Every gunsmith knows that using a headspace gage is the most reliable method to measure the length of a rifle chamber. "Headspace" refers to the distance between the face of the breech and the base of the cartridge when the action is closed. Excessive headspace not only compromises safety but can also impair accuracy. For example, unsupported brass fired in a rifle with excessive headspace may rupture, causing gas to blow rearward with dangerous force, much like rocket exhaust. For your safety, Forster Headspace Gages are meticulously crafted to exacting standards to ensure accurate chamber testing. If a rifle closes on a FIELD gage, it means the chamber is dangerously close to or already exceeds the SAAMI specified maximum chamber size, highlighting the critical importance of maintaining proper headspace for both safety and optimal firearm performance.