M249 squad automatic weapon

The M249 squad automatic weapon (SAW), formally Squad Automatic Weapon, 5.56 mm, M249, is an American version of the Belgian Fabrique Nationale de Herstal (FN) Minimi. The M249 is manufactured in the United States and is used by all branches of the U.S. Armed Forces. The gun was introduced in 1984 after being judged the most effective weapon to address the lack of automatic firepower in small units. It was the only one of a number of candidate designs to pass tests set by the U.S. government. Tactically, it is employed at squad level and is operated by a single designated automatic rifleman. The gun provides the heavy volume of fire of a machine gun with accuracy approaching that of a rifle.

The M249 is gas-operated and air-cooled. It has a quick-change barrel so that the gunner can rapidly replace an overheated or jammed barrel. A folding bipod is attached near the front of the gun, though a heavy fixed tripod is also issued. It can fire both linked ammunition and ammunition kept in magazines.

M249 SAWs have been issued during every major United States conflict since the 1991 Gulf War. Soldiers are generally satisfied with the weapon's performance, though there have been many reports of clogging with dirt and sand. Due to the weight and age of the weapon, the U.S. Marine Corps (USMC) is considering designs for an infantry automatic rifle, which is planned to complement and partially replace the M249 in its service.

Development

In 1965, the U.S. Army's primary machine guns were the M2 Browning and M60. The M2 is a large-caliber heavy machine gun, usually mounted on vehicles or in prepared emplacements. The M60 was a more mobile medium machine gun intended to be carried with the troops to provide heavy automatic fire. Both are very heavy weapons and usually required a crew of at least two to operate efficiently, meaning they were best employed by platoons or larger sized units. The Browning automatic rifle, the army's main individual machine gun since its introduction in World War I, was phased out with the introduction of the M14 rifle, which has a fully automatic mode, in the 1950s. Until the introduction of the M249, "designated riflemen" in every squad had been ordered to use their weapons on the fully automatic setting. Because army doctrine usually required troops to use a rifle's semi-automatic mode on most occasions to increase accuracy and conserve ammunition, the M14 and M16 rifles used by the U.S. Army had not been designed with sustained automatic fire in mind, and overheated or jammed regularly. The 30-round magazines of these weapons also limited their sustained automatic effectiveness when compared with belt-fed weapons.

The Army decided that an individual machine gun, lighter than the M60 but with more firepower than the M16, would be advantageous; troops no longer would have to rely on rifles for automatic fire. Through the 1960s, the introduction of a machine gun into the infantry squad was examined in various studies. While there was a brief flirtation with the concept of a flechette- or dart-firing Universal Machine Gun during one study, most light machine gun experiments concentrated on the Stoner 63 light machine gun (LMG), a modular weapon which could be easily modified for different purposes. The Stoner 63 LMG saw combat for a brief period in Vietnam with the USMC, and later on a wider scale with the U.S. Navy SEALs.

In 1968, the Army Small Arms Program developed plans for a new 5.56 mm caliber LMG, though no funds were allocated (5.56 mm ammunition was viewed as underpowered by many in the armed forces). Studies of improved 5.56 mm ammunition, with better performance characteristics, began. The earliest reference to studies of other caliber cartridges for the LMG did not appear until 1969. In July 1970, the U.S. Army finally approved development of an LMG, with no specified calibre. At this time, the nomenclature "Squad Automatic Weapon" (SAW) was introduced. Actual design of alternative cartridges for the LMG did not begin until July 1971. A month later, Frankford Arsenal decided upon two designs for the new LMG: a 6 mm cartridge and a new 5.56 mm cartridge with a much larger case. Neither design was finalized by March 1972, when the Army published the specifications document for the planned SAW. The 6 mm cartridge design was eventually approved in May that year. Prior to July 1972, SAW development contracts were awarded to Maremont, Philco Ford, and the Rodman Laboratory at Rock Island Arsenal. These companies produced designs with Army designations XM233, XM234 and XM235 respectively—X denoting "experimental". Designs were required to have a weight of less than 9.07 kg (20 lb) including 200 rounds of ammunition, and a range of at least 800 meters (2,600 ft).

When the time came for developmental and operational testing of the SAW candidates, three 5.56 mm candidate weapons were included with the 6 mm candidates: the M16 HBAR, a heavy-barrel variant of the M16 designed for prolonged firing; the Fabrique Nationale de Herstal (FN) Minimi; and the HK 23A1. The initial round of tests ended in December 1974. In February 1976, the Minimi and Rodman XM235 SAW were selected for further development. At this time, opinions of the 6 mm cartridge were beginning to sour due to the logistical implications of providing yet another ammunition type to the infantry. In June, it was requested that the SAW specifications document be revised to emphasize standard 5.56 mm ammunition. In October, the requested revisions were approved, and bids were solicited for the conversion of the Rodman XM235 to 5.56 mm. Production of the converted XM235 was awarded to Ford Aerospace, and its designation was changed to XM248. A new M16 HBAR variant, the XM106, was developed in 1978, and soon after, Heckler & Koch lobbied to include a 5.56 mm conversion of its HK 21A1 (instead of the standard 7.62 mm NATO ammunition it was built for) in future SAW testing. The latter model was designated the XM262. At this time, the Minimi received the designation XM249. Testing of the four candidates resumed in April 1979.

In May 1980, the FN XM249 was selected as the best choice for future development on the grounds of performance and cost. The HK XM262 reportedly came a close second. In September, FN was awarded a "maturity phase" contract for further development of the XM249. Testing of the new XM249 began in June 1981. The official adoption took place on February 1, 1982.

The new gun entered U.S. Army service as the M249 squad automatic weapon in 1984, and was adopted by the U.S. Marine Corps a year later. The M249 model is built in the FN factory in Columbia, South Carolina and has a different butt from the regular Minimi.

Although found to be reliable and accurate, the M249 was considered to present unacceptable hazards in the form of an exposed hot barrel and sharp edges. There were also complaints that the front sight required special adjustment tools. On August 23, 1985, Undersecretary of the U.S. Army James R. Ambrose suspended M249 production pending the development of the product improvement program (PIP) intended to fix these problems. Congress deleted funds for the M249 from the Fiscal Year 1986 defense budget, then retroactively set aside the program's prior year's funds from the M249 program for other purposes, including retirement and pay raises. Over 1,100 M249s already issued were to remain in use, but be retrofitted with the PIP kit when it became available. Over 7,000 remaining M249s were to stay in storage at depots until corrective changes could be made. The PIP kit was eventually developed and implemented; production of the M249 soon resumed.

Design details
A U.S. Marine Corps M249 being fired from a tripod mount.

At 1,041 mm (41 in) long and 7.5 kg (17 lb) in weight (10 kg (22 lb) including a 200-round belt and plastic ammo box), the M249 is a large and heavy weapon. It fires from an open bolt and is gas operated. When the trigger is pulled, the bolt and bolt carrier move forward under the power of the recoil spring. A cartridge is stripped from the belt, chambered, and discharged, sending a bullet down the bore. Expanding propellant gases are diverted through a hole in the barrel into a chamber. This pressure moves a piston providing the energy to extract and eject the spent casing as well as advance the belt and compress the recoil spring, thus preparing for subsequent shots. When the operator releases the trigger, the bolt carrier is held to the rear. When the belt is exhausted, the bolt carrier comes to a rest on an empty chamber.

The barrel has a rifling twist rate of one turn in 180 mm (7 in). Because firing heats up the bore, the air-cooled barrel is equipped with a mechanism to remove and replace the barrel assembly with a spare. A folding bipod with adjustable legs is attached near the front of the weapon, though there are provisions for hard-mounting to a tripod or vehicle mount. The M249 provides accuracy approaching that of a rifle, combined with the sustained volume of fire of a machine gun. Its original gas regulator offered two different gas port sizes, allowing cyclic rates of fire of 750 rounds per minute (r/min) or 1,000 r/min. The latter setting was intended for adverse conditions such as an excessively dirty firearm or cold weather. The two-position gas regulator was discarded as part of a product improvement program. Sustained rate of fire, the rate of fire at which the gunner can fire continuously without overheating, is approximately 85 r/min.

The M249 fires the 5.56x45mm NATO cartridge, usually a combination of one M856 tracer and four M855 ball cartridges fed from M27 linked belts. Belts are typically held in a hard plastic or soft canvas box attached to the underside of the weapon.