Rifle headspace is one of those topics that, in reality, is not nearly as mysterious as it seems. When customers buy a used firearm, especially a military surplus model, everyone’s advice is to check the headspace. If you’re not a gunsmith or avid reloader, the importance of headspace can seem like a daunting topic.

The concept of headspace and its ramifications is pretty simple. If we discuss what it implies and what it actually means separately, I think it’s a lot easier to digest.

While not the technical definition, you can think of headspace as the free space between the breech face and the cartridge base once it’s loaded in the chamber and the bolt or slide is closed. A cartridge can’t exactly match the length from the bolt face to the limiting forward portion of the chamber because there would be no wiggle room to allow for loading, unloading, or a bit of fouling. Also, cartridge cases expand in two dimensions when fired. The circumference increases and the case temporarily presses against the chamber walls. The case also stretches just a hair lengthwise. So, there always has to be a little bit of play between the breech face and the cartridge base. That’s the dimension that headspace impacts.

Now let’s talk about what headspace technically is and how it’s measured. To do that, we first need to understand that different cartridge designs fit into chambers in different ways. When I say fit I’m talking about the locked and loaded positioning. Something has to hold the cartridge in place before it’s fired to keep it in exactly the right position. Without that, the firing pin might simply push the cartridge forward without making a big enough dent in the primer to cause ignition. Or worse, the cartridge might just flop around and stay out of reach of the firing pin altogether.

There are three and a half approaches to locking a cartridge in the proper place in the chamber. To define each type, we’re going to use headspace as a verb and refer to how a cartridge “headspaces.” In other words, different cartridge and chamber designs use different brass case and chamber features to lock the cartridge in place, leaving just the right amount of space between the cartridge base and breech face.

Before we get into the different headspace measurement types, let’s clarify some supporting terminology. The breech face is the flat part of the bolt, slide or frame of a gun where the firing pin hole is located. With most common semi-automatic pistols, the breech face is part of the slide. For most rifles, it’s the bolt face. For revolvers, the breech face is part of the frame behind the cylinder.

Let’s take a quick look at each.

Straight-wall Cartridges

Pistol cartridges like 9mm, .40 S&W and .45 ACP just to name a few, headspace (or lock in place) based on the mouth of the cartridge case pressing against a ridge inside the chamber. Since these cartridges use bullets that fit entirely inside of the case, there’s a thin ledge of brass that circles the projectile at the mouth. This is what jams against the ledge inside the chamber and determines the depth at which the cartridge seats.

Rimmed Cartridges

Many revolver cartridges like .38 Special, .357 Magnum, .44 Special and Magnum, headspace using the rim around the base of the cartridge. While straight-wall cartridges are flush between the case body and rim, many revolver cartridges have a rim that’s wider than the case body. This rim butts up against the base of the chamber and stops forward motion of the case when it’s loaded in the cylinder or chamber. There are even some semi-automatic cartridges that use this approach like .32 ACP and .22LR. To be clear, the inside of the chamber is smooth as there is no need for a ledge when using rimmed cartridges.

Shouldered Cartridges

Many rifle cartridges and some pistol cartridges have a tapered case design where the case body has a shoulder that transitions between a wider body base and narrower mouth to fit a smaller caliber projectile. Think of .223 Remington, .30-06, .270 and many others that are similarly shaped. As for pistols, one shining example is the .357 Sig, although gunnies will argue eternally about whether those headspace on the case mouth or case shoulder.

Cartridges with a shoulder usually headspace on the shoulder itself. The inside of the chamber mirrors the cartridge shape and the chamber’s inverse shoulder is what stops the cartridge from moving forward.

Belted Cartridges

Example three and a half refers to belted cartridges like the .375 H&H Magnum and .458 Lott. These cartridges feature a “belt” near the base of the case but forward of the cartridge rim. The built-in belt in the brass isn’t for strength but rather headspacing. It’s the belt that catches on the chamber to precisely position the cartridge. The beauty of this model is that belted cartridges can feed from magazines well and can be shouldered or straight wall in shape.

Measuring Headspace

Now that we know the different types of headspacing, it’s easier to understand the different measurements. The points from which the measurements are taken depends on the cartridge type. For straight-wall cartridges, the measurement is from the cartridge case mouth to the breach face. For rimmed cartridges, it’s from the leading edge of the rim to the breech face. For shouldered cartridges, it’s from the catching edge of the shoulder to the breech face. Belted cartridges use the leading edge of the belted surface.

Headspace Consequences

If there is too much headspace a couple of things can happen. The case may stretch more than it’s supposed to, and that can lead to disastrous consequences. An over-stretched case may rupture or suffer a case head separation, allowing high-pressure burning-hot gas and case fragments to escape. Or, you might experience light primer strikes or misfires as the primer is not held close enough to the firing pin. On the other hand, too little headspace can prevent the bolt from closing or the gun going into battery properly.

So, to avoid potential gotchas, how do you check headspace to get it right? Simple tools called GO and NOGO gauges can give you a quick indication of whether headspace is within the specified range.

Go Gauges

A GO gauge tells you if the chamber has more than the minimum specified amount of headspace. After cleaning the chamber and bolt thoroughly, remove the ejector and extractor as you want to feel how much pressure is required to close the bolt on the GO gauge. The spring tension of those parts will add artificial pressure to the bolt closing force. The bolt should close fully with minimal effort. If the bolt won’t close or if it requires too much work to close, then headspace is at or below the minimum specification.

NOGO Gauges

The NOGO gauge is deliberately made too long to fit into a chamber with proper headspace. In this case, when you insert the gauge and attempt to close the bolt, it should not close. If it does, then there is too much headspace and you might be running the risk of light primer strikes, misfires, ruptures or case head separations.

There are a couple exceptions in the AR rifle world where a bolt closing on a NOGO gauge may be OK. Some Colt rifles are made with slightly long headspace, so if the NOGO fails, you also might need to verify the chamber with a maximum or field headspace gauge. Just something to consider.

Causes of Change

While new rifles are checked at the factory for proper headspace, used rifles have an unknown history and might have been exposed to many, many firing cycles. If you sell any military surplus guns, it’s always a good idea to verify that headspace is within spec. If you change bolts or barrels, that’s also a must-check occasion to break out the GO/NOGO gauges.

Especially if you sell used rifles and don’t have a gunsmith on staff, it might make sense to invest in a few sets of GO and NOGO gauges for common calibers. While those gauges won’t help you correct any underlying problems, they will allow you to communicate the high-level status of a used rifle to a potential customer. You can tell them that headspace appears to be in spec based on a quick evaluation with GO and NOGO gauges. Then it’s up to them to determine whether they want a gunsmith to perform a more in-depth assessment.

All Photos: Tom McHale