Twist Rate and Velocity Loss

I thought I would update this thread with a bit of personal experience using a fast twist barrel…. My .257 Monocoque was designed to use the 100 gr. and 113 gr. NOE Bob’s Boattails, which are designed for an 8″ and 7″ twist respectively…. Since the barrel supplier I deal with, TJ’s of Kentucky, only had 10 and 14″ twist available in that caliber, I asked Mike to make a mandrel for a 7″ twist with a 0.257″ groove and 0.250″ land…. He agreed, but decided that with such a fast twist, and using cast bullets, he should make the rifling deeper, so decided to make the barrel with a 0.248″ land diameter…. He sent me a barrel, and it was at that point he told me what he had done…. I was pretty concerned that the deeper rifling, with the limited power available in an airgun, wouldn’t achieve the velocities I was after…. He said “trust me, if it doesn’t work, I’ll do another mandrel and replace the barrel”…. I’m glad I did….

I recently completed the gun, and at 4000 psi it shoots the 113 gr. bullet at over 950 fps, and the 100 gr. at over 1000 fps, which met all my hopes and dreams…. I have set the gun up to be tethered to a regulator set at 3800 psi, at just over 200 FPE, and tuned to shoot a 4 shot string off tether…. The 3rd shot of the string is the same velocity as with it tethered, and occurs at just under 3500 psi on air…. At that pressure, the gun is still capable of achieving over 200 FPE, shooting 900 fps with the 113 gr. FN, and 960 fps with the 98 gr. HP version of the lighter bullet…. It does this with bullets sized to 0.257″ or 0.256″, but is about 20 fps slower if they are sized (oversize) to 0.258″…. Although I don’t have a slower twist barrel to compare this to, there is certainly nothing wrong with this performance…. I would say that fast twist barrels, if they are required to stabilize long, heavy bullets, can still be handled just fine by modern PCP airguns….

Bob