The M249 SAW (Squad Automatic Weapon) is an air-cooled, gas-operated, fully-automatic-only firearm that fires from an open bolt position. It can accept belts of linked 5.56 x 45 mm NATO (.223 inch) ammunition through the top-mounted feed tray or M16-type magazines through the side-mounted port. Linked ammunition can be fed from either a loose belt or from a plastic box (or cloth pouch) for 200 rounds, clipped under the receiver. The hard plastic box has issues with being insecurely attached and by producing noise with movement in its standard form. The M249 SAW features a built-in bipod and a tripod-mounting lug for supported fire, as well as a quick change barrel that helps prevent overheating during sustained fire. Barrels may be individually zeroed. The forearm is designed to contain a small cleaning kit for field use, though they may not be stored there in practice.
For training exercises, the M249 is used with the M15A2 BFA (blank-firing attachment), which is the same one used on the M16A2. For the M249 to function when firing blank rounds, the BFA must be used. The basic reason is that the pressure will not be high enough to cycle the gas system. U.S. M249s are sometimes fitted with the M145 MGO, a low-power optic that fits on via a top M1913 rail, when the rail is fitted. The regular M249 does not have a rail, however. The M249 mod kit increases the weight to 16.41 pounds.
United States military doctrine describes 3,600 m as the maximum range. Effective ranges include 600 m for a point target, 800 m for an area target, and 1,000 m for suppression. Tracer ammunition burnout is at 900 m, however. The advised rates of fire are 85 rounds per minute with no barrel changes. With a barrel change every two minutes, this increases to 200 rounds per minute. For a barrel change every minute, the rate of fire can go up to 850 rounds per minute, which is approximately equal to the cyclic rate of fire, at about 850 to 900 round/min.
Variant Summary Table
|XM249||5.56 x 45 mm FN Minimi machine gun; SAW candidate|
|XM249E1||XM249 variant; w/ 1:7-inch rifling|
|XM249E2/M249||XM249E1 variant; 5.56 x 45 mm NATO FN Minimi machine gun variant w/ Product Improvement Program (PIP) kit improvements, including the heat shield|
|M249E3||5.56 x 45 mm NATO FN Minimi Para machine gun variant; longer barrel|
|M249E4||5.56 x 45 mm NATO FN Minimi SPW machine gun; also incorporating PIP kit improvements|
|Mk 46 Mod 0||5.56 x 45 mm NATO FN Minimi SPW/M249E4 machine gun variant; fixed butt stock and improved rail hand guard|