Not an easy issue. I'm getting two distinct velocity distributions with my subsonic loads. Loads made at the same time with the exact same components. When I shoot I get the two different velocities occurring randomly during the shot string. I've meticulously checked the powder charge. I verified my chronograph is working by shooting standard velocity 22's at 1021 fps.
My AR pistol: 300 Blackout, 7 1/2 barrel, 1 in 8 twist
Loading: 209gr, coated cast, w/gas check, Federal 205 primer, seated 0.017" off rifling, LC brass trimmed +/- .001", no crimp, neck tension appears good - uniform bullet seating force although not measured. All rounds fired with suppressor-no evidence of hit(s).
With 9.4 grains of H110, V1ave = 1112, 10 STDev and V2ave = 987, 9 STdev: number of rounds = 15
I've replicated this exact issue at 9.0 grains with 10 rounds and get two velocity populations separated by 99 fps with the same exceptionally tight distribution.
I saw a similar issue with 195gr coated cast bullets but don't have as much data. Moved onto the 209's immediately.
I've order Pufflon case filler - that's the only thing I think might fix this.
Your thoughts?
