Apologies in advance, I don't have pictures because they said no copies. I have not handled the parts and pieces.

Case head blew out, in front of case head there is about a 1/8 wide ring of expanded metal around the circumference of the case, to me this indicates how far the case was out of battery when it fired.

Shooter had shot 5 mags, of 8 available, before mishap. When policing brass one piece was found near the shooter's firing point with no primer in the primer pocket. Likely, but not certain it came from shooter's rifle.

Bolt carrier was cracked on the sides and bent slightly downward near the bottom. Remarkably, at least to me the bolt seemed to be intact. The firing pin also appeared to be normal and intact (no measurements provided). The extractor was bent significantly outward and the case head from that area of the case was missing. There was significant flow around the ejector which helped locate the case's orientation. Bore of weapon was clear.

Shooter had shot 5 mags, of 8 available, before mishap. When policing brass one piece was found near the shooter's firing point with no primer in the primer pocket. Likely, but not certain it came from shooter's rifle. I was told FCG function-tested OK and there was no sign of primer in lower.

Any ideas as to what could cause such an outcome?