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 et cetera). 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 and other tolerances. In practice, chain guns come with two or three pre-set firing speeds.
The most commonly produced and used chain gun is the M242 Bushmaster. Versions of its 25 mm action are found on ships (the Mk38), Infantry fighting vehicles (the M2 Bradley), and LAV-25 around the world. Other examples of chain guns include the M230 30 mm Cannon, which is standard equipment on the Apache helicopter, as well as the Bushmaster II 30 mm, and the Bushmaster III 35/50 mm Chain gun.
It is a common misnomer to refer to Gatling guns (rotary cannons) or miniguns as chain guns; even in some video games, such as Wolfenstein 3D and Doom, the player carries a minigun referred to as a chain gun. In fact, most Gatling-type guns—weapons such as the M61 Vulcan, the M197, the M134, and the XM214—are externally-powered but are not chain guns. They instead function by directing the power source (usually electricity) to a unit known as a rotor, which contains each of the multiple barrel-and-breech assemblies that make up the distinctive shape of a rotary gun and is free to rotate within a fixed outer sleeve. A cam projecting from each breech unit runs in a shaped, recessed track within the sleeve. To fire, power is applied to rotate the rotor which in turn causes each breech to cycle as its cam is forced to follow the recessed track. Each barrel therefore fires independently.