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rsterne · Moderator

Nearly a year ago I assembled a 7mm version of my Hayabusa PCP.... Right after that my ShoeBox compressor quit on me, and by the time I had my new Freedom 8 up and running we were busy in the Motel and I never got a chance to even shoot a shot string.... Today I rectified that situation.... First, here is the gun....

I built it on the .308/.357 version of my Hayabusa test platform, using the same valve, which ends up being full bore size porting in 7mm (.284 cal).... The tank is a 22 CI, 3000 psi unit, and the tube above it is 100 cc, with a large passage between, so the reservoir is a total of 460 cc.... The main difference with the 7mm is that I am using a barrel that is tensioned to about 1000 lb. using Belleville washers, to make it super stiff.... I intend to shoot this gun most of the time tethered to my Great White 88 CF 4500 psi tank, through a Ninja regulator modified for inline use, and set to 2900 psi.... Therefore, the first step was to determine what the maximum velocity was at that pressure, and where the knee of the curve was, so that I knew about how to adjust the preload.... Here is a graph of velocity vs. preload at 2900 psi, shooting my 96.5 gr. Bob's Boattail bullets....

As you can see, the gun peaks at 950 fps, which is 193 FPE, just about exactly what I was hoping for.... with the knee of the curve from 4 to 6 turns out on the preload.... I then removed the regulator from the system, filled the gun to 3000 psi, and shot some strings at various preloads to see what the shot strings were like.... I did three strings with the FN 96.5 gr., and then one with the 91.0 gr. HollowPoint version, and you can see the results below....

With the hammer spring preload at 4 turns out from maximum, I got five shots averaging 181 FPE ending at 2520 psi for 0.98 FPE/CI.... At 6 turns out, I got 7 shots averaging 171 FPE ending at 2420 psi for 1.07 FPE/CI.... and at 8 turns out, I got 13 shots averaging 157 FPE ending at 2060 psi for 1.21 FPE/CI.... With the Holowpoints, at 4 turns out, I got five shots averaging 179 FPE ending at 2500 psi for 0.93 FPE/CI, with the first four shots averaging 948 fps (181 FPE) within an ES of just 17 fps (1.8%).... and of course tethered it will be even tighter.... So, I can shoot on the tether at right about 1.0 FPE/CI (just fine for that use), or get 4 full power shots off tether, at 180+ FPE.... which should be just fine for hunting, as I do that from my Rhino ATV with the tank in the back....

The plan is that once the snow melts I can get some proper testing done at longer ranges.... in preparation for Varmint season in the spring....

Bob