Forster .257 Roberts SAAMI Dimensioned Field Length Rimless Headspace Gauge

Every gunsmith understands that the proper use of a headspace gage is the most reliable method for testing the length of a rifle chamber. "Headspace" is the critical 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 negatively impact accuracy. For example, unsupported brass fired in a rifle with excessive headspace can rupture, causing gas to blow rearward with dangerous force, much like rocket exhaust.