Using a headspace gauge is the most reliable method to measure the length of a rifle chamber. Headspace refers to the distance between the breech face and the base of the cartridge when the action is closed. Excessive headspace can create safety risks and reduce accuracy. For example, firing unsupported brass in a rifle with too much headspace can cause the case to rupture, allowing gas to escape backward like rocket exhaust. Because safety is paramount, Forster headspace gauges are made with meticulous attention to detail and strict standards to provide accurate chamber measurements. If a rifle closes on a FIELD gauge, it means the chamber is dangerously close to or has exceeded the SAAMI maximum specification. 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