Chamber (firearms)

In firearms, the chamber is the cavity at the back end of a breechloader's barrel or cylinder, where the cartridge is inserted before being fired. The rear opening of the chamber is the breech, and is sealed by the breechblock or the bolt.
Function.
Rifles and pistols generally have a single chamber integral to their barrels, but revolvers have multiple chambers in their cylinder, and no chamber in their barrel. Thus, rifles and pistols can usually still be fired with the magazine removed as long as a cartridge is inserted into the chamber, while a revolver cannot be fired at all with its cylinder swung out.
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 generally in anticipation of firing the weapon, without need to "load" the weapon upon decision to use it (reducing the number of "actions" needed to discharge).
In firearms design or modification, "chambering" is fitting a weapon's chamber for a particular caliber or round, so a Colt Model 1911 is "chambered" for .45 ACP or .38 Super, or "re-chambered" for .38/.45 Clerke. While the majority of firearms are chambered for one caliber, some are chambered for multiple calibers; however firing an oversized or undersized cartridge can be hazardous.
Forensics.
The chamber is a key component to the practice of Forensic firearm examination. The chamber is known to imprint its surface striations irregularities on the cartridge case, in what are called chamber marks, due to the pressure produced when shooting. Such imperfections in chamber may be produced in the manufacturing process or through extensive use. Such chamber marks are even more pronounced on substandard firearms or when firing a from an undersized chamber. 
In recent years there has been a push to automate this process via the use of automated firearms databases. Ballistics identification has also seen the development of microstamping technology which purposefully creates chamber marks through engravings on the firing pin and breech face.