Ammunition

What Is Headspace (and Why Is It Important)?

What Is Headspace

Headspace is the distance between the back of a cartridge and whichever part of it controls the position in the chamber. The concept of headspace came with the advent of fixed cartridges.

At first, the concept was simple: cartridges were rimmed and headspaced on that rim.

  • Excessive headspace meant the cartridge would expand unevenly and sometimes crack on firing, with reduced obturation and accuracy.
  • Insufficient headspace would simply not allow the bolt to close, or it would close but jam during the attempt to extract.

Rimmed cartridges were quite forgiving of the chamber fit, some of them even using metal foil instead of drawn brass.

Semi-rimmed cases, possessed of a rim wider than the case body and an extraction groove, work the same way.

When rimless ammunition was introduced for the 1893 Borchardt automatic pistol, some other way to headspace was needed.

Types of Headspacing

Two methods are used for placing rimless cartridges consistently for firing.

Straight walled or tapered cartridges usually headspace on the mouth of the case, while bottlenecked casings use the middle of the case shoulder as the reference surface.

With belted cases, a projecting belt is added forward of the extraction groove specifically for headspacing.

This neatly gets around the problem of rimlock common to regular rimmed cartridges, strengthens the web of the casehead and provides a positive indexing surface.

 

Headspacing
Wikipedia image by Amendola90, used with permission.

 

Why Headspace Is Important

Why all this fuss about headspace? Ideally, all cartridges are the same size and always fit the chamber the same, leading to excellent accuracy and promoting consistent cycling.

In reality, heat changes headspace. Wear of the bolt and the barrel, and stretching of the receiver change headspace. Powder residue and dirt can cause the chamber to be too tight.

With some guns, too tight of a headspace merely causes a failure to go into battery, with others it allows firing out of battery with catastrophic results for the shooter.

Significantly loose headspace doesn’t just degrade accuracy, it promotes case ruptures and head separations.

The British Army actually used multiple bolt heads over the course of the life of Enfield bolt-action rifles, supplying large parts as the rest of the rifle wore.

Others replaced worn out parts with in-spec parts regularly. Obviously, the tolerances had to be tightest on sniper rifles, and less tight on infantry guns which might have to digest out of spec ammunition.

With machine guns, the specifications were looser still, since the chamber would change size significantly as it heated up.

Cases with Knocked Out Primers

Headspacing Best Practices

Since ammunition varies in size and brass cartridges are somewhat malleable anyway, armorer’s headspace gauges are usually made of steel.

“Go” gauges look a lot like a dummy round and verify that the space inside the chamber locked by the bolt is sufficient to load the weapon.

If the bolt fails to close on the “go” gauge, either the bolt head is too far forward or the chamber needs to be cleaned or reamed.

“No-go” serves the opposite goal, making sure that the chambered cartridge isn’t just bouncing around the cavernous, insufficiently supportive chamber.

Loose headspace is quite common with older guns and presents the most typical equipment-based danger to safe gun use.

A “field” gauge takes this a step further: if the bolt closes on it, the danger of a catastrophic case rupture becomes a certainty rather than a possibility.

Depending on the gun, the user might walk away with eyes and fingers intact (Mauser 98) or get a jet of hot gas in the face (Mosin-Nagant).

A set of gauges for common calibers usually runs around $60 and represents a very sensible investment, especially for high-volume shooters.

People who like AKs would be particularly well-served by checking even factory new guns, but I’ve encountered unsafely loose AR-10s from major manufacturers as well.

The tip-off is ejected cases with knocked-out primers. Collectors of classics would be likewise advised to check their find, though gauges for some of the older, odder cartridges are often hard to find.

What Is Headspace
Do you have any headspace-related stories or tips you’d like to share? Let us know in the comments below.

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Comments (9)

  1. I am unfamiliar with this definition of headspace. I have always understood headspace to be the distance from the face of a locked bolt to a given point on the cartridge case. I don’t see how you can have excessive headspace without considering where the bolt is in relation to the case.

  2. If the shoulder in the chamber is machined too deeply in the barrel won’t that be a headspace problem regardless of whether it’s an AR with a barrel extension/removable barrel, etc.? How will that automatically be compensated for?

  3. The text is correct but not the diagram. Bottle neck cartridges headspace on the datum line of the shoulder not at the base of the neck. This presents a camming surface at the middle of the case neck. Forgive me if I am wrong but that has always been my understanding.

  4. The British had this on all figured out at the turn of the 20th century. The bolt head could be unscrewed in the field, and the next size up bolt head screwed on until head space was right. If the bolt head over turned by more than 15 degrees it was time to go up a size. Firing pin, firing pin spring…all field serviceable. Ok…rimmed cartridge had it’s problems, but it doesn’t appear to be any worse than the other problems that rimless has…just have to know what your were doing.

  5. A better question might be “Why do we still have to deal with headspace issues at this late stage of industrial development?”

    Western industry more or less mastered interchangeable, standardized parts in the mid-19th Century. For understandable reasons, it took until the mid-20th to apply this reliably to the high-stakes field of firearms headspace – but they did, and it works. ARs weren’t the first, but they broadly popularized fully interchangeable barrel / extension assemblies. Not only did this eliminate the critical safety issue of headspacing, but also (by containing all firing pressure in the extension) de-stressed the receiver and tremendously simplified its manufacture.

    Patent protection may have been an issue at first, but that’s long since past. I guess it’s not too surprising why it’s not universal (manufacturers benefit when you buy a new firearm every time you want a new caliber, barrel length, etc.), but it is surprising how many competing (but far from competitive) systems of barrel interchangeability persist in the market.

    It’s bizarre to me why rotating-locking-bolt firearm or barrel customer in 2020 would accept anything falling short of 1840s manufacturing standards, requiring “Olde Worlde” filing and re-reaming and clocking and torqueing and shimming, etc. Stoner moved the bar over half a century ago, and it’s time other manufacturers step up.

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