I agree about the firing pin impression, what bugs me is the line from the impression over to the edge of the primer!
That looks like the pin was pressed into the primer when the bolt unlocked (or locked) I can't tell which, but I'm favoring locked because of the center hit on the primer cup.
If that's the case we're looking at a gas system malfunction not having enough dwell time before unlocking, or a shot locking shoulder or bolt locking face.
Allan Quartermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”