Re: ak
It is rare, but certianly not out of the question, to get an out of battery ignition with an AK. There is a tail on the bolt carrier that is designed to reduce this type of thing from happening, becuase of the large tolerances the AK desing uses, it is not fool proof.
2 Things that I would do. 1-purchase a brand new Tapco G2 single hook trigger group and 2- inspect the bolt carrier to insure the tail is intact and un damaged. Over many thousands of rounds, the tail will peen, consider minor amounts of that normal, if it is bent or missing, that is no bueno. While you are at it, insure that the receiver did not become bent.
The real question I have is, how was the rifle being shot when this occured, slow fire, rapid fire, bump fire etc. That answer may provide an idea of what exactly happened to allow this to occur.