While there's not much away from the muzzle end that can hurt in this hobby, out of battery or extreme head space can make some blood. I may put a trunnion in with toothpicks but the thought of an unsupported round going off in my face makes my skin crawl.
The open bolt sub guns fire before the round is fully chambered. A 7.62x25 sits 140 thousandths out of the chamber when it's all the way in. The gap between an AK74 bolt and the chamber face with a fully chambered round is 40 thousandths on a PPS43 barrel.
I've see no mention in other threads of machining the bolt face deeper although I may have missed it.
I think in terms of the bolt being held a few thousandths off the cartridge by the carrier on the trunnion but that leaves alot of cartridge unsupported.
Am I missing something here?