Skip to content
ARMAG

Search matches names, aliases and the text of every page. An arm found by its service designation is found by its alias, not its title.

Headspace

The distance from the closed breech face to the surface in the chamber that stops the cartridge going any further forward.

Which surface does the stopping depends on the cartridge. A rimmed case like the .303 British headspaces on its rim. A rimless bottleneck case like the 7.62×51mm NATO headspaces on its shoulder. A rimless straight case like the .45 ACP headspaces on the mouth of the case itself.

Get it wrong in either direction and the consequences are immediate. Too little and the action will not close on a correct cartridge. Too much and the case is unsupported at the rear when it fires: at best the brass stretches and separates, at worst gas escapes into the action.

It is normally a manufacturing tolerance rather than something a user sets. The M2 Browning is the exception on this site — headspace and timing are adjusted by the crew with a gauge, which is why the 2010 M2A1 with a fixed-headspace quick-change barrel was such a significant revision.