sure there's a way to know or what lesson there is to learn from this.
I wonder if some pellets are too hard? I love the big Eunjins, yet I've heard it said they are made of very hard lead (I always thought they were soft because of the way they expand at velocities above 750fps).
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Just to be completely sure it's not some random effect (like the close-range/long-range grouping "differences") - have you tried pushing maybe 3-5 pellets randomly chosen (shuffled or shaken up) through each barrel and comparing the rifling?
This would eliminate the possibility that one pellet had a different diameter from another. I couldn't tell from your post whether you've already done this, but if not it might be worth it before doing major barrel work.
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You say you used brushes for cleaning, what kind of brushes? ::)
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Bronze. I basically followed the advice here: https://www.pyramydair.com/blog/2005/11/is-your-airgun-barrel-really-clean/ (https://www.pyramydair.com/blog/2005/11/is-your-airgun-barrel-really-clean/) The only difference is I tried hard-core lead solvents first like Shooter's Choice and traditional solvents like Hoppes before I was brave enough to try JB.
For 1 1/2 years I never did anything to my gun but run patches through with a mild oil or cleaning pellets. About the middle of last year my accuracy went away and wouldn't be restored with any amount of patching. I lightly used a bronze brush and that restored my accuracy for a few weeks, but by late last year even a light brushing wouldn't do it. I then started using FS solvents and that helped for a little while. I was convinced that the problem was not using a strong enough solvent to cut the lead residue. CLP really seemed to do the trick for a month or two. Then by about December of last year nothing would restore accuracy. Someone recommended JB to me and it worked like a charm and my gun shot better than it ever did from the beginning of January to last week. During that time I patched it out twice but did no brush work.
For about 6-8 months there's been a gradual decline in my barrel's health that traditional AG light cleaning stopped fixing. I can't remember any variable that began about that time, except me hot-rodding the gun for an extra 100fps. Although I'm suspicious of the Eunjins in fairness to them I didn't start using them with the power tune until January. Previous to that, my power tune wouldn't shoot them well. I thought it was the speed until I gave the gun the JB cleaning in January and then they grouped great until last week.
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Here's my old thread where I was talking about my accuracy problems back in December.
http://www.gatewaytoairguns.org/GTA/index.php?topic=58428.msg565321#msg565321 (http://www.gatewaytoairguns.org/GTA/index.php?topic=58428.msg565321#msg565321)
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OK here's my .02c. Bronze brushes + slightly corrosive solvents = dead barrel. Air gun barrels are not much, if any harder than the bronze alloy in the brushes. Brass brushes are softer, but not by much. Shooters Choice is corrosive, as it has to be to dissolve copper and lead fouling. The so called "soot" is actually Iron compounds. One test of the barrel with the bad spot in the mid section, to see if it is just still fouled, would be to plug the muzzle and fill it with S/C lead remover and let it set over night. After all if it has no rifling left, what will it hurt. ::)
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... out of abundance of caution I don't use any bronze or brass brushes on my airgun barrels - just an occasional cleaning snake, cleaning patches and ballistol or kroil...
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Just to be completely sure it's not some random effect (like the close-range/long-range grouping "differences") - have you tried pushing maybe 3-5 pellets randomly chosen (shuffled or shaken up) through each barrel and comparing the rifling?
This would eliminate the possibility that one pellet had a different diameter from another. I couldn't tell from your post whether you've already done this, but if not it might be worth it before doing major barrel work.
I've tried some different pellets. The results are consistent, the new bore definitely leaves more defined rifling than the old. However, I found that the old bore will leave rifling on JSB Kings, albeit still not as defined as from the new bore. Interestingly the Kings in both bores receive rifling on the head as well as the skirt. Also interesting is that in my old barrel Kings shot the worst today.
OK here's my .02c. Bronze brushes + slightly corrosive solvents = dead barrel. Air gun barrels are not much, if any harder than the bronze alloy in the brushes. Brass brushes are softer, but not by much. Shooters Choice is corrosive, as it has to be to dissolve copper and lead fouling. The so called "soot" is actually Iron compounds. One test of the barrel with the bad spot in the mid section, to see if it is just still fouled, would be to plug the muzzle and fill it with S/C lead remover and let it set over night. After all if it has no rifling left, what will it hurt. ::)
You might be 100% right. At this stage I'm perplexed. I had another pair of eyes look at both bores tonight while I was in the shop. His opinion was that the old bore's rifling itself is fine and the problem is likely that my old bore is still fouled. He brought up that 10,000 pellets with light cleaning is going to leave a fierce coating and I maybe haven't got the bulk of it yet. However he's coming at it from a PB perspective so it may not apply.
Here's a comparison between the two crowns. The picture isn't the best but it was the best I could do. Barrel on the left is my old barrel, right is the new barrel. See how the fresh barrel from Crosman is black on the inside? See how my old bore on the left is not as dark with strips of silver shining through? On a PB barrel it would be said that the dark color of my barrel is fouling and the silver is the bore's proper finish peaking through. Yet if Crosman's barrel is black then perhaps I'm rubbing some sort of coating off of it. Not the rifling itself but whatever coating may come on it from the factory.
(http://i225.photobucket.com/albums/dd84/Bullgoblin/IMG_20140225_173224_673_zpsc42a394a.jpg) (http://s225.photobucket.com/user/Bullgoblin/media/IMG_20140225_173224_673_zpsc42a394a.jpg.html)
What it really comes down to is whether the inside of my barrel should have that dark coating or not. I think that's where the rubber hits the road. If its supposed to be there, its possible that I'm basically enlarging the bore by removing it. Not ruining the rifling per say, but I'm making the bore too big for the pellets to contact it. For the barrel to then be any good I'd have to start using slightly off-sized cast bullets after I've evened out the inside by removing ALL of the black.
On the other hand, I do distinctly notice that pushing pellets through the old bore makes a lot of turbulence especially near the middle of the barrel. The new barrel is completely without resistance except for the first push into the bore and then right before the crown. Not sure what this means. I guess it leaves open the possibility that the old bore is still fouled.
I've got a full barrel of lead solvent soaking that I'll leave in for 24 hours. I won't post any more updates unless I have some news. I'm going to hold off installing the new barrel until I work with this old one some more. Although the new barrel was cheap I'm very curious to get to the bottom of this. I also feel like I have the barrel transfer port just right. I drilled out the replacement port tonight and it came out a touch smaller in spite of using the same bit. If at all possible I want to save the old barrel.
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I'm super stoked. I can report some definitive results after a week of testing, although there's still some mysteries to solve.
First, I'm extremely pleased with how my gun is shooting. I replaced the barrel (more on that later) and conformed the barrel port to .18. Accuracy has been restored (with a major caveat, again, I'll talk about that further down the post). Here's my final tune:
31 grain Barracudas
2700psi
943.4
942.8
949.1
950.0
941.4
945.5
945.7
945.5
954.1
939.6
946.0
952.1
948.4
953.5
944.9
Error
891.0
Ending pressure 2000psi
17 shots. Average 943.3fps, 61.3fpe. Average 41 psi used per shot. 1.24 efficiency.
My gun behaves almost as if it is regulated. I can't explain that, but the behavior is consistent. Dime holes at 20 yards. I'm a happy man. Having equal sized transfer ports definitely fixed my efficiency problem. I'm comfortable that I could use bstaley buffers and lower my power if I wanted to and maintain or gain efficency.
Now here's what I've been doing this past week and some observations I've made. I spent the better part of the week cleaning the old barrel a couple of hours a day. Between scrubbing sessions I'd let is soak in whatever I could find. Hoppes, Bore Scrubber, Lead Remover, whatever. I kept getting soot out and mid week I noticed that the rifling returned when I pushed pellets through. As where there was hardly any rifling when I started, clear and defined rifling appeared all the way around the pellet. This encouraged me, but I couldn't get over wondering if I had now cleaned the barrel and was after a point simply removing bluing from inside the barrel along with fouling. The only thing that kept me going besides seeing an improvement in the rifling was the rough spot in the middle of the barrel. When I'd push a pellet through in the middle of the barrel I'd feel turbulence for a few inches. I knew that wasn't right. It also bothered me that the new barrel was definitely choked for the last couple of inches. I figured this would slow my pellets down. My old barrel doesn't' feel choked at the end. The rifling on the new barrel is definitely deeper but I attribute that to the choked crown. Between the crown and the breach there's virtually no resistance when a pellet is pushed through. Only at the crown does it take effort to push the pellet through. Compare that to my old barrel, where there is no resistance until the middle of the barrel, where it feels like turbulence and a slight resistance at the crown.
However, by Friday I realized I was slowly surpassing my costs in cleaners and brushes than a new barrel itself. Therefore I re-drilled the port in the new barrel to match the .18 port in my old barrel and installed it. The results were... the same. Pellets flying wildly. And yet sometimes I'd get a good group. Then I saw the pattern. Whether it be high velocity or low velocity, my pellets would scatter wildly when shot at 3200psi. My good groups with both the old and new barrel were always at the low bottom of my shot curve, even if velocities were still high at the bottom of the curve.
Therefore yesterday I tuned for a low fill, high velocity tune. Viola, complete accuracy restoration (with the new barrel mind you). Retuned for 3200 fill. Pellets sprayed again. It was clearly consistent. Tunes below 3000psi gave me accuracy, tunes above 3000psi were wildly inaccurate. I've almost always tuned my gun for 3200 fills and never had this problem until I drilled the ports out. Surely the bigger ports must have some effect on the pellet insofar as a fast, short puff of high psi air must be destabilizing the pellet. I can't explain it any other way and I can't understand the physics behind that. Yet the data shows a clear line in accuracy or lack thereof between high and low pressure fills. This also explains why I'd have the fluky good groups with my old barrel when all of this started. I noted then that the groups tightened up on the low side of my bell curve but I dismissed the idea when I turned the velocity down at 3200 psi and still got bad groups. I was still thinking in terms of velocity not pressure.
Wherefore, I can't answer what if anything was ever wrong with my old barrel. Maybe I cleaned the heck out of it for no reason. And yet surely it must have been fouled for the rifling to improve greatly over the course of the week. I'm going to continue to work on that old barrel, although I'm going to now do it at a leisurely pace. I'm happy enough with the performance of my new barrel.
Here's another observation. My velocity screw kept getting blown backwards during my shot strings. Because my velocity screw's end is concave, that means I got varying degrees of blockages during my shots. I would see this happen when my velocity would drop 100fps between shots. I finally had to blue loctite it in place. It seems to be holding. This told me that using the velocity port to restrict air isn't the way to go for my particular setup. I always lost power and air whenever the velocity screw was blocking the port.
Thanks for following along. The nature of my old barrel and how much cleaning is too much is still open to debate. I hope to have more answers as I work on that barrel for the weeks to come. I'd like to see if I can get that turbulence out of the center. Yet I'm not in a hurry to replace my barrel again. This new one is doing just fine. I don't want to tear into my gun again for anything the way its shooting now.
By the way, it is SO much quieter shooting the gun at the lower pressure. I have my hammer set about 6 turns CW and my #10 hammer spring about 3 turns CW. I'm using grey threading tape to keep the hammer from slipping.
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Very nice string! I think eventually I will start using the Cudas for hunting. I just have 9 tins of JSBs kings to go through. The accuracy of the kings is signicately betting than the cudas but I'm still getting 1in groups with the Cudas at 40 yards. Which is fine for hunting. They are also a lot more efficient. Right now I'm shooting JSB at 900 fps or about 45 fpe. The cudas at this same setting give for 50 fpe.
I'm feeling to 2700 and taking 16 shots. This give me a ES of about 12fps. My set up is producing very similar power as yours. Although, my porting is less than yours.
As far a accuracy an PSI, I notice the same thing with mine.
The main difference I have is that I'm using 12.5# spring. To get the power I'm getting is about 1.25 turns. I like that I don't have to have that much preload on the spring.
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I thought there was a setscrew to lock the position of the velocity screw, no?....
Bob
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I thought there was a setscrew to lock the position of the velocity screw, no?....
Bob
There is, but it doesn't hold. It blows back with the velocity screw. I had to loctite both the velocity screw itself and the set screw separately and then use the set screw to line up the concave velocity screw just right.
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Very nice string! I think eventually I will start using the Cudas for hunting. I just have 9 tins of JSBs kings to go through. The accuracy of the kings is signicately betting than the cudas but I'm still getting 1in groups with the Cudas at 40 yards. Which is fine for hunting. They are also a lot more efficient. Right now I'm shooting JSB at 900 fps or about 45 fpe. The cudas at this same setting give for 50 fpe.
I'm feeling to 2700 and taking 16 shots. This give me a ES of about 12fps. My set up is producing very similar power as yours. Although, my porting is less than yours.
As far a accuracy an PSI, I notice the same thing with mine.
The main difference I have is that I'm using 12.5# spring. To get the power I'm getting is about 1.25 turns. I like that I don't have to have that much preload on the spring.
I highly recommend the Barracudas for hunting. I've killed lots of coons with them. They kill coons better than anything else. I would have thought a polymag blowing up a coon's brain would kill them the cleanest. Yet there's something about that hard cuda tearing through them that puts them down better.
I'm glad to see I'm not the only one to observe a relationship between PSI and accuracy when dealing with custom ports. I'd like to know the physics behind why a higher PSI blast is destabilizing the pellets, even when the blast is small enough to send the pellet at a lower velocity. I don't understand why 750fps at 3200psi is less stable than 950fps at 2400psi.
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It maybe that the higher PSI causes the breech to twist on the air tube. It only has to move a thousandth of an inch or two at the breech to change your POI an inch or two at distance. Crosman now recommends fills less than 2600 PSI for accuracy. The breech is very thin at the bolt "Mouse hole" on the gen 1 marauder. The gen 2 has removed the mouse hole, I think to address this very problem. I've had good result with loosening the front breech screws 1/8 of a turn from full hand tight.
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Maybe the quick high pressure blast is deforming the pellet?
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Maybe the quick high pressure blast is deforming the pellet?
Couldn't say. Family Man is reporting the same inaccuracy problems at a higher fill pressure on his high-flow gun. Looks like a verifiable pattern. I'm wondering if it is barrel harmonics with the shroud. I forgot to report that last week in my accuracy testing I had one shot on a 3200 fill where the shroud got blown 2 inches down the barrel before the set screw caught again. I didn't have the set screw resting in the notch on the barrel, I had it just above the notch but tight (but I guess not tight enough). I wonder if that really strong blast is shaking the shroud and therefore the barrel?
It maybe that the higher PSI causes the breech to twist on the air tube. It only has to move a thousandth of an inch or two at the breech to change your POI an inch or two at distance. Crosman now recommends fills less than 2600 PSI for accuracy. The breech is very thin at the bolt "Mouse hole" on the gen 1 marauder. The gen 2 has removed the mouse hole, I think to address this very problem. I've had good result with loosening the front breech screws 1/8 of a turn from full hand tight.
What I'm thinking about in terms of barrel harmonics might be the very thing you're describing. Its very interesting that Pyramyd Air's description for the Mrod now includes language stressing a 2600 fill or less for best accuracy. Hmmmm....
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I added a 0.625 od carbon fiber tube over the full length of my barrel. Then I put 10 or 15 o-rings evenly spaced down the length in order to use the shroud as a barrel stiffener. I sprayed silicone down the shroud but none on the carbon fiber or o-rings, this way they can be tight but have more friction to the barrel than the shroud when assembling. The shroud is already attached to the barrel by an oring at each end so this does not affect the free floating design. I do not attach the shroud to the barrel band as some have suggested as this IMO directly connects the barrel to the shifting air tube. I shoot single shot as I found the magazine reduced my accuracy. I made a single shot tray to make it easier to load. I shoot benchrest at targets so this mayn ot be the preferred hunting set up. When I purchased it my 25 cal Marauder would not group at 75 yards, now I get 23 out of 25 within 1 inch ctc at 75 yards. I use JSB 25.4 Kings at about 910 FPS.
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I'm super stoked. I can report some definitive results after a week of testing, although there's still some mysteries to solve.
First, I'm extremely pleased with how my gun is shooting. I replaced the barrel (more on that later) and conformed the barrel port to .18. Accuracy has been restored (with a major caveat, again, I'll talk about that further down the post). Here's my final tune:
31 grain Barracudas
2700psi
943.4
942.8
949.1
950.0
941.4
945.5
945.7
945.5
954.1
939.6
946.0
952.1
948.4
953.5
944.9
Error
891.0
Ending pressure 2000psi
17 shots. Average 943.3fps, 61.3fpe. Average 41 psi used per shot. 1.24 efficiency.
My gun behaves almost as if it is regulated. I can't explain that, but the behavior is consistent. Dime holes at 20 yards. I'm a happy man. Having equal sized transfer ports definitely fixed my efficiency problem. I'm comfortable that I could use bstaley buffers and lower my power if I wanted to and maintain or gain efficency.
Now here's what I've been doing this past week and some observations I've made. I spent the better part of the week cleaning the old barrel a couple of hours a day. Between scrubbing sessions I'd let is soak in whatever I could find. Hoppes, Bore Scrubber, Lead Remover, whatever. I kept getting soot out and mid week I noticed that the rifling returned when I pushed pellets through. As where there was hardly any rifling when I started, clear and defined rifling appeared all the way around the pellet. This encouraged me, but I couldn't get over wondering if I had now cleaned the barrel and was after a point simply removing bluing from inside the barrel along with fouling. The only thing that kept me going besides seeing an improvement in the rifling was the rough spot in the middle of the barrel. When I'd push a pellet through in the middle of the barrel I'd feel turbulence for a few inches. I knew that wasn't right. It also bothered me that the new barrel was definitely choked for the last couple of inches. I figured this would slow my pellets down. My old barrel doesn't' feel choked at the end. The rifling on the new barrel is definitely deeper but I attribute that to the choked crown. Between the crown and the breach there's virtually no resistance when a pellet is pushed through. Only at the crown does it take effort to push the pellet through. Compare that to my old barrel, where there is no resistance until the middle of the barrel, where it feels like turbulence and a slight resistance at the crown.
However, by Friday I realized I was slowly surpassing my costs in cleaners and brushes than a new barrel itself. Therefore I re-drilled the port in the new barrel to match the .18 port in my old barrel and installed it. The results were... the same. Pellets flying wildly. And yet sometimes I'd get a good group. Then I saw the pattern. Whether it be high velocity or low velocity, my pellets would scatter wildly when shot at 3200psi. My good groups with both the old and new barrel were always at the low bottom of my shot curve, even if velocities were still high at the bottom of the curve.
Therefore yesterday I tuned for a low fill, high velocity tune. Viola, complete accuracy restoration (with the new barrel mind you). Retuned for 3200 fill. Pellets sprayed again. It was clearly consistent. Tunes below 3000psi gave me accuracy, tunes above 3000psi were wildly inaccurate. I've almost always tuned my gun for 3200 fills and never had this problem until I drilled the ports out. Surely the bigger ports must have some effect on the pellet insofar as a fast, short puff of high psi air must be destabilizing the pellet. I can't explain it any other way and I can't understand the physics behind that. Yet the data shows a clear line in accuracy or lack thereof between high and low pressure fills. This also explains why I'd have the fluky good groups with my old barrel when all of this started. I noted then that the groups tightened up on the low side of my bell curve but I dismissed the idea when I turned the velocity down at 3200 psi and still got bad groups. I was still thinking in terms of velocity not pressure.
Wherefore, I can't answer what if anything was ever wrong with my old barrel. Maybe I cleaned the heck out of it for no reason. And yet surely it must have been fouled for the rifling to improve greatly over the course of the week. I'm going to continue to work on that old barrel, although I'm going to now do it at a leisurely pace. I'm happy enough with the performance of my new barrel.
Here's another observation. My velocity screw kept getting blown backwards during my shot strings. Because my velocity screw's end is concave, that means I got varying degrees of blockages during my shots. I would see this happen when my velocity would drop 100fps between shots. I finally had to blue loctite it in place. It seems to be holding. This told me that using the velocity port to restrict air isn't the way to go for my particular setup. I always lost power and air whenever the velocity screw was blocking the port.
Thanks for following along. The nature of my old barrel and how much cleaning is too much is still open to debate. I hope to have more answers as I work on that barrel for the weeks to come. I'd like to see if I can get that turbulence out of the center. Yet I'm not in a hurry to replace my barrel again. This new one is doing just fine. I don't want to tear into my gun again for anything the way its shooting now.
By the way, it is SO much quieter shooting the gun at the lower pressure. I have my hammer set about 6 turns CW and my #10 hammer spring about 3 turns CW. I'm using grey threading tape to keep the hammer from slipping.
I'm updating this thread to report some changes to my gun's performance.
First, my gun kept performance as in the above listed string for several weeks. However, I had to take my gun apart for something a few weeks ago (can't remember why) and when I got it back together I was unable to duplicate this amazing tune and my efficiency dropped significantly. I also found that after reassembly, my gun would waste air again with light pellets, a problem that had previously been eliminated. Finally, the regulator-like numbers disappeared and my gun began shooting on a normal bell curve again.
Recently Bob and Tim suggested to me that the reason that my gun might not be able to shoot accurately above a 3000psi pressure is because perhaps my .18 ports are too big and causing a little bit of the pellet's skirt to be exposed in the transfer port. When the air is released during the shot, the high psi levels might warp the skirt while low psi levels may not. That made a lot of sense to me. As I've already lost my super tune anyhow for reasons unknown, I figured it would be good for me to test this to see if this was the problem.
Well, I never actually go around to testing the theory directly. The way I would have done this would be inserting a pellet and looking to see where the skirt was at after removing the barrel. Instead of doing this, I ended up tinkering with barrel alignment and discovered some interesting things.
First, I have completely taken for granted that the barrel's intake port will line up exactly with the transfer ports every time you set the barrel in the breech. That's completely wrong. There is a lot of room for play when you fix the barrel into the breech. The screws in the top of the breech that hold the barrel in place are not so precise that they automatically align the barrel when you tighten them down. Its very easy to position the barrel so that the barrel's intake port can be off center in any direction while still allowing the gun to function. However when its off center it affects the guns' performance.
For example, I've found earlier this week that having the barrel a little too far back would cause my gun to shoot 8 shots at a max 70fpe with an efficiency of .9-1.0 from a 3000psi fill. However, getting the ports aligned as perfectly as I can eyeball it raises the efficiency on the same tune and power level to 1.7-2.0 from a 3000psi fill.
Here's the other big change. I can now fill my gun as high as I want and maintain accuracy. I've tested as high as 3200psi and accuracy remains.
I'm wondering now if I had my gun's barrel set a little too forward when I had it set on my super tune. Having the barrel set too far forward might have been causing the pellets to not seat as far up the barrel as they should and therefore their skirts to have been exposed during the blast. When I dissembled and resembled the gun it may be that I aligned the barrel differently causing the drop in efficiency. I can only test this by moving my barrel forward and seeing where the pellet is. However I've got my gun shooting decent again and I don't want to move it.
Currently I have the gun set for 8 shots from 3000psi to 2600psi. Instead of waiting between shots like I normally do, I shot all my strings today in rapid succession. With 43.2 Eunjins I'm getting an average in the 860s within a 4% spread for an efficiency of 1.2. With Barracudas I'm getting an average in the 980s with a 2% spread for an efficiency of 1.11. Those are good numbers and I can't complain, but I do miss that awesome low pressure tune with the Barracudas. I also don't like the wasted air with the lighter pellets. I though I had solved those problems. For the record, I've checked my velocity screw and every other variable I can think of. I just can't figure what I changed when I disassembled the gun other than maybe the barrel alignment. I can't rule out that perhaps I accidentally changed my hammer spring. I have several 10# springs and some of them are more worn out than others.
If anything it just goes to show how fickle PCPs can be, especially when you start tearing into them and modding. The only thing I can say with some certainty is that there was apparently was some zonky way I had my gun assembled or tuned that was causing crazy efficiency and power at low psi and whatever that was was probably also causing my accuracy problems at a normal power-tune fill pressure.
I will also note that I was able to get some tunes this week at 2700 fills with light pellets for 50fpe for decent efficiencies. However, on my current 70fpe tune with the Eunjins I'd only have .8 or .9 efficiency with light pellets.
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When you tune for high power with heavy pellets, it is completely 100% normal and expected for the efficiency to go out the window with light pellets.... Tuning for heavy pellets requires a large increase in dwell.... Using that dwell with a light pellet keeps the valve open much too long, and much of the air is coming too late in the shot cycle (the pellet is too far down the barrel) for it to have good effect....
Bob
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I just reread my post. My tune efficency is 1.17-1.2, not 1.7-2.0.