Every gunsmith knows that proper use of a headspace gage is the most reliable way to test the length of a rifle chamber.
"Headspace" is the distance between the face of the breech and the base of the cartridge when the action is closed, and excessive headspace can be dangerous as well as impair accuracy. For instance, unsupported brass fired in a rifle action with excessive headspace can rupture, allowing gas to blow rearward like rocket exhaust. Because your safety is on the line, Forster Headspace Gages are made with painstaking care and exacting manufacturing standards to ensure accurate testing of your rifle's chamber.
If a rifle closes on a GO gage, the rifle chamber will accept any ammunition that is made within SAAMI (industry standard) specifications. If a rifle closes on a NOGO gage, the rifle chamber might have excessive headspace. Most gunsmiths chamber a rifle's headspace between the GO and NOGO dimensions. If a rifle closes on a FIELD gage, the rifle chamber is dangerously close to or already over the SAAMI specified maximum chamber size.