Wow. Well, I speculate that it would seem like a blown primer driving the striker back with enough force to break off the bolt at the stress point. I had a look at my SMLE bolt heads and although there is a vent hole in them the vent hole is largely covered buy the receiver side wall. The shooter didn't say anything about the primers but he did mention the first full power round being 'hot' (then he carried on firing the others!) It's easy to criticize someone after the fact, I know, but he should have stopped right there.

Anyway, I'm waiting for the guru's to come in to tell us what they think.