Tools That Confirm a Critical Dimension
Headspace is the distance from the face of the closed bolt to the chamber feature that stops the cartridge's forward travel. Headspace gauges are precision reference tools that confirm this dimension falls within safe limits. Their names describe the specific checkpoints they represent along that dimensional range.
What Headspace Actually Measures
Headspace is measured to whatever surface arrests the case: the shoulder datum on a rimless bottleneck case, the rim on a rimmed case, the belt on a belted case, or the case mouth on a straight rimless case. Correct headspace holds the cartridge with just enough support to fire safely without excess space. Too much space lets the case stretch on firing, which is dangerous.
Go and No-Go
A go gauge represents the minimum acceptable headspace. A properly headspaced action should close fully on the go gauge, confirming the chamber is at least long enough. A no-go gauge represents the maximum acceptable headspace, and the action should not close on it; if it does, the chamber is longer than the intended range.
The Field Gauge
A field gauge represents a slightly larger service limit than the no-go, used to judge whether a well-used chamber is still within safe operating bounds. An action that closes on a no-go but not a field gauge may be at the edge of specification yet still serviceable. Closing on a field gauge signals that headspace has grown beyond a safe limit.
Datum Reference
Because a bottleneck case has no single flat stopping surface, gauges reference a datum, a defined diameter on the shoulder used as the measuring point. Standardizing this datum lets gauges from different sources agree with one another. These terms and dimensions come directly from published cartridge and chamber standards.
Worked Example
Suppose a cartridge standard sets the go dimension at a defined datum-to-boltface length and the no-go at 0.13 mm longer. A gunsmith closes the bolt on the go gauge and it seats fully, confirming a safe minimum. Swapping to the no-go gauge, the bolt will not close the last bit, confirming headspace sits between the two limits, exactly where it should be for safe, reliable function.
A Common Misconception
Some assume the action should close smoothly on the no-go gauge, treating it like a routine pass. The opposite is true: closing on a no-go means headspace exceeds the maximum intended limit. The action should close on the go gauge and refuse the no-go; only that combination confirms the dimension is in range.