Chain gun

A chain gun is a type of machine gun/autocannon that uses an external source of power to cycle the weapon's action, rather than diverting excess energy from the cartridges' propellant as in a typical automatic firearm, and does so via a continuous loop of chain drive similar to that used on a motorcycle or bicycle.
Development history.
In 1972, Hughes Helicopters began a company-funded research effort to design a single machine gun to fire the U.S. Army's M50 round. By April 1973, the program had fired test rounds in more powerful WECOM linked ammunition, from a prototype (A model). In January 1975 a model "C" was added, a linkless version for the proposed Advanced Attack Helicopter YAH-64; the helicopter was later adopted as the Hughes Model 77/AH-64A Apache, with the model C (receiving the designation M230 chain gun) as standard armament. In 1976, Hughes Helicopters successfully patented the chain gun, and it has since been further developed into several other system of different caliber sizes.
, "Chain gun" is a registered trademark of Northrop Grumman Innovation Systems (formerly Orbital ATK, following several mergers and outright acquisitions after Hughes Helicopters) for "externally-powered machine guns".
Description.
A chain gun has a single barrel, whereas a Gatling gun and a Minigun have multiple rotating barrels with no internal chains to drive the weapon. Reliability and controllability are the advantages of chain-driven weapons over their recoil-actuated counterparts. Recoil-actuated firearms depend upon the sometimes unreliable firing of a cartridge to power the cycle of action, whereas a chain gun uses an electric motor to drive a chain that moves in a rectangular circuit via four sprockets that 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. A misfired round does not stop the functioning of the weapon, as it might with guns that use energy from a fired cartridge to load the next round; it is simply ejected. 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, said 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.
During each full cycle of four periods, two periods (passage along the "long" sides of the rectangle) control the time that the bolt takes 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 how long the breech remains locked while firing, and open to allow cartridge extraction and ventilation of fumes.
The time that the chain takes to move around a complete loop of the rectangle controls the rate of fire, so 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 1,000-rounds-per-minute version. In practice, chain guns usually have two or three set firing speeds.