These self-centering, changeable neck bushings let reloaders precisely set the amount of tension on the bullet when using RCBS® Gold Medal Match Series reloading dies. This reduces work hardening of the case neck brass, prolonging case life and increasing accuracy. However, getting these benefits requires the use of high-end brass with consistent neck thicknesses. Proper neck bushing size can be determined two ways. Measure the outside neck diameter of the loaded cartridge, subtract .002-inch to .003-inch.