.25 cal Disco Pumper Carbine

[Fastair]

Nice to see you are on to another project.
I liked what you did with your uber pumper and you shared all your ideas with everyone.
I will be following this project too and suggest that Frank print off all the info you post.

[rsterne]

Well I couldn't wait any longer.... I made a new bolt and transfer port and threw the gun together for a few shots.... no stock, harder than heck on the hand to pump and all.... First a string with the Disco hammer spring set just off coil bind.... varying the pump numbers and recording the pressure and velocity.... I used 25.4 gr. JSB Kings....



The gun started retaining air at over 10 pumps (1100 psi).... It delivered 490 fps (14 FPE) on 6 pumps (700 psi) and 590 fps (20 FPE) on 10 pumps (1100 psi) and peaked out at 667 fps (25 FPE) at 15 pumps (1500 psi).... To run the gun as a pump and dump, without changing to a heavier hammer spring, the most I can get is ~590 fps with 10 pumps to refill to 1100 psi....

I then started playing with the RVA (power adjuster), to find out how that affected the velocity and the pressure remaining after each shot.... It turns out that I can back the adjuster out about 3 turns and only lose 10 fps but the pressure drop for the shot is about 150 psi less, which means a big gain in efficiency....



At 7 turns out, I can get two shots at 590 fps (20 FPE), and still have ~350 psi remaining.... It takes 12 pumps to refill after the two shots.... or about 5-6 pumps after the first shot.... Compare that with the pump and dump, same power but 10 (easier) pumps to refill.... At 3 turns out, I get one shot at 656 fps and 7-8 pumps to refill to 1500.... with having a gauge, it is a simple matter to know when to stop pumping....

Now remember that these are preliminary results, using a stock Disco rear valve half, stock barrel port, and matching transfer port (all 0.150").... The next step is to bore out the valve and port everything out to 3/16".... While I'm at it, I have to get the stock done, this thing is killing my hand pushing on the 1/4" wide pump lever....

Bob

[rsterne]

I decided that I liked the way this gun responded to a lighter hammer, so I had another one kicking around that weighed only 40 gr. (30% lighter than a Disco hammer).... I installed it, and the gun responded in exactly the way it should.... I lost about 8 fps at the top end (with greater efficiency) with the Kings, and I needed one more turn of preload on the RVA to get back to my "3-shots at 31 FPE" tune.... and they are a bit more consistent, with a whisker less air use.... I got one additional benefit.... With the RVA at maximum, filled to 1500 psi, and shooting 42 gr. EunJin Points, I get two shots at 655 fps (40 FPE) that are only 2 fps apart.... Should I wish to tackle something larger and that requires extra penetration, that combination would be just the ticket....

Bob