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A telescoping bolt (also known as an overhung bolt) is a firearm bolt which telescopes over, that is, wraps around and past, the breech end of the barrel. This feature reduces the required length of a weapon such as a submachine gun significantly, and it allows compact designs to be balanced around the pistol grip in a way that gives "pointability" more like a pistol's.
While it would be simpler and easier to shorten the bolt to fit completely behind the breech, the bolt must have a certain amount of mass in order to operate reliably with a given caliber. The telescoping bolt moves some of that mass forward of the bolt face, resulting in a bolt which may be longer overall, but is shorter behind the bolt face.
Though technically a different, distinct concept, nearly all telescoping bolt submachine guns use a magazine located in the pistol grip used to hold and fire the weapon. However, there are blowback firearms with the magazine located in the grip that do not use a telescoping bolt, such as Kel-Tec SUB-2000.