Every gunsmith understands 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 dangerous and negatively impact accuracy. For example, unsupported brass fired in a rifle with excessive headspace may rupture, causing gas to blow rearward like rocket exhaust. Because your safety is at stake, Forster Headspace Gages are crafted with meticulous care and precise manufacturing standards to ensure accurate testing of your rifle's chamber. If a rifle closes on a GO gage, it confirms that the chamber will accept any ammunition made within SAAMI (industry standard) specifications, providing assurance of safety and reliability.