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. This allows approximately .001-inch of brass to spring back for correct neck tension. The other is to measure the neck wall thickness of the case, multiply by two, add the diameter of the bullet and subtract .001-inch. RCBS suggests that reloaders have neck bushings one to two sizes above and below their target diameter.