OP thanks for posting this. It's pretty common knowledge that new manufactured commercial ammunition can break the op rod on these guns so why this guy tried a max charge is beyond me. I have to agree that bullet setback is probably the culprit here.
Not crimping is a big no no in reloading for semi autos I've always heard. I only reload Mauser and arisaka fodder. Not that big of a deal for a bolt action.