Nope......., I most respectfully slightly disagree with your mechanics MrE but agree with the ammo observations. I don't happen to have a Bren BB with striker here and fully understand the geometry regarding the depth of the striker below the breech face. BUT, you're forgetting that it really doesn't matter how deep the striker is set back but how much it protrudes. If it did matter how much it was permitted to be set back, there would be a gauge for it. The depth of set back or return into the BB is all to do with the length of the recess for the transverse striker retaining pin in the striker - about 9mm or so from memory. As I said, I don't really understand the mechanics of the US re-welds so I could be talking pure horse shi..........er....., manure. In the meantime, it can ONLY be gas pressure or magic that is blowing those primers.

You could try different ammo to eliminate that OR a new striker OR a new BB.