I had an out of battery incident with my NM AR rifle. I was having problems with my reloads chambering, and thinking that I sized them wrong, used some new factory (not reloaded seconds) ammo. On the second shot the case head blew, got a big puff of gas in the face, and the rifle was locked up solid. We (it was a team effort) managed to get the bolt open by repeatedly banging the buttstock on the grass, and only the extractor was damaged(bent). My theory is that I had a piece of "something" (either a piece of a cleaning patch, or a piece of grass) stuck in the chamber, and that is why my reloads would not chamber. Replaced the extractor and it ran fine. Even a case head separation would not cause this with a regular load of powder, but an overload can blow the rear of the case off and cause damage.

My best guess is that either it was an overload, or the bolt was not fully in battery because the case was not sized properly.