Headspace Gauge
Also: Go Gauge · No-Go Gauge
A set of precision-ground gauges used to confirm that a chamber's headspace falls within the safe minimum and maximum limits for a cartridge.
A headspace gauge is a machined steel dummy, cut to a precise reference length, that a smith chambers in a barreled action to check headspace. The bolt should close fully on a go gauge, which represents the minimum safe dimension, and should not close on a no-go gauge, which marks the maximum, confirming the chamber sits within tolerance.
These gauges are how a chamber cut with a chamber reamer is verified before a rifle is fired. Correct headspace ensures the case is properly supported between the bolt face and the chamber shoulder or rim, which is essential for safe, consistent ignition. A field gauge is sometimes added to flag a chamber that has stretched beyond service limits with use.