The Hornady Match Grade Bushing is required for use with the Hornady Neck Sizing die. It is a self-centering, interchangeable bushing for extreme precision in neck sizing. Notes:Bushing fits in Hornady Match-Grade full length and neck sizing dieBushing is self-centering in die bodyUse of Hornady Match Grade Neck and Full Length Dies and Bushings:Hornady Match Grade dies have a separate, interchangeable neck size bushing (not included with die).The dimension of the bushing (inside diameter, listed) determines the neck size when the brass is inserted into the die.Standard dies size to SAAMI minimum specification and this is often not necessary, as a particular rifle, brass type and chamber may not need minimum dimensions. Less sizing results in less working of the brass.Using some loaded or dummy cartridges that fit the rifle's chamber without excessive play, measure the diameter of the case neck. Subtract .001" from that diameter and the result should be a reasonable starting po...