I have three 300BLK Uppers. 16", 12.5, and 9". All three run everything from 110 to 220g ammo, sub and super, suppressed or unsuppressed without issue. No adjustable gas block, they lock back on the last round. I can just change ammo. Mine are factory AAC uppers, I have seen issues with other combos, but it can be done without an adjustable gas block.