Thanks. I didn't realize exactly what happened until later. My AA kit is getting a little finicky in its old age, so I didn't think anything of it - just popped it out and ran a few more magazines before calling it a day. Then I took a closer look and saw what really happened. I have no idea what caused the OOB if that's what happened. That particular round was 3 or four into a 10 round mag that had been running just fine before. I ran another ten through that mag afterwards without any issue. It's possible there was some build up in there that got blasted out with subsequent rounds. Needless to say, it will get a thorough cleaning before getting used again.looks like an out of battery fire, but more importantly, what caused the OOB?
check your chamber, make sure there isn't anything lurking in there like a fired case that split around the circumference, leaving the front half in the chamber