In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct positioning, different cartridges have their datum lines in different positions in relation to the cartridge. For example,5.56 NATO ammunition headspaces off the shoulder of the cartridge, if the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the ammunition may not fit as intended or designed, the cartridge is said to be rimless if the extractor groove is machined into the head of the case so the rim is the same diameter as the adjacent part of the case. Most modern automatic firearms use rimless cartridges, straight-walled rimless cartridges often headspace on the forward lip of the cartridge as shown in the diagram at the top of this article. This necessitates careful attention to consistent case length, bottle-necked rimless cartridges may headspace on the conical shoulder between the narrow neck and the larger diameter portion of the case. All small arms cartridges have a flange at the base of the case called a rim, the part of the firearm called an extractor claw hooks over the rim to extract the spent case from the chamber after it has been fired. The cartridge is said to be of a type when the rim is of larger diameter than the remainder of the case. Most early cartridges were rimmed, and the ledge at the rear of the chamber prevents the cartridge from moving forward. Headspace for rimmed cartridges is the space between that forward ledge and the face when the action is closed. Some large rimless magnum cartridges have a belt formed above the extractor groove and this belt is of slightly larger diameter than the adjacent case, so the cartridge can headspace from the forward edge of the belt closest to the bullet. The original purpose of the belt was to give accurate headspacing for cartridges with shallow shoulder angles, where longitudinal precision of seating using such a shoulder presents difficulties. In effect this was similar to the function of a rim. Some more or less straight cartridges have no bottleneck to headspace on, such cylindrical shaped cartridge cases use the case mouth as a forward positioned flange used for headspacing. There are four types of control to consider based on the case type used. This style of case relies on the face of the case neck. It is from this circular conic section face, as a datum, examples include.308 Winchester.223 Remington, and 7. 92×57mm Mauser. For a rimmed case, where the rim extends beyond the diameter of the case, such as with.22 Long Rifle. 30-30 Winchester, a rimmed case can have straight, tapered, or bottlenecked case walls
A pistol cartridge which headspaces off the case mouth, e.g. .45 ACP
Headspace positioning of rimless, rimmed, belted and straight cartridges
Several different rimmed, .22 rimfire cartridges, which have a uniform forward diameter, and which have headspace on the rim, allowing any length of cartridge shorter than the maximum size to be used in the same firearm
Firearms chambered for tapered rimmed cartridges like this .303 British cannot safely fire shorter cartridges.