Proper use of a headspace gauge is the most dependable way to evaluate the length of a rifle chamber. "headspace" is defined as the distance between the breech face and the base of the cartridge when the action is closed. Excessive headspace can lead to safety hazards and affect accuracy. For instance, if unsupported brass is fired in a rifle with too much headspace, it may rupture, allowing gas to escape backward in a manner similar to rocket exhaust. Due to the critical importance of safety, Forster headspace gauges are crafted with meticulous attention and strict standards to ensure accurate testing of your rifle's chamber. If a rifle closes on a GO gauge, it signifies that the rifle chamber can accommodate any ammunition produced within SAAMI (industry standard) specifications. Extensive range of rimless gauges in large calibers Engineered from premium-grade, hardened steel and meticulously precision-ground to meet stringent ANSI and SAAMI specifications The headspace tolerance is precisely ground to an exacting specification of +/-0.00015 inches Each headspace gauge is meticulously inspected using NIST-certified measurement instruments