When possible, Manson Reamers Headspace Gauges are made to SAAMI specs. If only CIP lists a specific caliber, reamers and Headspace Gauges are made to those specs. Calibers not listed by SAAMI or CIP are made to the designer's or most common specifications. Headspace is the dimension between the surfaces within the chamber of a firearm that hold the cartridge when the action is closed. If the headspace dimension is too small, a cartridge won't chamber. If it's too large, the cartridge will not be held securely during firing- backed-out primers and separated cases can be the result. Headspace Gauges are used to determine if a firearm's headspace is within acceptable limits.Headspace gauges are fit into the chamber of a gun like a cartridge would be. For the headspace of a firearm to be within spec., its action must close on the shortest gauge("GO"), and not close on the longest gauge ("NO GO"). Notes:"GO" denotes the minimum headspace dimension on all headspace gauges."NO GO" denotes...