Headspace

In firearms terms, the headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum line) to the face of the bolt. Used as a verb, headspace refers to the mechanism by which the correct positioning is achieved. 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, whereas .303 British headspaces off the rim of the cartridge.
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Noun
1. the volume left at the top of a filled container (bottle or jar or tin) before sealing
(hypernym) indefinite quantity

The vapor mixture trapped above a solid or liquid in a sealed vessel.

headspace
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