Neck Turning
Shaving the case neck wall to a uniform thickness with a cutter, improving concentricity, bullet release, and clearance in tight chambers.
Neck turning is the reloading operation of trimming the outside of the case neck so its wall thickness is the same all the way around. Factory brass often varies slightly in thickness, which can seat the bullet off-center and leave uneven grip, so a lathe-style cutter removes the high spots to a set dimension. The payoff is straighter ammunition and more predictable bullet release.
Turning serves two related goals: it improves the concentricity you can verify on a concentricity gauge, and it evens out neck tension by making every neck spring back the same. Some chambers cut for tight necks actually require turning so the loaded round fits with proper clearance. Because the process removes material, reloaders measure carefully and turn only as much as the chamber and accuracy goals call for.