At least it's a mystery to me! Ar15 300 blkout, bolt doesn't lock on last round. I know this can be a common problem but the issue I'm having is it doesn't do it on all ammo. My last trip to the range I fired 25 rounds each of Fiocchi Range Dynamic 150 gn, Remington 200 gn. and my reloads of 15 gn. of H110. The Fiocchi load, eject fine about 4:00 oclock as all of round do but didn't lock the bolt back same as the Remingtons although I had one failure to eject as it tried to feed the next round and it locked the bolt once out of 25 rounds. My reloads, load, fire, eject and lock the bolt every time including a prior trip the range when I fired only my reloads. My reloads are very close to the Fiocchis in bullet type and FPS should also be very close based on published numbers have not chronoed them. The only difference that I can think of is my reloads have a longer C.O.L, Any ideas? Thanks in advance.