The Redding Instant Indicator Comparator with Dial is useful for quickly comparing headspace and bullet seating depths within 0.001 of an inch. The headspace measurement is from the case shoulder to the base. This information is critical for setting the sizing die to achieve the precise amount of shoulder bump. To compare bullet seating depths simply insert a standard Redding sizing bushing of the proper bore diameter for the caliber you are using. Supplied with instructions to check and quickly compare overall case lengths and bullet base to ogive.