Headspace is the measurement between two points in a rifle's chamber. These points are the boundaries of cartridge movement when a cartridge is chambered. In strict SAAMI terms headspace is the distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.

Why is headspace important with a mil-surp firearm?

To start with, most curio & relic mil-surp firearms are at least fifty years old. Most have had thousands of rounds fired through them (if not tens of thousands of rounds) and tolerances have most likely deviated from the original manufacturing specifications. These deviations can include but are not limited to: throat erosion, worn rifling, and headspace issues. 

In this article we are going to focus on: excessive headspace, insufficient headspace, and measuring headspace with commercial off-the-shelf headspace gauges.

Headspace is measured differently depending on what  firearm you are shooting. Most (but not all) twentieth century mil-surp rifles chamber and fire one of two types of metallic centerfire cartridges: rimmed cartridges and rimless cartridges. The table below shows common military calibers and their associated rifles categorized by rimless or rimmed cartridge.

Rimmed Case

For a rifle that fires a rimmed cartridge the headspace measurement is taken from the face of the bolt to face of the chamber or the maximum allowed space between the bolt face and the face of the chamber.

Drawing shows how headspace is measured in a rifle that shoots rimmed cartridges. The measurement is the distance between the face of the bolt and the top of the rim (face of the chamber) when the bolt is closed.

 

Rimless Case and Approximate Representation of Datum Line

For a rifle that fires a rimless cartridge this measurement is taken from the face of the bolt to the point on the cartridge's first shoulder that has been determined to be the datum line. The datum line specification for each caliber of cartridge is determined by SAAMI.

 

Drawing shows how headspace is measured in a rifle that shoots rimless cartridges. The measurement is the distance between the face of the bolt and about the mid point of the cartridge's shoulder (or the point where the front of the cartridge rests on its shoulder - "headspace on shoulder") when the bolt is closed.

Figure Legend: Rimless Cartridges

Part Part Name A Bolt Face B Middle of Shoulder (Datum Line) C Receiver D Bolt E Cartridge

 

Headspace Measurement Standards

There are two different standards for measuring headspace: Military and SAAMI specifications. Military specifications are more lenient in tolerances than the SAAMI specifications. Military firearms are purposely designed to have very lenient specifications so a variance in ammo supplies and adverse weather conditions would not cause the weapon to malfunction. Also military brass is thicker and can tolerate more stretching than a commercial case and because of this they can tolerate excessive headspace better than commercial ammunition, greatly reducing the chance for cases rupturing.

Because most all reloading components, commercial ammunition, and headspace gauges available today are based upon SAAMI standards we will focus on those standards of measurements in this article. The only downside to this is that firearms manufactured for the military are based upon the military specifications. This means that it is possible to deem a firearm unsafe to shoot using SAAMI specification gauges while the firearm may actually still be in military tolerances and somewhat safe to shoot. Still it is a very safe approach to use SAAMI gauges and accept the test outcomes because you would be much less likely to experience unsafe conditions while shooting your firearm.

The great thing about commercially available headspace gauges is that you don't have to know the prescribed distances and tolerances in measuring the headspace of your firearm. You can purchase a set of gauges specific to the caliber and cartridge you are wishing to measure for in your firearm. The gauges are manufactured exact to the SAAMI specifications. These gauges are very accurate and well made with heat treated steel that is actually harder than any of the firearm's metal parts.
 

Excessive Headspace

Excessive headspace allows movement of the case during firing. This can cause case stretching, case separation (ruptured case), and gas leakage. When the powder is ignited the base of the cartridge can move back while the sides of the case stick to the walls of the chamber. As a result the case can separate and rupture. If the bolt and receiver are not strong enough to contain and vent the blast you can at the very least damage the firearm or at worst you can injure or cause even greater harm to a bystander or yourself. Some mil-surp firearms are designed to handle a problem like case rupture. The ported holes on the side of Mauser bolts are an example of a design to vent off gases that may be inadvertently sent through the bolt to the rear of the firearm. If designs like this do not exist in the firearm you are shooting, then you could be in trouble.

The wayward pressure and gas has got to go somewhere!

Each Dave Manson gauge is very well made and labeled as to which gauge and caliber they are intended to be used to measure. They offer sets in almost all military calibers and would make a fine addition to your gunsmithing tools that would last well past your life time.

A "Go" gauge is the gauge that is used to measure minimum headspace. When you place the "Go" gauge in the rifle and close the bolt, the bolt should close and lock easily and completely.

If the bolt does close successfully then this tells you that the firearm has at least sufficient headspace. It may still have excessive headspace, but that is determined by the "No-Go" gauge.

If the bolt will not close on the "Go" gauge then you may not have sufficient headspace. This means you do not have enough headspace to chamber a cartridge properly and can damage the rifle if you force the bolt to close.

There are two potential fixes for this problem (at least there is on a Mosin-Nagant rifle):

1. Clean the face of the chamber. There may be unseen gunk and dirt at the face of the chamber. I have owned many Mosin-Nagant rifle and carbines that at first were very difficult to chamber a cartridge. Only later did I discover (with the help of a flashlight) that there was gunk and dirt fused to the face of the chamber. Gunk and dirt that is so dark and hardened that at a casual glance, and without the aid of a very direct and bright light, it can be easily overlooked. After it is successfully cleaned and/or removed then it is quite possible you have eliminated the headspace problem and the "Go" gauge check should be repeated. If the same results occur then.....go to the second fix (below).
2. Take it to a gunsmith. You need to have the rifle looked at by a gunsmith and the rifle will need to be modified or adjusted (possibly a chamber reaming, replace bolt parts, or moving the chamber face of the barrel closer to the bolt.) in some way to compensate for the lack of headspace.

A "No-Go" gauge is the gauge that is used to measure the maximum allowed headspace. When you place the "No-Go" gauge in the rifle and close the bolt, the bolt should not close and lock. If the rifle does close on the "No-Go" gauge you may have a problem that needs to be tended to by a gunsmith and may be an unsafe weapon to fire.

If the rifle fails the "No-Go" gauge then you should measure using a "Field" gauge. A "Field" gauge is used to measure the largest possible safe headspace dimensions. When you place the "Field" gauge in the rifle and close the bolt, the bolt absolutely should not close and lock. If the rifle does close on the "Field" gauge you have a problem that needs to be tended to by a gunsmith and an unsafe weapon to fire.

If the bolt closes on a "No-Go" gauge but does not close on the "Field" gauge this means you have a rifle that has excessive headspace but is safe to shoot. Basically it will stretch the heck out of brass and good luck trying to reload.

Each Yankee Engineers disc has lines cut into the extractor notch representing its function:

• One line: "Go" gauge;
• Two lines: "No-Go" gauge;
• Three lines: 'Field" gauge.

To measure headspace with the discs, make sure the bolt face and chamber face are clean and then drop the discs in one at a time. The bolt should close on the "Go" gauge, should not close on the "No-Go" gauge, and absolutely should never close on the "Field" gauge.

I found the discs very quick and easy to use. They would be a good field set to take with you to stores or gun shows when purchasing rifles. This would give you the ability to check headspace before the purchase thus saving yourself form headaches and heartaches only to be discovered once you get home.

Forster gauges are very well known and very well made gauges. Each is manufactured from hardened steel and well labeled as to their caliber and use.

jlm;)

WARNING - Failure to follow this procedure may result in accidental discharge, firearm damage and serious bodily injury. Always be certain that your firearm is functioning properly before use.

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