You will not get far into the ammunition or firearms world without hearing the term “Headspace”. I believe it is likely one of the most used terms that is understood the least by the firearm community as a whole. So what is it and why does it seem to matter so much?
Editor’s Note: I apologize this is a longer article so if you are on the porcelain throne reading this, you may be there for awhile 🙂
What is it?
Simply put headspace is the measurement from the breech face to the point or surface that stops the cartridge from moving forward in the chamber. There’s all sorts of ways to think about it, but I like to think of it as the measurement of the two points that compresses the cartridge and holds it in place prior to firing. Now this isn’t exactly true, but as you’ll see in the examples it is the dimension that determines how deep the cartridge will seat into a chamber in relation to the bolt face.
All headspace measurements start with the breech face, but the surface that the cartridge contacts first may be different. I.E it could be the rim, the shoulder, a belt or the case mouth.
You can usually tell how a cartridge headspaces by the cartridge’s design. However sometimes, the headspace dimension is not what you think it is and you should consult a SAAMI or CIP print in order to see how it is called out.
The 4 Types of Headspace
Headspace dimensions typically can be broken down into 4 different types listed below.
- Rimmed
- Shoulder
- Belt
- Case Mouth
We will review each of these headspace types.
Headspaces on the Rim
The First type of headspace is the rimmed cartridge. Examples of this cartridges can be found in many cartridges designed in the early days of brass cartridges. 45 Colt, 30-30 Winchester, 38 Special, and 303 British to name a few. There are some modern cartridges that are rimmed, such as the 327 Federal Magnum, 500 S&W and 460 S&W, but rimmed cartridges have largely fallen out of favor due to issues feeding in semi automatic firearms. (That is not to say you can’t but they do pose a challenge)
Above is an example of a rimmed cartridge design. This one is a 327 Federal. SAAMI Designates the headspace dimension as a circle with an “X”. I have highlighted the headspace dimension in the photo. When reviewing SAAMI drawings note that the head space dimension is only listed on the Min Chamber drawing, it is not found on the Max Cartridge drawing.
If we refer to the max case drawing we can see that the rim thickness on a 327 Federal has a max thickness of .055in which would give a .001 clearance between the breech face and the back of the cartridge. This clearance is needed in order for a revolver cylinder to turn freely.
After repeated firings the rim thickness will not change much, however you may run into a situation where the case length grows to the point where it is longer than the SAAMI Max case length of 1.475in. Should this happen, the case may start headspacing off the case mouth which is improper causing one of two things to happen.
- This may lift the case up, and prevent the cylinder of a revolver from freely turning causing the revolver to jam.
- Force the case mouth into the throat ( the angled portion of the chamber) which will effectively apply an addition crimp to the case. This can cause pressure to spike and in the worse cases may destroy the revolver.
Obviously both conditions are unfavorable. When inspecting a batch of cases, it is important to make sure the rim thickness falls within SAAMI in order for the case to headspace properly in the chamber, and to check case length to ensure that the case does not improperly headspace off the case mouth.
Headspaces off the Shoulder
Most, but not all, modern bottle necked cartridges headspace off the shoulder. This dimension can be a little tricky to understand for some as it’s not something that can be easily measure with a “Plain Jane” Micrometer. Examples of cartridges that headspace off the shoulder are .223 Rem, .308 Win, 30-06 Sprg, 357 Sig, 400 Cor-bon, and the 32 NAA. However the 357 Sig does not (I wouldn’t have guess it either, but that’s why we always check the SAAMI Print and not just assume)
Unlike the rimmed case there are two dimensions primary dimensions that define the headspace. The length, and then the diameter. The diameter is listed as a Basic (B) diameter with the circle “X” designating it a headspace dimensions. Basic dimensions are not toleranced, and are fixed dimension. The effective headspace is the measurement from the breech face to the circle (also called a datum) on the shoulder that measures .330in.
So you might ask, how do we measure that dimension in the real world? Well it is through the use of a comparator. A comparator is a ring, that is .330in in diameter. This attaches to a basic micrometer to measure the distance between the head of the cartridges to the .330in datum which establishes the headspace. There are several different types of tools that may be used to measure the headspace on a bottleneck cartridge. The easiest is probable the use of ring comparators such as these:
They lock on to a set of calipers easily, the trouble usually is finding the right diameter insert to pick up the datum. If you’re handy and you have a lathe you can make your own fairly easily, but I haven’t yet found a place that sells them. A future business idea, maybe.
Bottle neck cases whose headspace is too long will not chamber. It is usually a self regulating issue. If it’s only a few thousands to long you might force the bolt closed, but it will not cause issues otherwise, at least not that I am aware. However if the headspace is too short, several nasty things can happen.
If the headspace on a bottleneck cartridge is too short, it can lead to blown primers, case head separation, case failures, and in the worse cases can destroy the firearm. This is due to the high pressure hot gasses not being contained within the chamber and escaping through/around the bolt and into the firearm action.
Similar to the rimmed case if the case is allowed to stretch, and case mouth exceeds SAAMI Max case, you can have a situation where the cartridge will not chamber as it is not headspacing off the shoulder but rather off the case mouth. The likelihood of jamming a case into the throat of the firearm and causing it to crimp the bullet in place is higher in rifles, due to the leverage that exists in the action. This can cause an overpressure and a destroy the rifle.
Headspaces off the Belt
There are a few cartridges that headspace off a belt of brass that surrounds the head of the case. In this way they are dimension and behave very similarly to rimmed cartridges. There are no belted pistol calibers. The rifle cases are nearly all magnum calibers, 458 Win Mag, 458 Lott, 416 Weatherby Mag, 375 H&H Mag, and most notably the 300 Win Mag which is shown below.
The majority of the belted magnum cases are bottle neck cases, but they headspace from the breech face to the top of the belt. The dimensional callout is treated identically to a rimmed case. If things are done according to SAAMI the rest of the cartridge kind of “floats” in the chamber until fired as it is held in place by the belt. However not everyone follows SAAMI.
In 300 Win Mag there are some who will headspace the cartridge off the shoulder and cut clearance for the belt. That works just fine, it’s not how it’s called out in the SAAMI manuel, but the rifle is still safe to fire. The original intent of the belt was to strengthen the head of the cartridge for heavy magnum loads, this is largely obsolete and there are plenty of cartridges that have the same or higher pressures as the belted magnums without needing the additional material.
So what happens if the head space is out on the belt magnum? Potentially all the same things that have previously been mentioned with other headspace issues. A cartridge with too thick of a belt will not chamber in a SAAMI chamber. With too thin of a belt, there may be case head separation, primer blow outs, and a damaged rifle.
As previously mentioned with a belted magnum, you have the possibility of headspacing off the shoulder. In reloading it is possible to resize a case that doesn’t set back the shoulder far enough, so that the shoulder of the cartridge headspaces off the shoulder of the chamber before the belt. This possibly could result in trouble closing the bolt, however if the bolt closes there is likely not a significant safety issue associated. Indeed it is possible that many, many reloaders do this without even knowing it.
Headspaces off the Case Mouth
The vast majority of rimless straight wall cases headspace off the case mouth. Even cases with bottlenecks such as the 357 Sig headspace from the case mouth. Why is this? Well, the many pistol cases (except for the 357 Sig) are straight walled, and do not have a rim. So the only thing that can stop the cartridge from “falling” further down the chamber is the case mouth as it is generally not desirable for the bullet to touch the rifling prior to firing. (At least not in production ammunition.)
The above example is taken from the 45 Automatic, also known as the 45 ACP. The highlighted dimension is the headspace dimension. It is quite generous at .022in. The chamber has an abrupt step just prior to the throat that allows for the case to mouth to rest against. The case is effectively squeeze between the case mouth and the breech face.
Cases which are too long will simply not chamber if they headspace off the case mouth. If cases are too short, this can result in light primer strikes, misfires, or leaking and blown primers. Leaking and blown primers can cause damage to the firearm.
So why doesn’t the 357 Sig headspace off the shoulder? We believe this is due to the the shoulder on the 357 Sig being small, and compared to other bottleneck cartridges, weak. This may cause the shoulder to flex (this may sometimes be referred to as crush-up) when the firing pin strikes the cartridge absorbing some of the energy and resulting in a misfire due to a light primer strike. This is also a problem in cartridges like 300 Blackout that have a small shoulder. By headspacing on the case mouth the potential of light primer strike due to “crush-up” is eliminated.
In Closing
The term headspace is not something that everyone needs to know, however when it comes to ensuring ammunition is fit to fire, or that a firearm is chambered correctly, you need to know it. SAAMI and CIP both have different ways of calling out headspace dimensions. We reviewed the SAAMI method, and will review the CIP method in a future write up. Though is should be noted that the CIP cartridges have the same 4 ways headspace is defined, the dimensional callout is just different.