GTA
All Springer/NP/PCP Air Gun Discussion General => American/U.S. Air Gun Gates => Crosman-Benjamin Air Gun Gate => Topic started by: point177 on May 01, 2021, 03:30:06 PM
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Been thinking about this and came up with these posts here in this thread.
https://www.gatewaytoairguns.org/GTA/index.php?topic=183244.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=183244.0)
After discovering that two shots were possible with twelve or more pumps, I got out recently made springs from two different sources for my CO2 2300S project.
I tried the not so well made spring (second from top in pic #1 and it gave some encouraging results like more than two shots but low power and also it not being able to strike hard enough to even open the valve with fifteen pumps.
More testing and I finally settled for the cut down spring from the second source (fourth from top in pic 1)
Its giving me four shots (tried it more than twice). The first shot is low power and the next three are fairly high power shots with poi very close to each other as seen in the grouping picture.
Not having a chrony i was curious about what power levels are seen here so shot into an old pellet tin. The first shot dents the tin only so much that its hard to see. The next two penetrate and the third one makes a good dent.
As seen from the picture of rear of the tin, the second shot is the most powerful, followed by the third.
The testing was dine at a distance of about 7 yards with JSB Exact Express pellets.
I am more than encouraged and pleasantly surprised to have achieved a self contained pcp system so easily with just a spring change but now am curious as to what i need to do to make sure that the poi for all four pellets is as near to each other as possible. What must be done to achieve that ?
Thanks for reading and any advice offered.
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Sounds like an extreme case of retained air, but nice to have if it suits your shooting needs. I worked on a friend's vintage Benjamin pumper and found I could pump it 6 times, take a shot and then just 4 more pumps got me a shot with the almost same velocity. The actual trend was a slight increase over a few shots, but hitting a "steady state behavior" of about 730 FPS after about three or four shots. This rifle retained air on shots taken with four or more pump strokes. With six pumps, the shot would be about 700 FPS and a second shot on the retained air alone was about 500 FPS.
Some guns retain more air than others, but as you have found, you can influence the behavior. Makes me think of what a person can do to tune an unregulated PCP.
Without a chronograph you just won't know anything exactly. You could improvise a penetration test by shooting some medium. "Ballistic" gelatin probably isn't easy to come by, but others use regular food gelatin or even clear soap base. Maybe even multiple layers of cardboard? Anything that will let you at least compare each of the shots will be a start. I even used a laptop and a noisy target to do an acoustic chronograph. It required I zoom in on the actual time history of the audio of a shot and impact, but it was certainly useable data.
Once you get a way to estimate velocity, I wonder if you might find it better to look for a "steady state" behavior like what I found. Maybe you can use that retained air such that just a couple pump strokes per shot is all you need on a bunch of consecutive shots.
Here's the data I was getting before the piston seal I had made broke...Each string starts from dead empty. I forget the pellet weight, but they were probably average .177 pellets. The last two columns started with 6 strokes for the first shot, then 4 and 3 pump strokes respectively for consecutive shots.
consecutive pumping retained+pumps
shot no 1 stroke 2 stroke 3 stroke 4 stroke 6 (s+4) 6 (s+3)
1 324 481 549 652 716 693
2 352 498 586 648 719 675
3 371 500 597 675 734 661
4 348 502 610 676 746 651
5 350 505 619 692 734 649
6 347 507 608 695 738 651
7 356 504 620 705 732 648
8 363 496 606 717 735 654
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This is a great idea you proposed of topping up pumps for a given fps. I have 3d printed a power adjuster and will do some testing later in the weekend to see whats happening
Sounds like an extreme case of retained air, but nice to have if it suits your shooting needs. I worked on a friend's vintage Benjamin pumper and found I could pump it 6 times, take a shot and then just 4 more pumps got me a shot with the almost same velocity. The actual trend was a slight increase over a few shots, but hitting a "steady state behavior" of about 730 FPS after about three or four shots. This rifle retained air on shots taken with four or more pump strokes. With six pumps, the shot would be about 700 FPS and a second shot on the retained air alone was about 500 FPS.
Some guns retain more air than others, but as you have found, you can influence the behavior. Makes me think of what a person can do to tune an unregulated PCP.
Without a chronograph you just won't know anything exactly. You could improvise a penetration test by shooting some medium. "Ballistic" gelatin probably isn't easy to come by, but others use regular food gelatin or even clear soap base. Maybe even multiple layers of cardboard? Anything that will let you at least compare each of the shots will be a start. I even used a laptop and a noisy target to do an acoustic chronograph. It required I zoom in on the actual time history of the audio of a shot and impact, but it was certainly useable data.
Once you get a way to estimate velocity, I wonder if you might find it better to look for a "steady state" behavior like what I found. Maybe you can use that retained air such that just a couple pump strokes per shot is all you need on a bunch of consecutive shots.
Here's the data I was getting before the piston seal I had made broke...Each string starts from dead empty. I forget the pellet weight, but they were probably average .177 pellets. The last two columns started with 6 strokes for the first shot, then 4 and 3 pump strokes respectively for consecutive shots.
consecutive pumping retained+pumps
shot no 1 stroke 2 stroke 3 stroke 4 stroke 6 (s+4) 6 (s+3)
1 324 481 549 652 716 693
2 352 498 586 648 719 675
3 371 500 597 675 734 661
4 348 502 610 676 746 651
5 350 505 619 692 734 649
6 347 507 608 695 738 651
7 356 504 620 705 732 648
8 363 496 606 717 735 654
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It kinda works like a Nova Freedom pump up PCP. 8)