(Verschlussabstand): The space available for the cartridge between the face of the breechblock and the part of the chamber that keeps the cartridge from going forward. A critical dimension.
The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.
The fit of a cartridge in a chamber measured as the distance from breech face to that part of the chamber which stops the cases forward movement. Insufficient headspace hinders complete chambering; excessive headspace permits case stretching or separation.
Headspace is a measurement of the distance between the rear of a cartridge and the bolt or recoil shield. It is the amount of distance the cartridge can move during the firing of the gun. Headspace can be measured on a revolver using a feeler gauge, but go no-go gauges are used to determine if the headspace is acceptable on firearms that do not allow access to this area.
In small-arms weapons, the distance between the face of the bolt and the base of the cartridge when it is fully chambered and the bolt is locked.
This is the distance from the breech face to that part of the chamber which stops the forward movement of the cartridge case. Different cartridge designs obtain their headspace in different ways. A rimmed case, such as a .22 rimfire uses the case rim to position the cartridge within the chamber, whilst a rimless cartridge, like the 9 mm Parabellum uses the rim of the case mouth, seating on an annulus in the chamber (this means that 9 mm cases must be both of exact length and not use a rolled crimp to hold the bullet in place).
The distance from that surface of the barrel or chamber that prevents the cartridge from moving further forward into the chamber, to the face of the breech with the action fully closed and locked. This is the most important dimension governing the safety of the shooter.
In firearms terms, headspace refers to the distance between the bolt face and chamber necessary for reliable functioning of the weapon, or as a verb, the mechanism by which the correct positioning is achieved.