Headspace is one of the most critical measures in your rifle. A quick definition: the distance from the face of the locked bolt to a datum line or shoulder in the chamber that arrests the forward movement of the cartridge. The term originated when all cartridges had protruding rims, so the measure was initially taken only at the head. Now it includes other spans.
Headspace is measured from the bolt-face to the mouth of a straight rimless hull like the .45 ACP, whose mouth stops against a small, abrupt shoulder at the front of the chamber. In a belted magnum, the stop is the leading edge of the belt, in the back of the chamber. On a .30-30 case it’s the front of the rim. The datum line for rimless or rebated bottleneck rounds like the .270 and .284 lies on the shoulder. Semi-rimmed cartridges theoretically headspace on the rim, but sometimes (as with the .38 Super Automatic) the rim protrusion is insufficient for sure function. The case mouth then serves as a secondary stop. The semi-rimmed .220 Swift has a more substantial lip; but most handloaders prefer to neck-size the Swift, so after a first firing, the case actually headspaces on its shoulder.
If there’s too little headspace, the bolt won’t close on a chambered round. Too much headspace can shorten case life, even cause case ruptures and dangerous gas escape.
When you pull the trigger, many events follow. The blow to the primer crushes shock-sensitive priming mix, which detonates. The explosion shoots flame through the flash-hole in the primer pocket, igniting the gunpowder. The resulting gas expands rapidly, stretching the ductile brass case. The thin case wall up front is ironed against the chamber wall, but the solid rear section around the primer pocket stays close to its original diameter, slightly smaller than the chamber. Gas pressure shoves it back against the bolt face. Still expanding, the gas thrusts the bullet out of the case.
Because cartridges vary slightly in dimensions, and each must chamber easily, the chamber must be a tad bigger than the average case. If there’s too much distance between the bolt face and the point in the chamber that stops the forward motion of a cartridge, however, you have excess headspace.
Until the thick case head moves rearward to meet the bolt face, the bolt face isn’t supporting it. The striker has pushed the case to its forward stop. Excess headspace causes excessive stretching. After repeated firings, the “work hardened” case becomes brittle as well as thinner just ahead of the web. It no longer stretches easily and can crack at the web, or even separate.
A cracked case is dangerous because it spills powder gas into the chamber. That gas jets through the tiniest corridors at velocities that can exceed bullet speed. It may scoot along the bolt race, through the striker hole, into the magazine well. It can find your eye faster than you can blink.
Gunsmiths measure headspace with “go” and “no go” gauges. The “go” gauge is typically .004 to .006 shorter than the “no go” gauge for rimless and belted cartridges. The bolt should close on a “go” gauge but not on a “no go” gauge. Theoretically, if the bolt closes on a “no go” gauge, the barrel should be set back a thread and rechambered to achieve proper headspace. However, many chambers that accept “no go” gauges are still safe to shoot. The “field” gauge, seldom seen now, has been used to check these (mostly military) chambers. It’s roughly .002 longer than a “no go” gauge.
Minimum and maximum headspace measurements are not the same as corresponding minimum and maximum case dimensions. For example, a .30-06 chamber should measure between 1.940 and 1.946, bolt face to shoulder datum line. A .30-06 cartridge usually falls between 1.934 and 1.940. Case gauges perform the same check on cartridges that headspace gauges do in chambers.
An obvious difference: case gauges are female and don’t indicate headspace. They simply show whether a cartridge will chamber in a rifle that’s correctly barreled. Headspace is a steel-to-steel measure. Altering case dimensions changes the relationship of the cartridge to the chamber. Reducing head-to-datum line length of the round can result in a condition of excess headspace, even if the firearm checks out perfectly.