Headspace - The distance, or clearance, between the base of a chambered cartridge and the breech face (or bolt face) of a firearm. This is a critical dimension for all cartridge firearms, but particularly in high powered rifles. If there is too little headspace, the bolt will not close. If there is too much headspace the cartridge will not be properly supported in the chamber and the cartridge will expand upon firing and may rupture, blasting high-pressure gas into the action and possibly into the body of the shooter. Headspace should be .003" - .006" in a centerfire rifle. It can be checked with a set of "Go and No-Go" gauges specific to the calibre in question. (See below.) With a standard cartridge, the headspace is registered by the shoulder, with a belted cartridge, the headspace is registered by the forward edge of the belt.
Set of three headspace gauges for a belted magnum cartridge like the .375 H&H Magnum and .458 Winchester Magnum Set of three headspace gauges for the .30'06 cartridge and its derivatives .270, .25'06, etc
Headspace Gauge - Plugs of hardened steel, precisely machined in relation to the standard dimensional specifications of a given cartridge, normally in sets of three: "GO", "No-Go" and "Field". By loading these plug-gauges into the chamber in succession, one can check that the action should close on the "Go" gauge. It should not close on the "No-Go" gauge---but might were enough force to be used. And, it absolutely should not close on the "Field" gauge.
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