I'm a bit confused when you say "head space". Shotgun shells head space on the rim. The Darne breech block locks hard up against the barrels using a knuckle. Same principal as a Luger or injection mold presses or a zillion other mechanisms. In the Darne the rear end of the key pivots on to a replaceable shim or wedge or I don't know what you call it. In the event of wear, after about a million years, not a big deal to fit a new one.
nial