These self-centering, changeable neck bushings let reloaders precisely set the amount of neck tension on the bullet when using RCBS bushing reloading dies. This reduces work hardening of the case neck brass, prolonging case life and increasing accuracy. These benefits are increased when using high-quality brass and turning necks for consistency.
RCBS suggests that reloaders get their determined size as well as the sizes .001 inches above and below their determined size.