Barrel Shoulder
The flat face behind the tenon threads that squares the barrel against the action and sets headspace on a shouldered prefit barrel.
The barrel shoulder is the broad, flat surface machined directly behind the barrel tenon threads. When the barrel is torqued home, this face clamps tight against the front of the receiver (or against the recoil lug), squaring the barrel to the action and stopping it at a fixed depth. That stopping point is what fixes the chamber position, so the shoulder is the reference surface that determines final headspace.
On a shouldered prefit barrel the maker cuts the shoulder to a precise distance from the breech face, so the barrel hand-tightens to correct headspace without a gauge or a lathe. A clean, perpendicular shoulder is essential: if that face is not square to the bore axis, the barrel can index slightly cocked and degrade both accuracy and consistent lock-up.