Chamber
From Gunsopedia
In firearms, the chamber is that portion of the barrel or firing cylinder in which the cartridge is inserted prior to being fired. Rifles and pistols generally have a single chamber in their barrels, while revolvers have multiple chambers in their cylinder and no chamber in their barrel.
The act of chambering a cartridge means the insertion of a round into the chamber, either manually or through the action of the weapon (e.g. pump-action, lever-action, bolt-action, or automatic action.).
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