An important concept called “headspace” is often overlooked and unknown to many shooters who don’t realize how much they don’t know. I never realized how few shooters have even heard of, or been concerned about, headspace. That fact was pressed home when I was at the range one day and another shooter was having all sorts of issues with a new AR-type rifle.
The rifle would not fire more than one round. When it did fire, it would not cycle and would not go into battery. Knowing I am an instructor, he asked me my opinion. I said, he should return it to the manufacturer. He then said something that should not have surprised me. “I made it,” he declared.
I asked, “What do you mean, I made it? Did you manufacture the parts?” He clarified that he bought the parts and assembled them. In my naïveté, I asked who’s upper he used. He again proudly stated, “I put that together from parts also.” The first thing that leaped into my mind was, of course, the headspace.
I asked if the headspace was set correctly. That’s when the conversation took an interesting turn. He had the look of a deer in the headlights, a kid caught with his hand in the cookie jar, or your best friend caught in bed with your wife… HUH? “What is headspace?” he uttered feebly. At that moment, I told him to clear the weapon, put it in the case, and leave the range. He was not to return until the headspace was set properly.
How could someone be so ignorant as to think they could assemble a firearm and not even have heard of the term, headspace? Now that’s off my chest. I got some splanin to do, Lucy!
What Is Headspace?
Simply put, headspace is defined as the distance between the face of the bolt and a point in the chamber that prevents further forward movement of a cartridge. As an example, in the accompanying illustrations, we have cutaway views of a .22 rimfire, bottlenecked rifle cartridge, .45 ACP, and a belted magnum cartridge shown how they should appear in a chamber. On the .45, notice that the cartridge case is a bit larger than the bullet by the thickness of the brass case, and the chamber is shaped so that it fits the case correctly. Of course, depending on the shape of the cartridge, the headspace could be different for different firearms as seen in the illustrations.
Most of the earliest metallic cartridges were rimmed. This meant, they had a rim at the base of the cartridge that was larger (refer to the illustration of the .22 LR) than the diameter of the cartridge case. When this type of cartridge is pushed into a firearm’s chamber, the rim positions the cartridge correctly and prevents it from going too far into the chamber.
Because of that design feature, a rimmed cartridge’s headspace is determined by the thickness of the rim. The most popular cartridge today, the .22 Long Rifle rimfire cartridge is an example of a rimmed cartridge. It should be noted that it is common today to find that calibers intended for use in revolvers have rims that establish their headspace.
That means that most modern cartridge designs are of the rimless type. i.e., the diameter of the rim is the same or smaller (rebated) than the diameter of the case body. Modern cartridges also have other ways to ensure proper seating of the bullet in the chamber. One example would be pistol cartridges where the case diameter is slightly larger than the bullet and the chamber has a shoulder that the case mouth rests against as in the illustration of the .45 ACP.
However, it must be mentioned that there are other rimless cartridge designs, such as bottlenecked cartridges (which are mostly used in rifles), the shape of the firearm’s chamber is correspondingly tapered to achieve proper headspace. That is because the headspace on a bottlenecked rimless cartridge is the distance from the bolt face to the tapered section or shoulder.
In the case of belted cartridges, a design introduced by the British firm of Holland & Holland on its proprietary calibers (such as the .300 Holland & Holland Magnum), where the chamber is shaped to seat the forward face of the belt.
When the .300 H&H Magnum was first being developed, it was not possible to provide proper headspace for a cartridge with a shallow shoulder design. Holland’s solution was to add a belt around the cartridge body. This is similar in function to the rim of a rimmed cartridge, but gave a long enough surface to allow the cartridges to fit side-by-side in a magazine without interfering with the feeding of those cartridges. This design feature was later used on other magnum rifle cartridges designed by others as well.
As previously mentioned, headspace is the distance between the bolt face and a point in the chamber that prevents further forward movement of a cartridge. Depending on the firearm and the shape of the cartridge, this distance can be different for different cartridge types, as the images show.
Now, let’s look at what happens when the headspace is not correctly set, as in the anecdote mentioned earlier. There are two possible scenarios we must look at. The first is excessive headspace. Excessive headspace is best described as the condition where there is extra space between the bolt face and the cartridge in the chamber.
When that condition exists, the firing pin hits the cartridge and it will move forward into the chamber before detonating. When the propellant does ignite, the walls of the cartridge expand due to pressure and firmly stick to the walls of the chamber, preventing rearward motion of the cartridge. When that happens the thicker base of the cartridge will move backward.
This happens because there is a gap between the cartridge and the bolt face, and this will cause the walls of the cartridge to stretch. If the stretching is too much, the walls of the cartridge could rupture, release hot gases into the action, and potentially spray brass fragments out from the action of the firearm. In turn, this could be hazardous to the shooter or anyone standing next to him.
The second situation is created when there is insufficient headspace in the chamber, or too little space exists between the bolt and the cartridge. In this condition, the back of the cartridge will stick out and the bolt will not be able to close fully on the loaded cartridge. The user will not be able to properly operate the firearm when this happens.
If the user were to force the bolt to close on the cartridge, the bullet will be pushed tightly into the case neck. If the firearm is fired, this will cause excessive pressure to build up inside the cartridge case, leading to hot gases coming out of the cartridge’s primer pocket — with similar results to excessive spacing. In a worst-case scenario, the excessive pressure could cause the action to rupture and cause damage to the gun and its user.
So, the next question to ask is how to determine whether a firearm has proper headspace? We can do that by using a set of headspace gauges. These are measuring instruments that are precisely machined to the SAAMI, CIP, or military standards for a specific cartridge or caliber. Typically, headspace instruments are made of heat-treated steel and are machined to tolerances below 0.001 inches or so. They are made in various calibers and sold at reasonable prices. Typically, there is a “Go” gauge, a “No-go” gauge, and for military specification rifles, a “Field” gauge.
The bolt on a firearm must be able to close with no resistance when a Go gauge is inserted into the chamber. This signifies that the firearm is able to meet the minimum length specification for that particular cartridge. If the bolt does not close with the Go gauge inserted, the firearm has insufficient headspace. Another possible cause could be a dirty chamber or bolt face. The accumulated dirt may be thick enough to prevent the bolt from closing on the gauge. However, if the firearm is clean and the bolt still does not close on the Go gauge, it must be taken to a competent gunsmith for adjustments.
If a firearm successfully closes on the Go gauge, at a minimum, the firearm has sufficient headspace. However, it may still have excessive headspace. That can be determined with No-go gauge.
A new (or overhauled) firearm must not be able to close on a No-go gauge. If the bolt closes successfully on a No-go gauge, this means the firearm has excessive headspace and there is a risk of cartridge cases rupturing inside the chamber. If the firearm is new or recently repaired, it should be returned to the manufacturer immediately.
A used firearm may be able to close on a No-go gauge, due to wear of the bolt and chamber surfaces. This means it should probably go to a gunsmith for repair soon — it may be possible to fire new factory ammunition in it until then, but reloaded ammunition is probably a bad idea. The firearm may malfunction on slightly out-of-spec cartridges. Here’s where the test with the third gauge comes in (especially for firearms built to military specifications).
The bolt of any firearm, whether old or new, should not be able to close on a Field gauge. A bolt that closes on No-go, but not on a Field gauge, may be considered close to being unsafe, but may work on new cartridges. Likely, it should be sent to a gunsmith to have the headspace reset. However, if the bolt closes on a Field gauge as well, then it is not safe to fire and should be sent for repairs immediately.
Some calibers have a fourth gauge, known as a Field II gauge, for which the bolt should never lock on. This type of gauge is only used by certain rifles. For example, Colt uses it to reject M-16 rifles. It must be noted that gauges are usually manufactured to either SAAMI, CIP, or military standards, and therefore may have different dimensions, even for the same caliber cartridge. Therefore, it may be possible that a rifle manufactured to NATO specifications, may lock on a No-go gauge built to SAAMI specifications, but correctly not lock on a No-go gauge built to NATO specifications.
This is because military weapons are generally designed to operate with wider tolerances and military ammunition cases are generally thicker than commercial ammunition. That means they can tolerate more stretching without rupturing. Therefore, a military firearm may fail the test using SAAMI gauges, but still be deemed safe to fire per the military specification gauges. However, if it passes using SAAMI gauges, it is very likely to work correctly.
These gauges are relatively inexpensive. If you fancy yourself a firearms enthusiast and gun crank, do yourself a favor and get a set of gauges and learn how to use them for peace of mind.