A chain gun is a type of machine gun or automatic cannon that uses an external source of power, rather than diverting energy from the cartridge, 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 Alliant Techsystems Inc. for a chain-powered weapon.
Reliability and controllability are the advantages of chain-driven weapons over their recoil-actuated counterparts. Instead of depending upon the sometimes unreliable firing of a cartridge to power the cycle of action, a chain gun uses an electric motor to drive the chain that moves in a rectangular circuit via four sprockets which apply tension to it; one link of the chain is connected to the bolt assembly, moving it back and forth to load, fire, extract, and eject cartridges.
As with all guns which do not use energy from a fired cartridge to load the next, a misfired round does not stop the weapon; it is simply ejected.
During each full cycle of four periods, two periods (passage along the "long' sides of the rectangle") control the time taken for the bolt to drive forward and load a round into the chamber, and how quickly it extracts it. 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 while firing, and open to allow cartridge extraction and ventilation of fumes.
Since the time taken for the chain to move around a complete loop of the rectangle controls the rate of fire, varying the motor speed allows chain guns in principle to fire at a rate continuously variable from single rounds to the maximum safe rate imposed by the pressure drop rates in the barrel after firing a cartridge, mechanical tolerances, and other factors; for example the 7.62mm NATO version EX-34 was advertised to offer 570 rounds per minute, and developmental work was underway for a 1000 rounds per minute version. In practice chain guns usually have two or three set firing speeds.
The chain gun operating principle is inherently reliable. An unclassified report on the EX-34 prepared by the Naval Surface Weapons Center in Dahlgren, Virginia, dated September 23, 1983, stated that:
29,721 rounds of endurance tests were fired with no parts breakage and without any gun stoppages... It is significant that during firing of 101,343 rounds not one jam or stoppage occurred due to loss of round control in the gun or feeder mechanism... [this] is in our experience very unusual in any weapon of any caliber or type.
A commonly 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. Others are 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.
A 7.62 mm NATO (.308 Winchester) caliber chain gun is used on some armored vehicles as a coaxial machine gun, because of the inherently small amount of fumes from spent propellant discharged inside of the vehicle. A developmental version of this gun was named EX-34.