Tale of THREE BRods - .25, .30 and .357 Completed, Page 19 !

[rsterne]

Scott, you might have missed in Reply #66, I vented the rear cap and was able to back the SSG wayyyyyyyyyyy out once I eliminated the vacuum, and the ES went way down as well.... in fact the ES on the last 10 shot string I shot was 5 fps (0.5%).... I don't notice any difference in the smoothness of cocking, but then I'm comparing it to my own cylindrical steel hammer, which I took great care in fitting and polishing....

Bob

[Motorhead]

Scott, you might have missed in Reply #66, I vented the rear cap and was able to back the SSG wayyyyyyyyyyy out once I eliminated the vacuum, and the ES went way down as well.... in fact the ES on the last 10 shot string I shot was 5 fps (0.5%).... I don't notice any difference in the smoothness of cocking, but then I'm comparing it to my own cylindrical steel hammer, which I took great care in fitting and polishing....

Bob

Missed that ... my bad 

[grand-galop]

WOW !! I have been away one day and what a developpement here.. The try was WORTH IT foe sure.. The MAD SCIENTIST is shaking the ole community with some INEXPECTED RESULTS... 

[rsterne]

Today I hooked the gun up to a regulated 500 cc tank so that I could take pressure readings on that tank as I shot some short strings to check the efficiency.... I started with the tube with my valve in it, with the MDS hammer.... I adjusted the SSG to zero gap, shot one 8-shot magazine using the 34.2 gr. JSB Heavies, and recorded the average velocity and the pressure drop.... I then turned the SSG gap adjuster out one turn, repeated, and then did the same thing at 2 turns of gap.... I then changed to the 25.4 gr. Kings, shot another magazine with those, and continued, using the lighter pellets, for 2 more turns.... I then replaced the MDS hammer with the steel one, and starting from 4 turns out (where the starting velocity was the same, 945 fps with the 34 gr.), kept going, repeating the above with the steel hammer.... Then I calculated the FPE/CI based on each pressure drop and the total FPE of each string.... and plotted all the data as below....



The efficiency is very similar with the steel and MDS hammers.... The gauge I was using to monitor the 500 cc tank had 100 psi increments, so all pressure reading are only within about 10 psi at best, but I feel that the trends above are valid, even if the exact values are not perfect because of the 8 shot strings, which resulted in pressure drops from 120 to 240 psi over the 8 shots.... More accurate numbers would require longer strings..... I also averaged the FPE/CI values at the end of the 34 gr. string and the beginning of the 25 gr. string, because they were pretty close (the heavy pellets were a bit better).... The important numbers, of course, are what the efficiency is at the highest velocities tested (945 fps with the 34 gr. JSBs).... 8 shots used 240 psi with the MDS hammer and 230 psi with the steel hammer, so you can call it a wash (that's as close as I can read the gauge).... That works out to 1.08 - 1.12 FPE/CI at 68 FPE.... and that means I can get about 38 shots per tank from 3000 psi down to the 1850 psi setpoint I was using....

I then swapped tubes, fitting the one with the Cothran valve and the steel hammer.... I had to back the SSG wayyyyyyyyy out to find the cliff, because the last time I used that valve, I was getting a vacuum behind the hammer, which I have since cured.... With the steel hammer, the cliff occurred at just over 8 turns of gap on the SSG, and with the MDS hammer at just over 6 turns.... I followed the same procedure as above, but only using the 34 gr. pellets, with the following results....



I did not record the pressure drop, or calculate the efficiency, after the velocity fell off the cliff, as there was no point, you would never operate the gun there.... I did find that the difference between the valve opening properly and intermittently (fast and slow shots mixed) was only one flat (1/6 turn) with the MDS hammer, and then 1 more flat put the velocity in the basement.... With the steel hammer, just 1 flat went from full power over the cliff.... You can see that as you approach the cliff from the high-velocity end, the efficiency rapidly increases, particularly over the last turn.... A couple of turns too much on the SG, and the efficiency is a dreadful 0.45 FPE/CI, with the gun using about 80 psi per shot.... The best efficiency occurs just above the cliff.... For 8 shots, the best I could do was 290 psi drop with the MDS hammer and 310 psi with the steel hammer.... Again, those are close enough that there may not be any real advantage to either hammer.... It works out to 0.88-0.94 FPE/CI, which is significantly less than I got with my modded valve.... That works out to 30-31 shots per tank, compared to 38 with my valve.... Note, however, part of that will very likely be the fact that the velocity with the Cothran valve was greater.... 970 fps (72 FPE) compared to 945 fps (68 FPE) for my valve.... If they were tuned to the same velocity, the difference in efficiency may well disappear.... However, that would require running the Cothran valve at a lower pressure than my valve.... likely around 1700 psi instead of 1850, to drop it down to 68 FPE.... That would increase the headroom (pressure difference between fill pressure and setpoint) from 1150 psi to 1300, which in itself would raise the shot count to 34-35 shots per fill.... If there was any increase in efficiency, it wouldn't take much to get back to the 38 shots from my valve.... In all fairness, I would have to say that the efficiency of the two valves is VERY similar.... so much so that a tiny tweak one way or the other could make either valve come out on top....

There is another variable that will likely effect the ultimate FPE/CI numbers.... For all versions except my valve with the MDS hammer, there was a lot of gap in the SSG.... A better solution would be to fit a weaker hammer spring to reduce the gap.... Small gaps generally make for narrower ES values.... I would use the MDS hammer with the Cothran valve, in addition to a lighter spring.... the valve takes that little to open it.... One other thing that could be tried with the Cothran valve would be to artificially limit the lift of the valve by using a bumper on the back of the valve, and an adjustable, recessed hammer face, like Lloyd was experimenting with.... It seems to me that the Cothran valve, as is, simply releases too much air to be efficient in .25 cal. with pellets.... To get the velocity low enough you have to drop the setpoint pressure down a long ways.... I have a gut feeling that using a higher pressure, say 2000 psi instead of 1700, and then limiting how far the valve can open with a stop, may increase the FPE/CI.... I hope Lloyd continues his experiments with that at some point.... Using my valve, I think bumping the pressure up a bit, back to 1900 psi, or maybe even 2000.... and then operating the valve further down on the knee.... will improve the efficiency of that as well.... and may well buy me a few shots below the setpoint, giving me a similar shot count to what I have now, at my 70 FPE goal....

Bob

[ax57]

Wow, that is surprising to me.  I expected the lightweight hammer to be more efficient under the circumstances.  Thanks for taking the time to run the tests and post the data.

[rsterne]

The results don't surprise me, particularly at the 70 FPE level with 34 gr. pellets in .25 cal. in a 23.8" barrel.... Lloyd's spreadsheet tells me that the valve is open for about 10.5" of pellet travel with my valve (at 945 fps), and about 12.0" with the Cothran valve (at 970 fps).... The valve dwell is over 2 mSec. for both.... It has been my experience that in this power range (valve open for about half the barrel length), the long dwell governs the efficiency more than nearly any other factor.... You already have so much lift, that the flow is being "clipped" because the valve throat limits the area.... To get into the range where subtle changes make a big difference in efficiency, you have to get down to about half that much dwell, in the 1 mSec range, where the valve is closing before the pellet is only 20-25% of the way down the barrel, or less....

If I saw any trend at all, it would be that the MDS hammer seemed to have an edge in efficiency at the lower power levels, where the dwell is much less, and it has a chance to make a difference.... If anything, I think at the top end, with my valve, the steel hammer had an edge.... With the Cothran valve, there is no point in using a heavy hammer, you can max. out the velocity with a light hammer and light spring.... so why not use them....

Bob

[Motorhead]

With the Cothran valve, there is no point in using a heavy hammer, you can max. out the velocity with a light hammer and light spring.... so why not use them....

Bob

Yup ... having lower cyclic vibration ( lower mass in motion ) and reduced Lock Time ( Trigger pull to pellet exiting )  Smoother cocking with less wear & tear along with MDS being self lubricating ( No oil to attract and hold debris ) whats NOT to like 

[Buldawg76]

Well Bob those numbers are very interesting especially with the Cothran valve and the MDS hammer and tell me my build may work out quite well with the 26 gram MDS hammer with an aluminum core from Motorhead and a TSS from Travis with the Cothran valve and the reg set at 2000 to 2200 psi with a 500cc bottle.

I am still looking at using a CF bottle to up the pressure in it to 4500 with a 3K burst disc to protect the gun from reg bleed out. I know if I don't go to the 1/4" port I may have to use the 2200 psi on the reg to get close to the 930/950 fps range and is the driving reason for the 4500 bottle to compensate for the higher reg set point. Even if I can only get to 900 fps with the 3/16" ports and 2200 psi I will be happy for that. I am after good power with a high shot count being slightly more important then power.

I mainly am after a accurate gun for 100 yard bench rest shooting at our FT clubs sight in range that has spinners and reactive targets from 10 to 100 yards every 5 yards. We have some friendly long range competitions after the matches and soon will be holding long range silhouette matches as well. That and I would like to be able to show one member that you don't need a 2500 dollar RAW HM1000X to be able to shoot accurately out to 100 yards.

Mike

[3BI5]

I'm testing Cothran valve with stop buffer and adjustable hammer and it looks like it is not possible to adjust the velocity by limiting the hammer throw. The valve simply is either on or off. Too little throw and you're off the cliff like with hammer spring adjustments and with too much throw you're wasting air. The optimum setting for effiency seems to be less than a quarter turn more on the hammer throw from the point being off the cliff.

For example on my bottlepup, regulated at 1700psi with .25 24gr JSB's the fps stay at 920-935 range no matter how much hammer throw. Unless too little throw and then it drops down to 435fps.

[shorty]

Amazing stuff and my hats off to your sir. That was alot of work.

The only thing in my head keeps asking, why doesn't the Cothran valve close faster and how do you get it to close faster? In both cases heavy hammer vs light weight hammer, the efficiency was nearly the same.

Too me,
That indicates that the dwell will not be affected by an absorbing material like I messed around with (elastic collision/in-elastic collision) controlled lift. I bet, that your only gonna see (big swings) if the reg pressure is changed ( higher or lower) until the sweet spot is found for valve closing (dwell).

Really interesting about the balanced valve. I am also curious to know if the balanced valve can be adjusted (internally) to operate at different pressures to reduce dwell. There's gotta be a "mechanism" in the valve design to do this ( I would think?).

[rsterne]

Jari, the setup Lloyd was using did not adjust the hammer throw per se.... It used a recessed, adjustable striker, and the face of the hammer hit an energy absorbing bumper on the back of the valve.... The way he used it, the hammer ALWAYS hit the valve and stopped.... The valve stem protruded past the face of the bumper material.... The amount of recess for the striker governed how far the stem of the Cothran valve was pushed by the hammer before it stopped.... By increasing the recess in the hammer face, the valve opened less.... and vice versa.... If the striker was set flush with the hammer face, whatever the distance the stem was above the bumper, was the valve lift.... The hammer strike was always enough to operate the valve above the cliff.... The only thing that varied was how far the valve stem got pushed on firing.... It provided control over the power, allowing another way of tuning the Cothran valve without having to change the pressure to change the velocity....

Tim, I haven't been inside the Cothran valve.... Like Lloyd, I view it as a "black box".... a Proprietary design that works very well for what it was designed to do.... flow a LOT of air with a light hammer strike.... The only way to tune the velocity, other than what I describe above, is to vary the operating pressure.... More pressure, more velocity.... This results in a constantly decreasing shot string in an unregulated PCP, with the first shot (at the highest pressure) the fastest.... This meets its original intended goal of a high powered hunting valve extremely well.... with way over 200 FPE available in a Disco or MRod at 3000 psi, with a standard hammer and likely a lighter than stock spring.... The amount of power it can generate with light cocking force is quite amazing....

The Cothran valve has a "cycle time" which you can consider the dwell.... It is basically either ON or NOT.... That is why hammer weight, or spring preload, has almost no effect on the power, because it doesn't change the dwell much.... I have no idea if that time is relatively fixed, or if it changes with air pressure.... I know from previous testing that the valve cycling DOES vary with backpressure, it works differently with different pellet weights.... If you tune the gun just above the cliff, and change to a lighter bullet, it "falls off the cliff" and shoots a low velocity shot.... Likewise, if you shoot without a bullet in the barrel, the valve closes very quickly, with a much reduced report, ie it doesn't cycle as intended.... If you want to really understand the Powerhouse valve better, I suggest you study my thread on the tests I did in my Disco Double with one.... http://www.gatewaytoairguns.org/GTA/index.php?topic=117089.

Don spent a lot of time getting the internals to work the way he wanted.... I have no idea what changing things around in there would do to the way it operates.... I'm betting you could custom taylor it to work differently.... Perhaps you should get one and tear into it to satisfy your curiousity....

Bob

[rsterne]

I just posted about casting the new 68 gr. BBTs using the RG series moulds in the Big Bore Gate....  http://www.gatewaytoairguns.org/GTA/index.php?topic=120719.0

I'm looking forward to testing these in my BRods once I get the .30 cal TJ's barrel installed....

Bob

[rsterne]

Today I moved back the port in the .30 cal barrel that I had to line up with the port in the MRod receiver.... I was able to get everything to line up without having to cut another O-ring groove, shorten the barrel and machine a new chamber, so that saved me a LOT of time.... The new barrel port is 0.22" wide by 0.28" long, so it is the same area as the 1/4" transfer port.... and the new spot face around the port is 3/8" to match the receiver, valve, and the OD of the transfer port.... The barrel was already machined for a Hatsan Air Stripper, so with just the change in port location it was ready to fit with a Carbon Fibre sleeve to stiffen it.... The barrel is a TJ's with a 1/2" OD and I had some high modulus CF tubing of that ID that was 16mm OD, the same as the .25 cal LW barrel I used, which I had already machined the receivers to fit.... By insetting the CF tube into the receiver, there is no weak area at the front of the receiver for the barrel to bend, and the high-modulus CF the tube is made of is 50% stiffer than normal CF, which puts it at least as stiff as a steel barrel of the same diameter.... Instead of being able to flex the barrel with your hands, once sleeved, it's like a rock....

I sanded the barrel OD and the ID of the tube with 220 grit to give a bit of "tooth" for the adhesive.... I used Loctite 638 (green) which is high-strength, high-viscosity, intended as a retaining compound for slip-fitted parts with a gap of 0.010" or less.... It has a shear strength of 4500 psi once cured, stronger than most epoxies, has a working time of 4 minutes (or less) and a curing time of 24 hours.... Once cured, I will be giving it a post cure in my wife's oven at 175*F for 3 hours to assure full strength.... You MUST assure that the parts slide together nice and easy, if they stick and you have to force them, you may run out of time, because the tighter the fit, the quicker 638 sets to the point you can't move it.... I used two nuts on the threaded muzzle portion, and an O-ring, as a stop for the sleeve, to insure proper placement.... but a few wraps of masking tape will also work just fine.... Make sure you have the CF tube exactly the length you want, and know where you want to position it, before you apply any glue.... Make sure you plug the ends of the barrel and any ports, with wadded up paper towel, etc., to keep the glue out.... Here is what it looks like with the CF tube glued in place....



The portion of CF tube in the receiver is about the same length as the steel part where the port is.... I had to drill out the cone of the Hatsan Stripper to 11/32" to have ample clearance for the .30 cal bullets.... This one started out drilled for a .177 cal.... The other job I did today was to change the cocking and lockup pin in the bolt to 10-32 for safety with the extra loads from the .30 cal.... At the same pressure, the force pushing the bolt back is 44% greater than on a .25 cal, and going up from 8-32 to 10-32 increases the shear area by 45%.... In the .30 cal, at 3000 psi, there is 212 lbs. force on the bolt, and although the stock 8-32 should be able to handle that, if you ever fired the gun without having the bolt handle down, that force would accelerate the bolt backwards, with only that 8-32 bolt to stop it before it hits your face.... We have never heard of Crosman having a problem, but the MRod only goes to .25 cal from the factory.... Using a 10-32 gives me the same safety margin with my .30 cal.... Here is a photo of the receiver showing the new pin and the wider slots for it....



The MRod bolt is case hardened, so the first thing I had to do was heat it red hot around the threaded hole for the pin, and from there forward.... By letting it cool slowly, it anneals the steel, making it much easier to machine.... although there is still a bit of a tough skin.... I carefully lined up the 8-32 hole with my lathe chuck, using a long screw in the chuck, and clamped down the bolt.... I then removed the screw from the chuck, and drilled the hole out and tapped it 10-32.... One tap I had was a bit older, and I thought I might break it, as the metal was still hard to thread.... but my other tap, a nice new one, cut the threads no problem.... I then used a 1/2" long 10-32 SHCS, and loctited on a hex nut, tightened hard against the head of the screw.... I turned the nut, and the head of the screw down to 1/4" OD, and shortened the head about 0.025" so that it was just below flush in the receiver when installed.... I had to mill out the cocking slot in the receiver wider as well, along with the cross-slot on the left side (I will only be installing the handle on the right) so that the new, larger locking pin would fit....

The last thing I did today was to grind off the probe and the nose of the bolt, leaving a 1/4" stub about 3/4" long.... I will make a new .30 cal nose with probe that will be slid over the stub and loctited in place to complete the conversion of the receiver to .30 cal....

Bob

[grand-galop]

Where do you get those sleeve???  I saw that you have used it in another build..  Would they work on the crosman barel???  Your rifle is comming along nicely and the end product will be one of a kind...

[rsterne]

There are a few companies that sell CF tubing online....

http://www.clearwatercomposites.com/Products/carbon-fiber-tubes/round

https://dragonplate.com/ecart/categories.asp?cID=263

https://www.rockwestcomposites.com/

Lots of listings on eBay, most out of China (not a problem, I have bought a few, but usually Metric), and a few "surplus" CF tubes from US suppliers.... lots of sources online.... You would need a 7/16" ID tube for many of the common Crosman barrels.... I haven't (yet) dealt with dragonplate, but I like the variety of wall thickness and material they have listed....

Bob

[grand-galop]

And would you suggest the addition for a crosman barel...  I always pay close attention to my 2240 while hunting with it but noticed how easy it is to move around.... The addition is it enouf to STABILIZE with a steel breach??

[Buldawg76]

And would you suggest the addition for a crosman barel...  I always pay close attention to my 2240 while hunting with it but noticed how easy it is to move around.... The addition is it enouf to STABILIZE with a steel breach??

Not sure how much it will help without the CF shroud being inside the breech like Bob does his, The 2240 steel breech would not allow for the shroud to be fitted inside it since its only slightly bigger than the barrel itself. If you had a aftermarket breech that could have the hole drilled out to allow 2 inches or so of the shroud to fit tight inside the breech it would certainly help stiffen it up. Otherwise the barrel is still the only thing inside the breech to provide any real support for flexing at the breech. It would stiffen the barrel outside the breech but could allow the barrel to bend at the breech if hit hard enough on a branch or tree trunk.

Mike

[rsterne]

Alain, the barrel on a 2240 pistol is so short I don't know if I would bother.... Also, there is no room for a CF sleeve between the barrel and the main tube anyways, the clearance is about 1/32".... As Mike says, for best results the CF sleeve needs to be set into the receiver at least 2 diameters, and 3 would be better.... Mine is about 2-1/4 diameters, but picks up extra stability because the barrel goes that far again into the receiver.... Believe me when I say the barrel on this gun doesn't flex relative to the receiver.... 

I finished up the .30 cal Bolt conversion today by increasing the diameter of the forward section.... Now you can clearly see the larger cocking / locking lug....



I ground off the probe, leaving a straight 1/4" diameter shank 3/4" long.... and then made a sleeve to slide over the 1/4" diameter shank from a piece of 5/16" drill rod.... I turned the front part down to form a probe that is 7/64" in diameter, which just fits inside the base of the JSB 45 gr. and 50 gr. pellets.... The flow area remaining around the probe is the equivalent of a 0.280" port, and the ports in the barrel and Cothran valve are only 1/4", so it should not restrict the flow.... I turned down (filed and sanded, actually, in the lathe) the OD so that it would just slide into the chamber of my .30 cal barrel, checked the alignment, and glued it onto the stub of the bolt with Loctite 638 (green) and allowed it to set....

The shoulder on the bolt nose is located so that the outside edge of it is flush with the back of the barrel port.... and the probe will just push the skirt of the JSB pellets past the front of the barrel port.... When you are making a longer, thinner probe, you are limited in the overall length by how far it will withdraw to clear the back of the magazine.... With the hammer and cocking pin in place, I temporarily install the receiver, with the bolt in place, and pull it back as far as the hammer will allow it to go.... and then mark and cut the probe off flush with the back of the magazine notch.... That will still be a bit long, once you install the trigger, the hammer when cocked will sit slightly forward of that point, and you may have to shorten it a bit more.... The problem is, that if you do, it may not push pellets past the longer barrel port, so you may be stuck with having to hold the bolt back to remove the magazine.... On my .25 cal, I am just able to remove the mag. with the gun cocked, I won't know on this one until the final assembly.... Worst case, I don't mind holding the bolt back to remove the mag. if that is the only way to get the pellet past the barrel port....  Like with many conversions, there is often just enough (or not quite enough) room for things like this....

Bob

[grand-galop]

Thanks for the precision about the barel and will consider trying if i change my breach for a magazine fed..  Your project is riding really smooth..

[MJP]

Nice looking work Bob, don't you just love to turn those small probes on a lathe that doesn't spin fast enough.
I'm limited to around 1800rpm on my lathe and anything smaller than 3mm is files and sandpaper.
Is there any room to extend the cocking slot if the mag won't fit?

Marko