When the bolt does not initially lock up completely, the FP can  not fully protrude out the bolt face, and most of the hammer contact to the firing pins is just driving the carrier forward to lock the bolt up; with not enough FP initial force to the primer once it reached it to ignite it.

If flip the B/C over with it in hand, pull the bolt forward like it not fully locked up, you will see that the tail of the bolt is not protruding out the back of the carrier, and why this fail safe happens (so the rifle will not fire when the bolt is out of battery).

Normally when this happens (bolt not fully locked up) you should be able extract the unfired case with not problems (making sure that the bullet came out with the case, and not still embedded in the bore to case a KB if another round is loaded and fired with a second bullet obstructed bore)


The fact that is a bitch to cycle the live unfired round out is screaming of one of two problems.

The first, is that the gas tube is not correctly aligned with the carrier key.  
To check this, remove the bolt from the carrier and dry fit the carrier to the upper receiver.  The sides of the key should not drag on the sides of the receiver, and about an inch before the front of the carrier touches, the back of the barrel extension, the gas tube should cleanly enter the carrier key.  If the gas tube is binding when entering the carrier key, pull the hand guards off the barrel and tweak the gas tube over the center of the barrel to get to align correctly with the carrier key.

The second, is often the failure to clean the rig correctly, and could be either a sluggish fouled action, or just too much crap in the chamber from not scrubbing the chamber with CLP by hand with a chamber brush.  Here, it really breaks down to not only correctly cleaning the rig with the correct solvents (CLP mostly, with the exception of the bore that takes a good copper solvent like Sweets instead), and then making sure to lube the upper receiver bearing areas with CLP as well.
Note, if you are shooting steel case ammo, scrubbing the chamber every cleaning, and running the upper more the wetter side with CLP will go a long way between problems/rifle starting to foul choke out and need to be correctly cleaned again.

Keep in mind that when the gas tube gets hot, it going to distort a touch on it own.

This is the reason that the barrel nut has to be correctly indexed with the upper receiver channel for the gas tube to allow it to float in the channel, and not be bound tight between the barrel nut void and the upper receiver gas tube channel instead.

So a gas tube that is not correctly indexed to begin with, will cause a touch of binding on lock and release, and if the tube is not free floating in the upper receiver gas tube channel (gas tube should wiggle in the channel when you touch it), the binding of the tube to key will become worse as the tube heats up instead.


Once we can determine that the gas tube is not the issue of the bolt  failure to lock up/ hard to cycle out a live round, then we can focus on other items that may be causing the problem instead.

You state you are using a Bushmaster BCG, over the years Bushmaster has has a bad (deserved) reputation of improper staking of Bolt Carrier Key Screws. The results wind up being the Carrier Key Screws work loose and you wind up with short stroking and finally you have a manually operated AR.

Check the torque on the Carrier Key Screws.

Carrier key bolts either becoming loose, or just snapping with the head of the bolt retained in the channel via the key peen'g to the sides of the bolt head spines.


The torque to the  carrier key allen bolts when they are being installed is only 37inlbs (3.8ftlbs), so just check the bolts to make sure that they are tight with a standard L shaped allen wrench.  If a bolt is snapped, then it going to spin with just light hand pressure to the wrench that will break it free from the side staking.


But the above will only cause short stroking problems for the most part, and not the hard unlock of the bolt to manually extract a live round instead (read still either a gas tube not correctly indexed with the key, or the chamber fouled all to hell/needs to be chamber brush with CLP scrubbed clean instead).