The Accu-Ring is a sizing/seater die lock-ring which provides reference marks in increments of thousandths of an inch (0.001"). Sizing die (primary use) - Designed mainly for sizing dies, the Accu-Ring helps you zero in on what you want the headspace length of your cases to be. Seater die (secondary use) - The Accu-Ring also allows you to make fine adjustments to bullet seating depths.