On an AR15 I have, the bolt wont always hold open on the last round but it did not have any feeding issues either. I used everything from a regular carbine buffer up to a heavy spikes h3 buffer. Regardless of the buffer, the gun would eject brass at 4 o'clock and would sometimes not lock back on the last round (it locks back fine when cycling the bolt by hand). The only time I achieved lock back every time was with 62 grain ammo. Question here is... if the gun was not getting enough gas, why would it run fine with both the carbine buffer and h3 buffer yet experience the same thing when locking the bolt back? Could it be the BCG? Looking for ideas on where to go from here.