Different type cartridges Head Space in different ways.
Rimmed cases space on the Rim (raised rim) at the base of the brass
Rimless cases space on the front rim of the case
Bottle neck cases space on the neck of the case
Belted cases space on the belt at the base of the case
Head spacing stops the case from going any further into the chamber and allows the bolt, or in the case of a break open action, the face of the action to contact the rear of the case with a standard space. This space prevents the case upon firing from backing out of the chamber and allowing the case to rupture or the primer from blowing out of the primer pocket.
Space Gauges are cartridge like in their design for each caliber and are inserted into the chamber. There are actually 3 gauges, also a Field gauge to carry with you.
The below table illustrates the function of each gauge:
GaugeMeasurementIf Bolt Closes on GaugeIf Bolt Does Not Close on Gauge
GoMinimum factory-spec spacing"Go". Headspace is greater than minimum spec."No-Go." Headspace is too short. Bolt will not close on factory-spec cartridges, resulting in jammed brass, and firing
out-of-battery, both dangerous conditions.
No-GoMaximum factory spec spacing"No-Go." Headspace is greater than the maximum factory spec. Risk of case ruptures if used."Go". Headspace is below maximum factory spec.
FieldAbsolute maximum safe spacing.Rifle is unsafe to fire. Headspace is greater than what is considered safe to fire. High risk of case rupture if fired.Generally "go". Headspace is below the maximum for what is considered safe to fire when using factory spec ammunition. Rifle should be evaluated by a gunsmith for possible re-headspacing.
Sets are availaible in popular sizes, also in single.