Light Machine Gun M249

The M249 light machine gun (LMG), also known as the M249 Squad Automatic Weapon (SAW) which continues to be the manufacturer’s designation and formally written as Light Machine Gun, 5.56 mm, M249, is the American adaptation of the Belgian FN Minimi, a light machine gun manufactured by the Belgian company FN Herstal (FN). The M249 is manufactured in the United States by the local subsidiary FN Manufacturing LLC in Columbia, South Carolina and is widely used in the U.S. Armed Forces. The weapon was introduced in 1984 after being judged the most effective of a number of candidate weapons to address the lack of automatic firepower in small units. The M249 provides infantry squads with the high rate of fire of a machine gun combined with accuracy and portability approaching that of a rifle.

The M249 is gas operated and air-cooled. It has a quick-change barrel, allowing the gunner to rapidly replace an overheated or jammed barrel. A folding bipod is attached near the front of the gun, though an M192 LGM tripod is available. It can be fed from both linked ammunition and STANAG magazines, like those used in the M16 and M4. This allows the SAW gunner to use rifle magazines as a source of ammunition in case they run out of belts. However, it should only be used in emergencies due to its high malfunction rate.

M249s have seen action in every major conflict involving the United States since the U.S. invasion of Panama in 1989. Due to the weight and age of the weapon, the United States Marine Corps is fielding the M27 Infantry Automatic Rifle with plans to partially replace the M249 in Marine Corps service.

Development

Background

In 1965, the U.S. Army and U.S. Marine Corps‘ primary machine guns were the M2 Browning and M60. The M2 was a large-caliber heavy machine gun, usually mounted on vehicles or in fixed emplacements. The M60 was a more mobile general-purpose machine gun intended to be carried with the troops to provide heavy automatic fire. Both were very heavy weapons and usually required a crew of at least two to operate efficiently. The Browning automatic rifle, the army’s main individual machine gun since its introduction in World War I, was phased out in 1957 with the introduction of the M14 rifle, which had a fully automatic mode. “Designated riflemen” in every squad were ordered to use their weapons on the fully automatic setting, while other troops were required to use their rifle’s semi-automatic mode on most occasions to increase accuracy and conserve ammunition. Because the M14 and M16 rifles had not been designed with sustained automatic fire in mind, they often overheated or jammed. The 20-round and 30-round magazines of these weapons limited their sustained automatic effectiveness when compared to 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 would no longer 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, a modular weapon that could be easily modified for different purposes. The Stoner 63 LMG saw combat for a brief period in Vietnam with the Marine Corps, 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 caliber. 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 on two cartridge 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).

Design details

The M249 is a belt-fed light machine gun. It fires the 5.56×45mm 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. The M249 can also fire rifle grenades.

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. 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 cumbersome weapon.

The M249’s air-cooled barrel is equipped with a mechanism to remove and replace the barrel assembly with a spare, this makes it easy for operator to easily change the barrel on the field when it gets too hot during extensive amounts of fire. The barrel has a rifling twist rate of one turn in 180 mm (7 in). A folding bipod with adjustable legs is attached near the front of the weapon, though there are provisions for hard-mounting to a M192 Lightweight Ground Mount tripod or vehicle mount.

The M249’s original gas regulator featured two different gas port sizes, normal and adverse. The adverse setting increases the cyclic rate of fire from 700–850 rounds per minute to 950–1,150 rounds per minute and is used only in extreme environmental conditions or when heavy fouling is present in the weapon’s gas tube. The two-position gas regulator was discarded as part of a product improvement program, which made the M249’s that received the product improvement kit can no longer fire at higher cyclic rate. The sustained rate of fire, the rate at which the gunner can fire continuously without overheating, is around 100 rounds per minute. Whilst the rapid rate of fire is around 200 rounds per minute.

Future replacement

An extensive maintenance program intended to extend the service lives of M249s has been carried out to refurbish rifles, especially units that suffered from wear due to heavy use. In particular the warping of the receiver rails on the early-models was a defect that occurred in heavily used first-generation M249s. This defect however has been completely eliminated on later models and is no longer present on the current-issue M249, which has reinforced rails and full-length welding rather than spot welding. A replacement of the M249’s buttstock that is redesigned to be adjustable in length is also available.

The U.S. Marine Corps tested the M27 Infantry Automatic Rifle, a lighter, magazine-fed rifle to supplement and partially replace the M249. With plans to buy up to 4,100 IARs to complement and partially replace its 10,000 M249s (of which 8,000 will remain in service) held at platoon level, it acquired 450 of the Heckler & Koch HK416–based weapons for testing. The Marines started fielding the M27 in 2010, but kept both weapons in the inventory due to the M249’s greater ammunition capacity and higher sustained fire rate; rifle companies are typically issued 27 IARs and six SAWs. The U.S. Army does not plan to introduce the IAR. Colonel Robert Radcliffe of the U.S. Army Infantry Research and Development Center stated that an automatic rifle with a magazine would lower the effectiveness and firepower of a squad. While the Marine Corps has 13-man squads, the Army organizes its soldiers into squads of nine and needs considerably more firepower from the squad machine gunners to make up the difference. The U.S. Army does, however, want to replace aging M249s with newer weapons.

In early 2017, the U.S. Army posted a notice soliciting bids for the Next Generation Squad Weapon-Automatic Rifle (NGSW-AR or NGSAR) to replace the M249. In July 2018, the Army awarded contracts to six companies including Textron, head of the preceding LSAT program where they made development leaps with cased telescoped (CT) ammunition, for NGSW-AR and ammunition prototypes. These first prototypes will be delivered to the Army for evaluation in June 2019. The stated requirements include:

Maximum weight of 5.4 kilograms (12 lb), including sling, bipod, and sound suppressor
Maximum total length of 89 centimeters (35 in)
Engage pinpoint targets up to 600 meters (2,000 ft), and suppress (area fire targets) to a range of 1,200 meters (3,900 ft)
Compatible with next-generation Small Arms Fire Control systems

Variants

M249 PIP Kit: The product improvement program kit replaced the original steel tubular stock with a plastic stock based upon the shape of the heavier M240 machine gun. The change in stocks allowed for the addition of a hydraulic buffer system to reduce recoil. In addition, the dual gas port settings were reduced to only one; variants with the product improvement kit can no longer fire at a higher cyclic speed. A handguard was added above the barrel to prevent burns, and the formerly fixed barrel changing handle was swapped for a folding unit. Certain parts were beveled or chamfered to prevent cutting soldiers’ hands and arms. Other changes involved the bipod, pistol grip, flash suppressor, and sights. Over the years, additional modifications have been introduced as part of the Soldier Enhancement Program and Rapid Fielding Initiative. These include an improved bipod, 100– and 200–round fabric “soft pack” magazines (to replace the original plastic ammunition boxes), and Picatinny rails for the feed tray cover and forearm so that optics and other accessories may be added.
M249 Paratrooper: The M249 Paratrooper, often called “Para”, is a compact version of the gun with a shorter barrel and sliding aluminum buttstock based on that of the Minimi Para, so-called because of its intended use by airborne troops. It is much shorter and considerably lighter than the regular M249 at 893 mm (35 in) long and 7.1 kg (16 lb) in weight.
M249 Special Purpose Weapon: The M249 SPW is a lightweight and shorter version of the M249 is designed to meet USSOCOM special operations forces requirements. The barrel changing handle, magazine insertion well, and vehicle mounting lug all have been removed to reduce weight. As a result, the SPW cannot be mounted in vehicles or use M16 magazines. Picatinny rails were added to the feed cover and forearm for the mounting of optics, lasers, vertical foregrips, and other M4 SOPMOD kit accessories. The SPW has a detachable bipod. The SPW’s lightweight barrel is longer than that of the Para model, giving it a total length of 908 mm (36 in) and a weight of 5.7 kg (13 lb).
Mk 46: The Mk 46 is a variant of the M249 SPW that was adopted by USSOCOM. The program that led to both the Mk 46 and Mk 48, that was headed by the US Naval Special Warfare Command (NAVSPECWAR). Like the SPW, the barrel changing handle, magazine insertion well, and vehicle mounting lugs have been removed to save weight. However, the Mk 46 retains the standard M249 plastic buttstock instead of the collapsible buttstock used on the SPW. The Picatinny rail forearm differs slightly from the SPW. The Mk 46 has the option of using the lighter SPW barrel or a thicker, fluted barrel of the same length.
Mk 48: This is a 7.62×51mm NATO version of the Mk 46, used by USSOCOM, when a heavier cartridge is required. It is officially classified as an LWMG (Light Weight Machine Gun) and was developed as a replacement for the Mk 43 Mod 0/1. The M60 based machine guns are a great deal more portable than the heavier M240 based designs used elsewhere in the US military in the infantry medium machine gun role. However, the M60 based designs have a long history of insufficient reliability. Trials conducted through the mid-1990s led the US Army to replace its M60 with the M240B GPMGs. The M240B, however, weighs in at ≈27.5 lb and is about 49″ long with the standard barrel. NAVSPECWAR was reluctant to give up the increased portability of the M60 (≈22.5 lb, 37.7″ OAL with the shortest “Assault Barrel”) designs in spite of the M240’s increased reliability. A request was put in for a new machine gun in 2001, and FN responded with a scaled-up version of the M249 weighing in at ≈18.5 lb with an OAL of ≈39.5″. The new design achieved much better reliability than the M60-based weapons while bettering its light weight and maintaining the same manual of arms as the already in-use M249. USSOCOM was slated to begin receiving deliveries of the new gun in August 2003.
M249S: This is a semiautomatic version manufactured for the civilian sport shooting and collector’s market. Derived from the fully automatic military firearm, this version shares most of the major components of the military models with the exception of the firing mechanism and the addition of welded internal components to prevent conversion to a fully automatic mode. Notably, this version retains the ability to be belt fed, an uncommon feature in civilian firearms.

Specifications

Mass	7.5 kg (17 lb) empty 10 kg (22 lb) loaded
Length	40.75 in (1,035 mm)
Barrel length	465 mm (18 in) 521 mm (21 in)

Cartridge	5.56×45mm NATO
Action	Gas-operated long-stroke piston, open bolt
Rate of fire	Sustained: 100 rounds/min Rapid: 200 rounds/min Cyclic: 700–850 rounds/min (gas setting 1: normal) 950–1,150 rounds/min (gas setting 2: adverse)
Muzzle velocity	915 m/s (3,000 ft/s)
Effective firing range	700 m (2,300 ft) (point target, 465 mm barrel) 800 m (2,600 ft) (point target, 521 mm barrel) 3,600 m (11,800 ft) (maximum range)
Feed system	M27 linked disintegrating belt, STANAG magazine
Sights	Iron sights or Picatinny rail for various optics