The other day I was at my range shooting some lake city .223. I was getting ready to fire another shot and the trigger would not move. Of course I freaked out and said a few choice words. I walked back to the house and took it apart. I couldn't find anything wrong with it. I put it back together and still no click. So, I called bushmaster. The guy on the other end of the line had never heard of such and thing. He transfered me over to there head gunsmith to see if he knew anything. I told him what was wrong, I think he could hear the distess in my voice and began to chuckle. He said he knew exactly what it was. He told me that a primer cup had blew off the round and went down into my trigger. So, I took it all apart again with him on the phone and low and behold, there was a primer cup stuck between the trigger spring and trigger. He said he has only heard of half a dozen or so instances that this has happened. I was very thankfull they had a good gunsmith that could get my baby back up and firing again. Thought I would share this in case someone else runs into the same problem.

Last time  I looked, the only time a primer cup will blow out of a fired round, especially mil spec ammo w/crimped in primers, is if headspace is grossly oversized, or cartridge pressure is grossly over max.

Prevention of this occurance is precisely why theay crimp the primers in the first place.  I've had fired primers come out of handloads. particularly when I'd used military fired cases and over reamed the primer pockets, but never seen this on milspec ammo that had the crimp.  Still, I suppose there's a first time for everything.

It happens.  Fire enough ammo, and sure enough, it will happen.