Every gunsmith knows that using a headspace gage is the most reliable method for testing 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 can be both dangerous and impair accuracy. For example, unsupported brass fired in a rifle with excessive headspace can rupture, causing gas to blow rearward like rocket exhaust, which can be hazardous. To ensure safety, Forster Headspace Gages are made with precision and rigorous manufacturing standards, providing accurate testing of your rifle's chamber. If a rifle closes on a GO gage, it indicates that the rifle chamber will accept any ammunition made to SAAMI (industry standard) specifications, ensuring safety and reliability for the shooter.