How To Measure Your Rifle's Headspace

To many shooters, headspace is a mysterious term, one to which many evils can be attributed. I've had shooters come into my shop expressing the belief that their rifles had too much, not enough, none at all and even the wrong kind--I'm still not sure what that guy was talking about. In simple terms, headspace is the dimension of the chamber of your rifle, the gap between the face of the bolt and the stopping surface for the cartridge. To be precise, it is the distance between the face of the bolt and the datum line, which is a circle of stated diameter, along the slope of the shoulder of the cartridge.

On a rimless cartridge such as the .308, the stopping surface is the shoulder. On a rimmed cartridge like the .30-30 or .303 British, the headspace is the gap for the rim. On a belted magnum, headspace is the gap for the belt. Since not every rifle or cartridge can be made to exact dimensions, with perhaps the best custom rifles being one exception, headspace is deemed to be correct if it is within a certain range.

The specs for a cartridge involve a minimum chamber/maximum cartridge drawing. Headspace for a cartridge is the minimum dimension of the chamber plus .006. If your rifle's chamber is not cut within this range, some extreme situations can be dangerous. If your chamber is below minimum, and thus too small, you can have problems with bolt function. The cartridge, being crammed into the short chamber, prestresses the locking lugs of the bolt. Once fired, the bolt handle may be hard to lift. Or, in a self-loading rifle, empties do not eject far, or at all. The extra force can wear the engagement surfaces of the locking lugs, or even wedge the action closed and locked.

A chamber that is over maximum may not show any signs of trouble at all. Unless you reload, you may never know it. A rifle with excess headspace can eat reloaded brass quickly. Upon firing, the expansion of the case blows the shoulder forward. When you resize and set the shoulder back, you set up the excess headspace condition again. The stretched brass will eventually crack and break. A prime example of this is the .303 British. It headspaces on the rim, and the shoulder is left with large amounts of space for reliable function under adverse conditions (I've seen rifles with more than .050 extra). I've never been able to get more than three firings from reloaded .303 brass before the case separates. You can have the same problem with belted magnums if the belt headspace is correct but shoulder space is excessive.

There is an easy way to measure the headspace of your rifle--even if just for peace of mind. You'll need headspace gauges, precision-ground cartridge-shaped measuring tools. There are three kinds: three-gauge sets, multi-gauge sets and micrometer gauges.

The three-gauge set consists of "Go," "No Go" and "Field" gauges. The Go gauge is the one which corresponds to minimum chamber dimensions, and your bolt should close on it. The No-Go gauge corresponds to maximum dimensions, and your bolt should not close on it. The Field gauge measures the largest safe dimensions, and any bolt that closes on it should be immediately tended to.

Multi-gauge sets are used by gunsmiths who work on super-accurate rifles for match or long-range shooting. A multi-gauge set will have a precision-ground gauge at each .001-inch interval along the headspace spread. Using such a set, a gunsmith can ream a chamber to an exact specification. The micrometer gauge is a chamber-shaped adjustable measuring tool not commonly seen outside of factories or arsenals.

To measure headspace with these gauges, first strip and clean your rifle. Chamber and bolt must be squeaky-clean. Then disassemble the bolt; if your rifle has a spring-loaded ejector, you must remove it. Luckily, they are usually held in place by a cross pin which can be drifted out. If the extractor can be removed easily, do so. The extractor on a Remington 742, for example, is staked in place and cannot be removed. But get the ejector out. On a bolt-action rifle, remove the cocking assembly. You do not want the reading to be muddled by the spring pressure of the striker.

Slide the Go gauge into the chamber, and gently close the bolt. On a bolt-action rifle you should be able to close the bolt handle without feeling any resistance. On a multi-lugged bolt, like the AR-15, you should be able to turn the bolt by hand without resistance against the locking lugs. If there is resistance, or the bolt handle stops before it is completely closed, headspace is under minimum dimensions and must be opened up. For that you'll need a finish chambering reamer, which is a subject to be covered in a future column.

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Next, remove the Go gauge and slide the No-Go gauge in place. Again, attempt to gently close the bolt. You should not be able to do so. Do not try to force the bolt--use only fingertip pressure. The bolt handle will usually stop halfway down. On an AR-15, you might be able to catch the leading edge of the locking lugs under the shoulders of the barrel extension. If you had not removed the ejector, you could not feel the resistance, as the force needed to overcome the ejector spring would be too great for the delicate feel needed.

If the bolt closes on a No-Go gauge, you'll need a Field gauge to check it further. If the bolt closes on the No-Go but not the Field, you simply have a rifle that is going to be hard on brass. Be careful that you don't set the shoulder back while resizing, and you can get many years of use out of both rifle and brass.

If the chamber swallows a Field gauge, you'll have to have a professional look at it. Most likely, you'll have to have the barrel set back and rechambered by someone with riflesmithing experience.

And the rimmed and belted magnum rifles? Dealing with the rim or belt and the shoulder at t

he same time is a reloading problem, involving expanding the neck and resizing it down to create a false shoulder, then fire-forming the brass to your chamber. On a .303, I wouldn't waste time. When I tried it, I got one extra loading out of my brass. On a .300 magnum, it is definitely worth the extra effort.

Caution

Using a cartridge in a rifle for which it is not chambered presents a real danger. For example, if you were to, say, inadvertently slip a .308 cartridge into a .30-06 rifle, the extractor might hold the rim, and then the case would be blown out to .30-06 length. This is extremely hard on the brass. If the cartridge slipped past the extractor and the firing pin still set it off, you could end up with a pierced primer and jets of gas raging through your rifle. I've actually seen this type of situation, and it's scary. Make sure you use the correct cartridge for your rifle.