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Achain gun is a type ofautocannon ormachine gun that uses an external source of power to cycle the weapon'saction via acontinuous loop of chain, similar to that used on amotorcycle orbicycle, instead of diverting excess energy from thecartridges'propellant as in a typicalautomatic firearm.[1][2]
In 1972,Hughes Helicopters began a company-funded research effort to design a single machine gun to fire theU.S. Army's M5020 mm round.[1][2] In April 1973, the program fired test rounds in more powerful30 mm WECOMlinked ammunition, from a prototype A model. In January 1975, a model "C" was added, a linkless version for the proposedAdvanced Attack Helicopter YAH-64. The helicopter was later adopted as theHughes Model 77/AH-64A Apache, with the model C receiving the designationM230 chain gun as its standard armament.[1][2]
In 1976, Hughes Helicopters patented the chain gun,[3] and it has since been further developed into several other systems of different calibers.[1][2]
As of 2019[update], "chain gun" is a registered trademark ofNorthrop Grumman Innovation Systems, formerly Orbital ATK, following several mergers and acquisitions after Hughes Helicopters, for "externally-powered machine guns".[3]
There are several differences between chain guns and other types of autocannon. Whilerotary guns can also use an external source of power to cycle the weapon's mechanism, they have multiple rotating barrels, unlike chain guns. The necessary actions are performed by complex rotatingcam mechanisms, not a chain.
Recoil-operated guns, e.g. manymachine guns, theBofors gun, andgas-operated guns, e.g. mostassault rifles, theShVAK cannon, depend upon the firing of thecartridges of the weapon's ammunition to power the cycle of action, instead of an external power source. As a cartridge may misfire - completely fail to fire, hang fire - or discharge with insufficient force to cycle the mechanism, this fundamental dependence affects the reliability of such weapons.
In contrast, in a chain gun the action of the firearm is cycled by aroller chain, driven by anelectric motor. The chain moves in a rectangular circuit around foursprockets that apply tension to it. One link of the chain is connected to thebolt assembly, moving it back and forth to load, fire, extract, and eject cartridges.
Each full cycle consists of four different periods of the key link travelling along the circuit. Two periods, the passage along the "long" sides of the rectangle, control the movement of the bolt: the time that the bolt takes to drive forward and load a round into the chamber, and how quickly the bolt retracts and extracts the spent cartridge after firing. 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.
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. Thus, the chain gun operating principle is inherently reliable. An unclassified report on the EX-34 prepared by theNaval Surface Weapons Center inDahlgren, Virginia, dated 23 September 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.
The time that the chain takes to move around a complete loop of the rectangle controls therate of fire. Accordingly, varying the motor-speed allows a chain gun, in principle, to fire at a continuously variable rate from single rounds to the maximum safe rate. The maximum rate depends on the pressure drop rates in the barrel after firing a cartridge, on mechanical tolerances, and other factors. For example, the7.62mm NATO versionEX-34 was advertised to fire 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.
| Chain gun | Calibre | Uses |
|---|---|---|
| L94A1 | 7.62×51mm | AFVs such asChallenger 2 andFV510 Warrior (coaxial gun) |
| AAI In-Line | 7.62×51mm | Prototypemultiple-barrel externally driven machine gun using a chain |
| Profense PF 50 | 12.7×99mm | |
| Northrop Grumman Sky Viper | 20×102mm | Experimental derivative of the M230 |
| M242 Bushmaster | 25×137mm | AFVs such as theM2 Bradley andLAV-25, Mk 38 mount on warships |
| Mk44 Bushmaster II | 30×173mm | AFVs such asCV90, as theDS30M and Mk 46 mounts on warships |
| M230 | 30×113mmB | AH-64 Apache |
| Bushmaster III | 35×228mm | AFVs such as CV90 |
| Bushmaster IV | 40×365mmR | Uses the same ammunition as theBofors 40mm L/70 cannon |
| XM813 Bushmaster II | 40×180mm | |
| XM913 Bushmaster III | 50x228mm | Designed to fire the programmableXM1204 High Explosive Air Burst round[4] |
with a chain gun