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A chain gun is a type of machine gun or automatic cannon that uses an external source of power, rather than recoil, to cycle the weapon, and does so via a continuous loop of chain similar to that used on a motor or bicycle. Chain gun is a registered trademark of McDonnell Douglas for a chain-powered weapon. The primary advantages of chain-driven weapons over their recoil-actuated counterparts are their reliability and controllability. Rather than being dependent upon recoil to actuate the system, which is usually derived from the detonation of a cartridge and is thus inherently uncontrollable, a chain gun instead depends on an external motor to produce power. The motor drives the chain, the chain moves in a rectangular loop via four sprockets which tension it, and one link of the chain is in turn connected to the bolt assembly, thus the bolt moves back and forth to load, fire, extract and eject cartridges. As with other externally-powered guns, this provides a degree of reliability. In addition, and again as with all externally-controlled guns, a misfired round does not stop the weapon - it is simply ejected. The speed of the motor also controls how fast the weapon fires, and thus provides controllability. During each full cycle of the chain link attached to the bolt assembly, two periods (passage along the 'long' sides of the rectangle') control the time taken for the bolt to drive forward and chamber a round and how quickly it extracts it, whilst the other two periods - when the chain moves across the 'short' sides of the rectangle, sideways relative to the axis of the barrel - determine for how long the breech remains locked (during firing) and open (allowing extraction and ventilation of fumes etc). Since it is the time taken for the chain to move around a complete loop of the rectangle that controls the rate of fire of the gun, chain guns can theoretically operate at an infinite number of firing rates from single shot to the maximum imposed by mechanical etc tolerances. In practice, chain guns come with two or three pre-set firing speeds. (from Wikipedia)
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