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 in 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 used 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.
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.
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.
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.
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.