GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Engineering- Research & Development => Topic started by: Pidgin Popper on December 08, 2012, 06:28:39 PM
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OK, I’m not sure if this is the right place to ask the question, I’ll try here and if no answer I’ll try on the dark siders gate.
I’ve got my 1322 Crossman pump up .22 pistol shooting at 572 FPS with CPHP (14.3 grains) @ 16 pumps. If I pump it 18 times I can get 2 shots, first one @ 526 FPS and the second one at 481 FPS. That’s the second shot with no additional pumping.
So, my question is: what would I monkey around with to get 2 shots at the same velocity (with in reason)? Thanks for any help.
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more pumps maybe slightly lighter hammer spring
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Put an RVA (power adjuster) on it so you can play with the hammer spring preload.... You should be able to find several tunes with two equal velocity shots depending on the spring setting.... I have a 14" barreled .22 cal Carbine based on a 2289 (1322) and it gets two shots at 600 fps with 14.3 gr....
Bob
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I’ve got my 1322 Crossman pump up .22 pistol shooting at 572 FPS with CPHP (14.3 grains) @ 16 pumps. If I pump it 18 times I can get 2 shots, first one @ 526 FPS and the second one at 481 FPS. That’s the second shot with no additional pumping.
I'm messing around with a 1377/22 project (change from stock 1377 to 24" 1377, Crosman steel breech and barrel). The other day I was fooling around with it and got it to pop twice with one pump and simply pulling the bolt back 'til it caught on the sear by friction, squeezing the trigger then doing it again. It got me to thinking a bit, but I hadn't thought clear to the point you mention - getting two full power shots with one pumping session. As I'm mining the site for info on this and other projects I have percolating I came across your post. My question is:
What mods have you done to your 1322 pumping mechanism? Is it a flat top piston/valve? Mine is stock for the most part at this time, other than polishing the sear, swapping the receiver and a home made trigger spring guide. Your results intrigue me as I was contemplating what it would take to enlarge the valve to get a mini-PCP. It sounds like you've got the start of what I was contemplating.
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Once you pump a 13XX enough (particularly easy with a flat-topped piston in place).... you will put so much pressure in the valve that the hammer won't open it enough to let all the air out.... The valve "retains" some air, which can be used for a second shot.... If you want to tune for that, here is the procedure.... Once you pump enough to get two shots:
If the first shot is faster, pump more and try again.... Adding pressure will REDUCE the velocity of the first shot, leaving more air for the second, which will then increase....
Once you get to the point where the second shot is faster than the first, you have gone too far.... Try one pump less....
If you can't get the two shots almost exactly the same speed, you will need to fine tune it with the hammer spring preload.... This is where an RVA (rear velocity adjuster, aka "power adjuster") comes in very handy....
If you have an RVA, you can actually vary the velocity where the two shots will come into balance.... I have a 13XX based gun set up for two shots of 600 fps with a 14.3 gr. pellet on 18 pumps of a 2200 pump (50% bigger swept volume).... I can get one 600 fps shot on 10 pumps, also....
Bob
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Thanks, Bob. I understand the basic theory, and I'm pretty sure you've mentioned it before - it's just that it wasn't until the past week when I got to fooling around a bit, trying to make up my mind which way to go with this, that the seeds started to germinate. Thanks for the thorough explanation.