Due to the interest in, and abundance of military surplus rifles and older commercial rifles on gun racks across the country, headspace is a topic which should be understood by anyone planning on shooting one of these rifles. Headspace is the distance between the face of a closed bolt and the point on the chamber that stops the cartridge from moving forward. For a rimmed cartridge, headspace is essentially the thickness of the rim. It's a little greater in the case of a belted round, where the measure is taken from the leading edge of the belt. Rimless cases like the 30-06 headspace on a "datum line" about halfway up the shoulder. "Go" and "No Go" headspace guages are used in rifles to check this critical dimension. These guages are readily available through most reloading supply houses, and they are shaped like part of a cartridge. The bolt should close on a "Go" guage, but not on a "No Go" guage, if the chamber was cut within acceptable SAAMI tolerances. Chamber dimensions can vary a bit from one rifle to another, even in rifles of the same manufacturer, which can give a cartridge a little more end play in one than another. Too much end play means trouble. Here's why:
When you squeeze the trigger and the sear releases the striker onto the primer, the cartridge moves forward under the blow. It won't move much if the case fits the chamber properly. As the powder ignites and gas pressure builds inside the case, that pressure expands the front of the hull (where the brass is thinnest) to the sides of the chamber. The thick-walled rear of the case expands to a lesser degree, and consequently is forced back against the bolt, stretching the case. If there's too much headspace, that stretch becomes more than the brass can manage, and the case may crack or even separate just in front of the web, forward of the extractor groove. By neck-sizing only, stretch is minimized after the first firing due to pressure -forming the case to the larger chamber dimensions. That's okay. Full-length resizsing on the other hand, works the brass a lot if there's too much headspace, thereby making premature case failure a certainty.
Because excess headspace can cause case ruptures, it is dangerous. Sometimes, however, shooters don't immediately recognize the problem for what it is. A case with a severely flattened primer may be misinterpretted as a case which was loaded "too hot". A flattened primer accompanied by a bright ring forward of the web, however, is not an indication of a hot load so much as excessive headspace. When the striker falls the case is powered forward, and gas pressure blows the primer to the rear a millisecond before the case head moves. When the base of the hull kicks back, it smashes the expanded primer flat. Gas jets out of the hairline crack forward of the web
and shoots back through the bolt. Severe damage to the eye's and face are a good possibility, which is why wearing shooting glasses is extremely important. Anyone suspecting their rifle has too much headspace should have the rifle checked, rechambered, or rebarreled.