Where the Cartridge Sits
The chamber is the enlarged rear section of the barrel that holds a cartridge during firing. Its shape is defined by industry drawings so that ammunition and firearms made by different companies remain interchangeable. Understanding a few dimensions explains why cartridges are engineered to a tolerance window rather than one exact size.
Headspace
Headspace is the distance from the closed breech face to the surface in the chamber that stops the cartridge's forward travel. It ensures the cartridge is held in a consistent position when the action closes. Too little headspace can prevent the action from closing, while too much leaves the case unsupported, which is why it is a controlled dimension checked with gauges.
The Datum Reference
Which surface stops the cartridge depends on the case design. A rimmed case headspaces on its rim, a belted case on the belt, a rimless bottleneck case on a defined point on the shoulder, and a rimless straight case on the case mouth. Drawings specify a datum line, a reference point on that stopping surface from which headspace is measured.
GO and NO-GO Gauges
Headspace is verified with precision gauges rather than by measuring directly. A GO gauge represents the minimum chamber length and the action should close on it; a NO-GO gauge represents the maximum acceptable length and the action should not close on it. This pass and fail check confirms the chamber falls inside the tolerance window.
Minimum and Maximum Tolerances
Dimensional standards publish a minimum chamber and a maximum cartridge, with a deliberate gap between them. This gap guarantees that any in-spec cartridge fits any in-spec chamber, even when both are at opposite extremes of their tolerances. Reading these paired limits explains why interchangeability works across makers.
Worked Example
Consider a rimless bottleneck cartridge that headspaces on its shoulder. If a GO gauge measures 1.630 in to the shoulder datum and the NO-GO gauge measures 1.636 in, a correctly cut chamber will close on the 1.630 in gauge but stop short of closing on the 1.636 in gauge. The roughly 0.006 in span is the acceptable headspace window for that chamber.
A Common Misconception
Headspace is often confused with the overall length of the cartridge or the depth to which a projectile is seated. Those are separate measurements; headspace concerns only the distance to the surface that arrests the case, measured to a defined datum. A cartridge can vary in overall length while still headspacing correctly, because the stopping surface is unchanged.