Thread Protector
A small cap that screws over a threaded muzzle to guard the threads and crown when no brake, flash hider, or suppressor is mounted.
A thread protector is a short metal cap that threads onto the bare muzzle threading of a rifle whenever no muzzle device is installed. Exposed threads are easy to ding against a truck bed or a rocky position, and a bent or burred thread can make a brake or suppressor sit slightly off axis. The cap takes those knocks instead, keeping the threads clean and true.
Just as important, many thread protectors are shouldered so they also shield the crown, the precise edge at the mouth of the bore where the bullet finally releases. Damage to that crown can open up your groups, so capping a muzzle when the rifle is bare or in transit is cheap insurance for a precision barrel.