Barrel Tenon
Also: Tenon
The threaded shank at the breech end of a barrel that screws into the action, cut to set headspace and align the chamber with the bolt.
The barrel tenon is the short, externally threaded section at the breech of the barrel that mates with the receiver. A gunsmith machines its threads, its length, and its shoulder so the chamber sits at the correct depth, which is how headspace gets dialed in when the barrel is torqued into the action. The tenon also centers the bore on the axis of the bolt, so any sloppiness here shows up as runout and inconsistent ignition.
Because tenon dimensions follow a published thread spec for a given action family, barrel makers can cut a prefit barrel that drops onto your receiver with little or no fitting. On a traditional gunsmith-fit barrel the tenon is cut slightly long, then the shoulder and breech face are turned back until headspace gauges read correctly, locking the chamber to the bolt with no slack.