“What is headspace, and when / how do I check it?” is an often asked question posed to professional gunsmiths.

What . . .

First let’s talk about the “What is headspace, …” part of the question. The easy answer is that the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) defines headspace as “The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.” If you are looking at that definition while scratching your head and thinking, “that sounds nice” but you’re not really sure what it means, you are not alone.

So in the interest of ‘muddying the waters’ a little more let’s look at some specific examples of what is measured when checking headspace. In guns chambered for rimmed cartridges (e.g., .22 Long Rifle, .30-30 Winchester) the measurement is made from the closed breech face to the front of the rim cut. In guns chambered for rimless bottleneck cartridges (e.g., .223 Remington, .30-06 Winchester) the measurement is made from the closed breech face to a specified datum reference point (an unpublished point on the shoulder at a specific diameter).

As you may have been able to tell, the actual measurement of headspace means something a little different based on the cartridge case type that the gun is chambered for (e.g., rimmed, rimless straight-walled, rimless bottleneck, belted magnum, and so on). However, headspace is telling us the same thing regardless of chambering; how far is the distance from the closed breech face to the part of the chamber that stops the forward motion of the cartridge during chambering.

Keep in mind that while correct headspace is a very important component of a safe firearm, it only checks one aspect of a gun’s suitability for safe use. A gun can have correct headspace and still be unsafe to fire due to other issue(s).

Headspace is a little like Goldilocks from the children’s story. It cannot be too big, it cannot be too small, it must be just right.

Cartridges may not chamber, the action may not go into battery, the gun may slam fire, or there may be excess pressure (which can cause very serious damage to the gun and/or the shooter) if headspace is too small. If the headspace is too big the gun may fail to fire, misfire, be inaccurate, and the cartridge cases my excessively stretch, have a short life (for reloading), or rupture (including case head separation).

When . . .

Now that we have a little better understanding of what headspace is, let’s look at when we would need to check it.

When a gun is manufactured, re-barreled or re-chambered it would need to have its headspace checked to ensure that the chamber is meeting the breech face as intended. If the chamber was cut too deep it would result in a headspace that was too large. If the chamber was cut too shallow it would result in a headspace that was too small.

When repairing or modifying a gun in a manner that alters or replaces parts in the action it can change the way that certain types of guns lock up. As an example, look at a bolt action rifle; the distance the bolt face (breech face) is from the chamber is directly related to the placement and size of the locking lugs. Thus, anything that changes the size of the locking lugs could alter the distance between the bolt face is and the chamber.

Classic American Gunsmith recommends checking headspace before any significant gunsmithing work is done to modify a gun. If a fault is found, it allows the client to correct the headspace issue before making an investment in a gun that, in its current state, may not be suitable for the purpose intended. We also require that any job involving action work must be checked for headspace and test fired before the gun is returned to the client.

How . . .

SAFETY: Do not use this post as a how to guide on checking headspace. It was written to inform our clients about what headspace is and what generally happens when they have headspace checked. If you check the headspace of some types of actions incorrectly you can damage the gun. If you do not know what you are doing, take your gun to a competent, qualified, professional gunsmith.

Now that we know why we check headspace, let’s discuss how it is measured. Again, we will start with the easy answer. Most people measure headspace by taking their gun to a competent, qualified, professional gunsmith and letting them do it.

That is not to say that the serious gun enthusiast can’t check headspace; sure they can. However, the gauges cost on the order of $30 to $60 each (depending on manufacturer, caliber / gauge / bore) and there are two or three per set (depending on your situation). That coupled with the fact that most professional gunsmiths will check headspace on a gun for $35-$70 pushes most enthusiasts to the gunsmith based on pure economics.

There are three types of headspace gauges. They are GO, NO-GO, and FIELD. All gauges, that will be used to check headspace on a gun, should be made by the same manufacturer. Do not mix and match gauge manufacturers on the same gun.

The GO gauge measures the SAMMI minimum chamber length. The NO-GO gauge measures an agreed to (by the industry) length between The SAAMI minimum and maximum chamber length. The FIELD gauge measures the SAMMI maximum chamber length or slightly less.

I was taught in gunsmithing school that unless you are in a combat situation do not use FIELD gauges to check headspace. As stated earlier, cartridge case head separations and wall ruptures can occur in a gun that will close on a NO-GO gauge and FIELD gauges are larger then NO-GO gauges. I do not use FIELD gauges in my shop and I strongly recommend that you don’t either.

Like the measurement of headspace is dependent on the type of cartridge, the technique for checking headspace is dependent on the type of action or cartridge (in certain cases) (e.g., bolt action, rimfire, revolver, lever action, pump action) the gun has or is chambered for. This post will cover the general concepts of checking headspace. It is not a how-to guide for checking headspace

In general, to check headspace:

1. As with all things, you first must ensure that the gun is not loaded. When working on any gun there should never be any live ammunition in the gun or at the workbench.
2. Rid the action of parts that can interfere with the gauges that will not be used during the test (e.g., extractor, ejector, firing pin). In some gun types, we also remove certain springs while checking headspace. If parts are riveted in place we strip the action the best we can.
3. Clean the chamber, and any other part of the gun that will come in contact with the gauge.
4. Cycle the action several times getting a feel for how it closes when empty. Go slowly. Do not force anything. Understand how much force it takes to close. Measure or mark the position of breech faces that do not have a physical locking mechanism (e.g, blowback actions). If it is possible, only use one finger to operate the action.
5. Place the GO gauge in the chamber. Slowly close the action. Do not force it. Does it lock / does the breech face return to the same position? Does it feel the same? If the action is the same as when empty, remove the Go gauge and;
6. Place the NO-GO gauge in the chamber. Slowly close the action. Do not force it. Does it lock / does the breech face return to the same position? Does it feel the same? It should not. It should not lock up. The breech face should not return to the same measurement.  The NO-GO gauge should be holding the action open. If the gun closes on a NO-GO gauge it is not safe. Even if it does not close on a FIELD gauge, if a gun closes on a NO-GO gauge do not do anything with it (e.g., shoot it, sell it) until the headspace issue has been corrected.

Generally speaking, If the action closes on the GO gauge but does not close on the NO-GO gauge the gun has headspace that is within limits.

Now you know what is headspace is and generally how it is checked. When building, repairing, or modifying guns, always keep in mind that headspace is one of the very important tolerances that cannot be overlooked. I recommend that you make sure that whoever is checking the headspace on your guns is detail oriented, knowledgeable and careful. This is an area where a small error can lead to irreparable damage to a gun or worse, the injury or death of a shooter.

Footnotes / Sources:

The Sporting Arms and Ammunition Manufacturers’ Institute, SAAMI, is an association of the larger US manufacturers of guns, ammunition and components. At the request of the US Government, it was founded in 1926 and tasked with creating and publishing US firearms industry standards. For more information on SAAMI, their standards, etc, please visit their website at www.sammi.org ⇒.

Classic American Gunsmith uses Dave Manson ⇒ headspace gauges for all calibers / gauges / bores they are available for.