The Lock-N-Load Headspace Comparator extends brass life, improves accuracy, and enhances safety. The gauge measures variations in brass before and after firing or re-sizing. It allows for headspace comparison between fire-formed brass and re-sized brass.The five bushings that come with the Lock-N-Load Headspace Comparator provide the ability to check most bottleneck cases, from .17 Rem through belted magnums. Ackley Improved chambers use the same bushing size as the parent case. See chart provided or consult product instructions for cartridge/bushing size details.