My nephew has/had a Mark 2 M-77 in 308. My brother called last evening and told me Dan had blew up his rifle and they were headed to the hospital. We later met at mom's and Dan was ok, some metal in his thumb and some nasty zits on his face but not in his eyes thanks to glasses. Anyway he'd bought some rounds a couple years ago at a gun show, 110 gr FMJRN. That is what took it apart. Anyway we got the bolt open, I have the mother of all flattened primers and the case head is in pieces.
I took the other 7 rounds he had left, not counting the ones in the yard at the farm that blew out of the mag.
Found a full case of what appears to be 748 in 4 of them, 53 grs. Right at max. But the other 3 had about a 1/3 real fine ball powder mixed in. looks like AA2 or similar. Gonna have the rifle checked out, I think it can be fixed witha few parts like extractor, collar, mag well, and floor plate, A chambering reamer should remove the case, he wanted a synthetic stock any way.
I am nothing less than impressed as to how the gun vented the gas and everything considered, it could have been a lot worse.