9" 7 twist AAC.
2.120 OAL measured from the tip. In a bolt gun you could go WAY longer.
These were all shot with flip up BUIS since my optic hadnt shown up yet, but I was just function/speed checking anyway. 10.7 seemed good. 10.9 was also a good load in my gun. I started at 11 and worked down .1g at a time. I need to re-work with my optic and suppressor to see just how slow I can go and still get complete function.
I could, in theory, shoot 240g from my gun if I modified a p-mag to be able to load longer. Only problem with them is cost and that you are only buying 50 for what you can buy 100 208 AMAX or 225g MATCH.
So far in subsonics I have shot 220g SMK, 208g AMAX and 225g Hornady Match. The 220g SMK was a good bullet but just way to damn expensive. Both Hornady's are about 10-15c a piece cheaper than the SMK for bullets that work equally(or in the case of the AMAX) BETTER than the SMK load. We messed with the 245g Missouri Bullet Company cast bullets, but we could never get them to feed right, I think because the flat tip is wider than the feed ramps in the AR platform. They also were VERY inconsistent speed wise. Loaded all the same we were seeing some down at 900 and some up over 1100. That coupled with the crap feeding we abandoned them. Now, in a bolt gun with a 7 twist barrel, I would probably go back and try them.
Another thing for those loading 300blk is crimping. It seems to help with accuracy. I believe(and its been discussed over on 300blktalk a bunch) it is due to the VERY small case mouth contact on the bullet compared to a lot of other cases. I put a medium to firm crimp on my rounds(all my rounds get the same, I dont adjust my LEE FCD at all). Annealing is another subject some guys over there discuss. I dont do it, but I also dont necessarily care about case longevity since I have an excess supply of .223 I can turn into 300blk. Some guys say they get better accuracy by annealing because it gives more consistent case tension. I dont know.