Necessary when finish chambering a barrel or when checking the chamber of a used gun to be sure the headspace dimensions are within safe limits. This is a No Go Gauge only, and both a Go Gauge and a No Go Gauge are required to properly headspace a barrel. These No Go Gauges represent the upper limit of SAAMI specifications for proper headspace dimensions. If the action closes with this gauge inserted, then the firearm may not be within acceptable maximum headspace dimensions and should be examined further by a knowledgeable gunsmith.
- Duplicates the distance over the maximum allowable, from the case head (or boltface) to the datum line of the cartridge's shoulder, allowed within the chamber to secure a loaded cartridge and allow the breech to be closed in a safe manner for firing.
- If the firearm locks up in battery when the gauge is inserted, then it is not within acceptable maximum headspace dimensions. The firearm should not be used and should be examined further by a knowledgeable gunsmith.