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Author Topic: 1752 fps (yes > Mach 1.5) with a .25 cal Disco  (Read 471 times)


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1752 fps (yes > Mach 1.5) with a .25 cal Disco
« on: February 03, 2017, 10:56:02 AM »
This post was made on the GTA on Apr. 17, 2016, in a thread by Lloyd Sikes documenting his shots of over 2000 fps.... The thread ran to many pages, debating the controversial idea that the maximum velocity that could be achieved by a PCP at room temperature running on air is 1650 fps, because that is the average molecular velocity.... This was solidly disproven, not only by Lloyd and others, but by my simple experiment documented in Reply #490, and copied to the Guild below to preserve it for posterity....

The original thread on the GTA.... .... and my contribution to airgunning history....

Here is a little more confirmation that 1650 fps is not the limit.... The rifle is my .25 cal Discovery, with a 24" TJ's barrel, running tethered at 2900 psi.... The gun is tuned to shoot 50.6 gr. cast bullets in a 3 shot string at 960, 966, and 956 fps (104 FPE) starting from that pressure, and was not retuned for this test.... I did, however, shoot a couple of JSB pellets over the Chrony to confirm the performance.... The 34.2 gr. Heavy shot at 1152 fps (101 FPE), and the 25.4 gr. King reached 1254 fps (89 FPE).... Here is the setup I use, and have used for the last 7 years, to Chrony all my guns.... The muzzle is 2 feet from the first sensor and 3 feet from the second, and this setup is rock solid and reliable from under 500 fps to over 550 FPE, the velocity readings are steady, reliable, and consistent with what I get outside.... The lighting kit is a factory Chrony setup, running two long 40W incandescent bulbs.... Without a pellet in the gun, it NEVER registers anything, so muzzle blast is having no effect.... The Chrony is on an angle to be parallel to the path of the pellet, because my target is on the floor, 20' away.... When shooting, the butt of the gun is slightly higher than in the photo, and I am shooting right through the center of the triangle formed by the rods and diffusers....

This gun holds air perfectly, and when I took it down off the rack, where it has been stored for over a month, the gauge still said 2900 psi.... Likewise, the Great White tank it was tethered to, was filled at the same time, about a month ago, and both were at room temperature, 68*F.... I hooked it up to the regulated tether, ran the shots with the JSBs to confirm it was performing normally, and then proceeded with the testing.... The first test was a pellet made from a 1/4" piece of Delrin rod that was 3/8" long, rounded at the front, and counterdrilled at the back to resemble a cylindrical skirted pellet.... It weighed 6.9 gr, and crossed the Chrony at 1595 fps.... I then changed over to something much lighter, based on a 6mm Airsoft BB, as shown in the photos below....

The BB, which weighed 1.8 gr., was too small for the .25 cal barrel, and rolled out the muzzle, although a quick test firing (muzzle raised, lowered just before firing) produced a velocity of 1740 fps.... I assume the Airsoft BB was already rolling down the barrel, so didn't get the benefit of the entire 24" for acceleration.... In order to make the BB stay in place until fired, and fit the bore a bit better, I wrapped it in tinfoil, as shown in the photo above.... The piece of tinfoil was a bit over 1" square, folded over the front and twisted 1/2 turn at the back while stretching it as tight as possible.... I then cut the back of the foil off with scissors, to produce the pellet shown.... As you can see in the photo below, it weighs 2.2 gr....

The pellet was loaded normally, with the twisted part of the foil at the back, and slid into firing position with the bolt.... I fired a total of 10 of these pellets, and the velocities varied from 1722 to 1752 fps except for one shot at 1706 fps, where I noticed the tinfoil following the pellet to the target (I assume it dislodged in the barrel).... I made up each pellet between shots, so the gun had a couple of minutes to stabilze in temperature, although all that was happening between shots was for the regulator to top up the gun about 200 psi after each shot.... There was NO observable change in temperature of any part of the system during the tests.... The average over the 10 shots was 1734 fps (14.7 FPE).... I had two shots at 1750 fps, and one at 1752 fps, as shown in the photo below....

All 10 shots hit my target backstop, 20' away, producing a group size of about 3".... As you can imagine these shots were extremely loud, since the high velocity meant the pellet was exiting while the valve was still open, even though the gun was not retuned.... The calculated residual muzzle pressure was nearly 2500 psi.... I cannot stress enough that there were NO changes made to the gun, it was shot as tuned to deliver 104 FPE with 50.6 gr. cast bullets.... It didn't have any special firing chamber or valve, the barrel was only 24" long, and the pellet was a lousy fit, shimmed up with tin foil....

I would suggest that anyone having a .25 cal PCP that is capable of hitting 100 FPE should be able to duplicate this experiment and prove to yourself that the long-proposed 1650 fps velocity limit for PCP running room temperature air is a flawed theory.... I have no doubt that a smoothbore barrel of the proper diameter for airsoft BBs, that was 36" long or more, and running higher pressures, could achieve much higher velocities, up to 2000 fps or more....

« Last Edit: February 03, 2017, 11:01:21 AM by rsterne »

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