Barrel Nut
A threaded collar that clamps a barrel to the action and lets the smith set headspace, common on prefit and switch-barrel systems.
A barrel nut is a large threaded collar that tightens against the front of the receiver to lock the barrel in place. Instead of relying on a fitted shoulder, the nut allows the barrel to thread in and out freely while the nut, once torqued, holds it at the chosen depth. This is what makes the design so friendly to home builders and busy gunsmiths.
Because the barrel can be screwed in to any depth before the nut is snugged, the builder simply runs the barrel against a go gauge and locks the nut to set headspace without a lathe. This is the heart of most prefit barrel systems and the reason a switch-barrel rifle can change calibers in minutes while holding its zero.