Progress on my 2260 HPA Project

Progress on my 2260 HPA Project

Joined: February 17th, 2010, 3:30 am

August 18th, 2012, 7:07 pm #1

After a rather long enforced break due to busy season in the Motel, I got a chance to get back to this project over the past couple of days.... I sold off some of my earlier 2260 HPA parts, which forced me to commit to the "tank under" conversion of that gun, which saw the destruction of about 100 ground squirrels and marmots this spring.... I used the tank mounting block, extended valve, and hammer spring guide found in this thread:http://www.network54.com/Forum/275684/m ... +2560+HPAs and reassembled the gun using a 13 CI tank regulated to 1600 psi and carrying a 1.8K burst disc.... The hope was that the increased plenum volume of the full length 2260 tube between the regulator and valve would allow me to achieve the same power levels on less pressure than I was using previously (1900 psi), and I'm please to say that was the case.... The initial tests were done without the spring guide (which increases the hammer weight by 11 gr.) with the following results....

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As is typical with a regulated gun, the velocity plateaus if you have more hammer strike than needed for the regulator output.... In this case, there is no loss in velocity with reducing the preload one turn from coil bind, and only a 20 fps loss at 2 turns out.... That setting gives me 943 fps with 21.0 gr. H&N Baracudas (41.5 FPE), and 1000 fps (40.2 FPE) with the 18.1 gr. JSB Exact Heavies.... One more turn out drops the 'Cudas to 890 fps (36.9 FPE) and the JSBs to 954 fps (36.6 FPE), which is right on the velocity that proved so deadly with those pellets in this gun in the spring.... One more turn out, and the power level drops to 30 FPE....

The purpose of using the full length 2260 tube and reversing the tank to sit underneath it, was to greatly increase the volume of the "plenum" between the regulator output and the valve seat.... The extended front end on the valve allows more air to enter the valve than can escape during the shot, so the pressure at the valve seat drops less because of that, raising the AVERAGE pressure during the shot.... My calculations show that the air usage (33 CI at 1 bar) when the gun is tuned for 37 FPE drops the pressure to 1480 psi when the valve closes at the end of the shot, making the average 1540 psi during the shot.... That means the valve is releasing 4.9 cc of HPA to do the work.... This rifle is a .22 cal with a 24" barrel, which places the bore volume at 14.5 cc, so that air can expand by nearly a factor of three in the barrel, which should lead to reasonable efficiency.... Here is a graph of what is happening in this case....

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You can see that the efficiency tracks virtually parallel to the ratio of the bore volume divided by the volume of HPA used for the shot.... This is an important piece of information I learned from Steve in NC, and I thank him for that "lightbulb moment".... After doing the baseline work, I added the 11 gr. hammer spring guide, and found that I could now back off the hammer spring preload an additional 3/4 turn and still get the same velocities.... At this point I also changed to a new tin of JSB Heavies, and it turns out they were 0.1 gr. heavier, so I fiddled with the preload to bring the velocity to 950-960 fps and shot a complete string, filling to 3000 psi....

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As you can see, the results are an extremely flat string, with an ES of only 12 fps over shots 2-45.... I'm still under the hex of the "low first shot after a fill" gremlin, but at 938 fps, it was only 2% below the average velocity of 957 fps (37 FPE).... The bottom line is that I had 44 shots within a 12 fps range, and 47 shots with 2.5% (24 fps).... The pressure after the last shot was 1280 psi, meaning that I can shoot 300 psi below the regulator setpoint without a noticable drop in velocity.... That shows a VERY good balance between pressure and hammer strike, which is confirmed by the efficiency of 1.12 FPE/CI.... All that remains is to do some long range testing to find the highest velocity at which the JSB Heavies will hold good groups.... and to test a few other pellets to see if any others are up to scratch....

Bob

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Joined: April 28th, 2010, 12:23 am

August 19th, 2012, 4:03 pm #2

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Joined: February 17th, 2010, 3:30 am

August 19th, 2012, 5:01 pm #3

with EunJin 32.4 gr. Pointed pellets with the hammer spring set near coil bind.... not many shots, though....

Bob
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