M-242 Autocannon
Caliber:	25MM NATO.
Action:	Chain gun.
Barrel Length:	85.6 in (2.175 m).
Effective Range:	1.86 miles (3,000m).
Maximum Range:	4.23 miles (6,800m).
Maximum ROF:	200 rounds per minute.
Muzzle Velocity:	1,100 m/s (3,610 ft/s).
Total Weight:	243 lb (110 kg).

The M242 Bushmaster is a 25MM (25×137MM) belt-fed autocannon. It is used extensively by the US Armed Forces, as well as by NATO’s and some other nation’s forces in ground vehicles and watercraft. It is an externally powered, chain driven, single-barrel weapon which may be fired in semi-automatic or automatic modes. It is fed by a metallic link belt and has dual-feed capability, and uses a roller chain that drives the bolt back and forth. The weapon can destroy lightly armored vehicles and aerial targets (such as helicopters and slow-flying aircraft), and suppress enemy positions such as exposed troops, dug-in positions, and occupied built-up areas. The standard rate of fire is 200 rounds per minute. The weapon has an effective range of up to 3,000 meters, depending on the type of ammunition used.
History
The Bushmaster project started as an offshoot of the US Army’s MICV-65 program that was attempting to introduce a new infantry fighting vehicle to replace their existing M113s. Part of this program called for a new scout vehicle to replace the M114, a parallel development taking place under the XM800 Armored Reconnaissance Scout Vehicle. Both the XM800 and the cavalry version of the XM701 MICV vehicles were armed with the M139, a US-built version of the Hispano-Suiza HS.820 20 mm autocannon.
During the testing phase the Army eventually rejected the XM701 and started work on a newer design known as the XM723. Soon after the XM800 was also rejected. This led to the combination of the two programs, moving the scout role to the cavalry version of the XM723.
At the same time, the M139 proved to be disappointing (for reasons that are not well documented), and a contract for a new weapon to replace it started in 1972 at Hughes Aircraft as the Vehicle Rapid-Fire Weapons System-Successor, or VRFWS-S. This was essentially a power-driven gun firing similar ammunition as the HS.820, the power driven mechanism would ensure operation even in the case of a misfire.
Progress on the VRFWS-S was slow, and eventually resulted in a switch to a much more powerful 25MM round. Similar delays in the MICV program meant the ultimate vehicles descending from their efforts, the M2/M3 Bradley Fighting Vehicle, did not enter production until 1981, by which point the Bushmaster had matured. Since 1990, there have been several enhancements made upon the weapon, resulting in the Enhanced 25MM gun.
To date, more than 10,000 weapons are in service. One of the major reason for this popularity is the extremely reliable nature of the weapon. It has a rating of 22,000 mean rounds between failure (MRBF), much higher than many comparable devices.
Description
Unlike most automatic firearms, the M242 does not depend on gas or recoil to actuate its firing system. Instead, it uses a 1.0 hp (750 W) DC motor, positioned in the receiver to drive the chain and dual-feed system. This system uses sprockets and extractor grooves to feed, load, fire, extract, and eject rounds. A system of clutches provides for an alternate sprocket to engage and thus allows the gunner to switch between armor piercing and high explosive rounds.
The weapon assembly consists of three parts: the barrel assembly, the feeder assembly, and the receiver assembly. The three-part structure makes it possible for a single person to install or remove the system despite its considerable total weight.

The Mk-38. Note the M-242’s characteristic fluted gun barrel to reduce weight and assist cooling.

Variations
The M242 is an extremely popular weapon. It is currently in use by the The M242 is an extremely popular weapon. It is currently in use by the U.S. Army, the U. S. Navy, the U. S. Marine Corps, the United States Coast Guard, the New Zealand Army, Royal New Zealand Navy, the Norwegian Army, the Malaysian Army, the Spanish Army, the Swiss Army, the Canadian Army, the Australian Army and Navy, and the Israeli Navy as well as several others. The wide usage results in several variations and modifications on the standard M242 weapon system.
Ground Vehicles: The M242 is standard equipment on the U. S. Army M2 and M3 Bradley fighting vehicles. It is also in use on the LAV-25. Currently, the Bushmaster II 30 mm chain gun (a successor to the M242) is also in use on the Marine Corps’ Expeditionary Fighting Vehicle (EFV). The M242 is also a popular choice of primary armament for armoured fighting vehicles manufactured around the world.
Enhanced 25MM gun: Work on an upgraded weapon began in 1990. In doing so, three major systems and seven minor systems were improved. The modifications began with introducing a chrome-lined barrel, an enhanced feeder, and an enhanced receiver. The weapon systems also received minor upgrades such as quick-detachable link covers, a larger breech assembly, a high efficiency muzzle brake, longer recoil, an integral round counter, an extended life firing pin and spring, and a triple-spring drive clutch. It was first put to use on the M2A3 Bradley, the third version of the M2 Infantry Fighting Vehicle.
Aircraft: The M242 is not used on aircraft. The AC-130U and the AV-8 Harrier II use different 25MM weapons (GAU-12/U). The sister weapon of the M242, the M230 30MM chain gun, is standard equipment on the AH-64 Apache attack helicopter.
Naval: In 1977 the U. S. Navy realized that it needed a replacement for the Oerlikon/Hispano-Suiza 20MM Mk 16 series of guns. In 1986 this need was satisfied with the introduction of the Mk 38 Mod 0 weapons system. A derivative of the M242 system, the Mk 38 consists of the M242 chain gun and the Mk 88 Mod 0 machine gun mount. It provides ships with defensive and offensive gunfire capability for the engagement of a variety of surface targets. Designed primarily as a close-range defensive measure, it provides protection against patrol boats, floating mines, and various shore-based targets . The US Navy also developed a high ROF variant under the designation Mk 46 Mod 0. Recently, several US Navy platforms have been outfitted with a newer version, Mk 38 Mod 2 which includes an Electronic Optical Sight, Laser Range-Finder, FLIR, and a more reliable feeding system, enhancing the weapon systems capabilities and accuracy.
Operation
The M242 weapon system has both electrical and manual fire control and can be operated electrically or manually. In doing so, the gunner can choose from three rates of fire: (1) Single Shot Semi-Automatic, in which the gunner can shoot as fast as the trigger can be operated; (2) Low Rate Fully Automatic, in which the weapon fires 100 rounds a minute, plus or minus 25 rounds; and (3) High Rate Fully Automatic, in which the weapon fires 200 rounds a minute, plus or minus 25 rounds.
Ammunition
A wide range of ammunition has been developed for this weapon, providing it with the capability to defeat the majority of armored vehicles it is likely to encounter, up to and including some tanks. The ammunition used in the M242 may also be used in a variety of weapons such as the French Giat M811, the GAU-12, or the Oerlikon KBA weapon system. It has the capability to fire U.S. manufactured ammunition as well as the NATO equivalents thereof. Primarily though, it fires six types of rounds: the M791, M792, M793, M910, MK210, and M919.