Muzzle Threading
Also: Thread Pitch
Threads cut on the end of the barrel, such as 5/8-24, that let a brake or suppressor mount concentric to the bore for clean bullet passage.
Muzzle threading is the machined thread cut into the end of the barrel so a muzzle device can attach securely. Common patterns include 5/8-24 for many .30-caliber rifles and 1/2-28 for smaller bores, where the first number is the thread diameter and the second is threads per inch. Cutting those threads true to the bore axis is the whole point, because anything mounted there has to sit perfectly concentric.
That concentricity matters because the bullet has to pass through whatever is screwed on without clipping it. A muzzle brake or a silencer mounted on threads that are off-axis can cause a baffle strike or throw a rifle’s accuracy off badly. For that reason quality threading is usually indexed off the same setup used to true the crown, so the threads and the muzzle face share one true axis.