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rsterne · Moderator

I thought it would be nice to have some hard data about how barrel length and hammer springs affect the velocity using a stock 22XX valve.... I drilled and tapped the front end of a valve to accept a male Foster fitting and then made an internal spacer so that the tension on the valve spring and the internal volume was exactly the same as a stock valve.... The CO2 piercing pin is still in place, with the usual clearance at the front end.... so the only volume available for each shot is that inside the valve.... exactly the same as on CO2.... I was hoping that the testing I did would give useful information for a CO2 powered gun as well as on HPA.... That may or may not be the case.... you will have to draw your own conclusions about that....

The gun is a 2250 with the CO2 cap removed and a remote line attached directly to the modified valve.... I used a CO2 tank which I could pressurize with air safely to 1800 psi.... It was equipped with a gauge and a bleed valve so that I could adjust the pressure before each group of five shots.... JSB 14.3 gr. Express pellets were used for all testing.... I tested with a 14.6" barrel from a 2289 using a 2240 hammer spring, a 2289 hammer spring, and a Disco hammer spring.... The tests were then repeated with a 24" barrel from a Disco.... I took great care to record the average velocity of the 5 shots that were centered over the test pressure desired.... The results are shown in the graph below....

The first thing that you will note is how much extra velocity the 24" barrel (solid lines) delivers compared to the 14" (dotted lines) with any given hammer spring.... Using a 2240 hammer spring (red lines) the 24" barrel was about 50 fps faster.... Using a 2289 hammer spring (black lines), the longer barrel was about 60 to 90 fps faster.... With a Disco hammer spring (blue lines), the 24" barrel was about 90 to 100 fps better.... This trend is what I would expect.... with the bigger gains made at the higher velocities for the longer barrel.... You will also note that the velocity peak was different with the three springs.... The 2240 spring peaked at 1200 psi.... the 2289 spring peaked at 1400 psi.... and the Disco spring peaked at 1600 psi.... No RVA was used, so all springs were fitted at "stock" preload....

The 14" barrel exhibited what I would consider a "classic" response to changing the hammer spring.... ie heavier springs produced no improvement in the velocity at lower pressures.... Only when the weaker spring was not strong enough to dump all the air in the valve was there any advantage to increasing spring tension.... The 24" barrel did show an increase with the 2289 (and Disco) spring over the 2240 spring even at lower pressures.... I think this is due to the hammer bounce which was present at 700 psi and extreme at 500 psi.... The extra hammer hit(s) were able to release more air (likely being replaced from the hose) into the long barrel before the pellet exited.... at least that's my theory.... Wink

I am rather surprised at how high the velocities were at 850 psi and below.... My stock 2260 (24" barrel) produces about 575 fps at 70*F where the CO2 pressure is (in theory) 850 psi.... This test gun produced 620 fps on 850 psi of HPA.... using the exact same barrel length, valve volume and porting (stock).... I suppose that the difference could be because the CO2 has a higher viscosity and is also more dense.... both of which would make it less efficient in propelling the pellet.... I have no other explanation.... If a proportional difference of 40 fps is applied to the 14" barrel then we would expect about 520 fps on CO2 which is probably about right for a stock 2250....

I did not test a barrel between 14" and 24" which some consider optimum for CO2 in .22 cal.... Based on these results, I have my doubts if a barrel of 18-19" would outperfrom a 24" as some claim.... I was originally going to cut down a 24" barrel in 2" increments to test this idea.... but since the velocities I got on HPA are about 40-45 fps higher than the same barrel and valve on CO2.... it wouldn't prove anything for a CO2 based gun....

SPECIAL NOTE FOR CANADIANS:
There is one very encouraging conclusion that CAN be drawn from this testing, however.... It should be possible to make a non-PAL regulated HPA rifle (or carbine).... that can achieve ~495 fps on about 500-700 psi.... If you started with a Disco and replaced the front half of the valve with a regulator set to about 530 psi.... you should be able to achieve over 100 shots at 7.8 FPE from a 2000 psi fill.... Using a 2260 tube and a 14" barrel.... with an extended valve front including regulator set to about 600 psi.... a carbine producing about 50 shots should be possible.... It might take some playing with the hammer spring or an HDD to avoid hammer bounce.... and you might need slightly more pressure as a result.... These shot numbers assume an efficiency of just 1 FPE/ci (16.4 Barcc/FPE) which should be easy to achieve.... I offer no guarantee of these numbers.... but my experience indicates they may be, if anything, conservative.... Running a regulator pressure of 700-850 psi and restricting the velocity with a smaller transfer port and/or weaker hammer spring could produce an even more efficient gun that is in addition much quieter.... The danger of doing that in a non-PAL gun is that when the pressure drops below the regulator setpoint the velocity might climb above 500 fps.... The possibility of producing a non-PAL PCP based on a Disco or 2260 is very tempting and we can only hope that one of our skilled machinists will step up to the challenge.... Mr. Green

Bob

rualert · Member

Looking forward to your next installment. Always interesting to read your posts, as the technical advise is worth it's weight in gold.

Casey

sniper · Silver Status Member

Bob, I've come to love your flooring as much as your projects. Shocked

tho' flooring makes more sense to me. Laughing

Alstone · Moderator & Site Supporter

Nice piece of work Bob, I’ll have to read through it a few times to get it into my head, but there’s certainly some valuable information regarding the effect of changing a barrel, Plus the bit about hammer spring changing and dumping the volume of the valve, that’s something I’ve had problems with.

Good Stuff! Thumb Up!

Al