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Headspace: Why It's Important & How To Check It

By The Brownells Editorial Staff
Headspace gauges help you make sure your gun's chamber dimensions are safe.
Headspace gauges help you make sure your gun's chamber dimensions are safe.
Headspace is one of the most critical dimensions on your firearm. It is the distance from the bolt or breech face to a point on the chamber; the exact location of that point varies from one type of cartridge to another.

For a rimless bottleneck rifle cartridge, that point is where the shoulder of the case rests against the shoulder of the chamber. For semi-auto pistol cartridges, it's a ridge at the front of the chamber that the mouth of the case rests on. Rimmed cartridges like .38 Special and .45-70 are retained by the rim resting on the back of the chamber, just as belted magnum rifle cartridges "headspace" with the belt resting on a shoulder in the chamber.
This Wilson Combat Classic Supergrade's magazine has an extended polymer base pad
that flows smoothly into the lines of the grip frame.
Different types of cartridges headspace on different parts of the chamber. (Illustration courtesy of Forster Products.)
When headspace is set correctly, there is a tiny bit of extra length in the chamber to accommodate the slight variations in cartridge length that occur in even the most rigorously manufactured commercial ammunition.

But why is headspace so important? Let's take a look at what can happen inside the chamber when you fire a round.

THE PROBLEM(S)

When the primer ignites the powder in the cartridge, enormous pressure builds up rapidly and makes the case expand against the chamber walls to create a seal. This keeps the hot gases from escaping, except by pushing the bullet out of the case mouth and down the bore. At the same time, the base of the case also expands backward and presses against the bolt or the breechface.
Igniting a cartridge releases enormous pressure in the chamber that pushes the
bullet down the bore.
Igniting a cartridge releases enormous pressure in the chamber that pushes the
bullet down the bore.
Excessive headspace can lead to bulged cases or even outright failures - cracks, case head separation, and splitting of the case neck. It can also cause light primer strikes, failure to fire, primers popping out of the primer pocket, and of concern to reloaders, shortened case life. A catastrophic case failure can allow hot gas and even bits of brass to escape from the receiver and injure the shooter or bystanders.
Igniting a cartridge releases enormous pressure in the chamber that pushes the
bullet down the bore.
Excessive headspace can lead to split case necks...
Igniting a cartridge releases enormous pressure in the chamber that pushes the
bullet down the bore.
...or separation of the case head. Neither is good.
Insufficient or excessively tight headspace can be just as dangerous. It can prevent the gun from going into battery, leading to a failure to fire or damage to the case caused by attempting to force the breech closed. Forcing the breech closed can jam the case neck too far forward, causing it to grasp the bullet too tightly, which delays the release of the bullet and leads to excessively high chamber pressures.

You can see the critical importance of correct headspace. What you don't know can hurt you. But how do you know if your gun's headspace is correct?

THE SOLUTION: HEADSPACE GAUGES

The good news is headspace is one of the easiest dimensions to check. This is typically done with headspace gauges. Any professional gunsmith will be equipped to check headspace for you, usually for a modest fee. Gauges are not expensive, compared to the cost of a gun. If you own more than a few guns, you may want to get your own set of gauges.

Headspace gauges are usually cartridge specific, though cartridges that use the same parent case, like .30-06 and .270 Winchester, use the same gauge. You'll need a GO and a NO-GO gauge for each cartridge. In some situations, you'll need a FIELD gauge as well.
The Freed Designs Grip Extender (#100-003-163) enables use of full-size 8-round Government Model magazines in an Officers model pistol.
Each cartridge requires a different GO/NO-GO gauge pair.
Gauges are precision machined from hardened steel to very exacting tolerances. But be careful not to drop one on a hard surface because any damage or deformity will render a gauge useless.

You should always use GO and NO-GO gauges from the same manufacturer. Here's what the Brownells Big Book says about mixing brands of headspace gauges: "DON'T DO IT!"

"That's the short answer; here's why. It boils down to tolerance stacking. Each headspace gauge manufacturer works within a range specified by SAAMI. Manufacturer A may work at the high end of the range while Manufacturer B's gauges are in the middle of the tolerance range. Mixing the two could give an inaccurate reading. By sticking with one brand within a particular caliber, you will eliminate a variable. If you have a Forster .308 GO gauge, get a Forster .308 NO-GO. Use Clymer with Clymer and Manson Precision with Manson Precision, etc."

Brownells offers a complete array of headspace gauges covering just about any firearm chambered for a commercially loaded cartridge. Gauges cost around $30 apiece. If, for example, the .308 Winchester is your favorite cartridge and you buy, sell, trade, and work on a lot of rifles chambered in it, a pair gauges quickly pay for themselves.

WHEN TO CHECK HEADSPACE

Headspace should be checked after performing any action work such as chambering a barrel, changing a barrel, or changing the bolt. You should also check it on any used gun you purchase at a gun show, a store, or from a friend - before you fire it.

In most cases, a brand-new rifle will have been checked for headspace, and maybe even test fired, before it leaves the factory. Even so, mistakes sometimes do happen at even the best-run factories, so it really doesn't hurt to check the headspace on a new gun, right out of the box.

HOW IT'S DONE

Let's use a typical bolt action rifle - such as a Remington 700, Winchester 70, Mauser, Mosin Nagant, or Ruger® American - as our example. Start by thoroughly cleaning the chamber, locking lugs, and bolt with your favorite bore solvent. Make sure everything is dry - but DON'T OIL IT!
This Springfield Armory .45 customized by John Harrison of Harrison Custom Design sports a mag with an extended bumper that’s ideal for a competition gun.
Thoroughly clean the chamber, locking lugs, and bolt before you check headspace..
Remove the ejector and/or the extractor from bolt. Refer to your gun's owner's manual or one of the many available assembly/disassembly guides like this one or this one or others.
The Freed Designs Grip Extender (#100-003-163) enables use of full-size 8-round Government Model magazines in an Officers model pistol.
Remove the ejector and/or extractor from the bolt.
Insert the GO gauge into the chamber and try to close the bolt. Use light pressure but don't force it. You don't want to damage the bolt or the gauge. The bolt handle should close normally. If it won't, you most likely don't have enough headspace. The gun should be taken to a qualified gunsmith for further checking and possible repair.
This Springfield Armory .45 customized by John Harrison of Harrison Custom Design sports a mag with an extended bumper that’s ideal for a competition gun.
Bolt closes on GO gauge - that's good.
Next, insert the NO-GO gauge and try to close the bolt. It should not close. If it doesn't, you're good to go to the range. If it does, the gun probably has excessive headspace and should be inspected by a gunsmith.
1911 magazines come in a huge variety of finishes, capacities, and styles, such as these from (left to right) Brownells (#078-000-027), Ed Brown (#087-000-051), and Chip McCormick (#207-000-002).
Bolt will not close on NO-GO gauge - that's good too.
The "magic formula" is actually very simple: Closes on GO + won't close on NO-GO = Safe To Shoot

Anything else may be a problem, and you should take the gun to a gunsmith. Otherwise, you're ready to head to the range!

A NOTE ON OLDER MILITARY RIFLES

An old military surplus rifle like a Mauser, Mosin Nagant, '03 Springfield, Enfield, Arisaka, and others may close on a NO-GO gauge. If it does, check it with a FIELD gauge. A FIELD gauge measures the absolute maximum allowable, safe headspace. If the bolt does not close on a FIELD gauge, and the gun is in good condition, it is generally safe to shoot.

However, the strength of these old guns can vary a lot depending on when and where they were manufactured. Headspace is not the only factor in deciding whether one of these guns - or any gun for that matter - is safe to shoot. When in doubt, take it to a qualified gunsmith to have it checked out.
Checking headspace on old military rifles like this well-used Mauser may require the use
of a FIELD gauge.
Checking headspace on old military rifles like this well-used Mauser may require the use
of a FIELD gauge.
Editor's Note: For a "hands-on" demonstration of the concepts discussed in this article, check out these Brownells Tech Tip videos, "What is Headspace" and "Bolt Action Headspace".