So I’ve been working on some subsonic loads for my 300 blackout and I have run into a number of silly things I don’t understand. One thing I learned was that after loading 5 rounds without sizing they grouped .6” at 50 yards vs the standard 1.5-2” I’ve been getting. All I did was just run these upside down to swage on the gas check without touching the driving bands, Then seated them like the rest. These are 206 grn 311299 gas checked falling at 9bhn, sized to .310 being fired out of a 16” barrel over 11.5 grains of 300blk. The barrel slugs at .3081”.
My first thought was that perhaps they were getting swaged down because the expander wasn’t expanding the mouth enough. I made a dummy round from a .310 sized boolit and pulled it, still measures .310. What gives? I ordered a .312 sized and an expander plug from NOE just so I can quit wasting time and fix both issues of undersized boolits or excessive neck tension. Also oddly with the .310 sized projectiles the velocities were not very consistent, with the as cast rounds all 5 were 6 FPS apart +-. Anyone understand an explanation for this. I will add that I sure as hell don’t anneal the necks and the brass is from converted lake city .223 brass.