The M249 is a gas-operated, air-cooled, belt-and-magazine-fed, shoulder-fired automatic weapon. It feeds from either a 100-round or 200-round ammunition box, each round linked by a split-link disintegrating belt. It can also use AR-15 STANAG magazines; however, this is not advisable, as the weapon will exhibit frequent failure to feed malfunctions.
The M249 has a hooded and semi-tied front sight. The rear sight assembly mounts on the top of the cover and feed mechanism assembly, and is adjustable from 300 meters to 1000 meters. Range changes are made on the rear sight by rotating the elevation knob to the desired range setting.
Rotation of the rear sight aperture is used for fine changes in elevation or range adjustments, such as during zeroing. One click of the rear sight for elevation or windage represents a half mil change, or .5 centimeters at 10 meters.
This was the first M249. This variant was made of steel with a side folding stock, similar to the FN MINIMI.
This variant is an upgraded version of the M249E1; the use of a PIP(Product Improvement Program) kit allowed the M249E2 to be more lightweight, due to the fact that some upgraded parts were plastic. This variant is used in the US military. Because the gun is an upgraded M249E1, it is usually referred to as M249.
The M249E3 has all the same features of the M249E2. This variant is also used in the US military. The M249E3 features a collapsible buttstock instead of the solid buttstock on the M249E2. It also features a shorter barrel for airborne, armored infantry, and CQC (Close Quarters Combat). When the stock is collapsed, it is 10inches shorter than the M249E2, it is usually referred to as M249 PARA.
A lightweight shorter version of the M249 designed to meet USSOCOM requirements.
Mk 46 Mod 0
This is a variant of the special purpose weapon adopted by USSOCOM. The program, which led to both the Mk 46 and Mk 48, was headed by the US Naval Special Warfare Command (NAVSPECWAR). Like the SPW, the carrying 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 also 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. The Mod 0 variant has been discontinued in favor of the Mod 1.
Mk 48 Mod 0
This is a 7.62x51 mm 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 M240B GPMGs. The M240B however, weighs in at ~27.5lb and is about 49" long with the standard barrel. NAVSPECWAR was reluctant to give up the increased portability of the M60 (~22.5lb, 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 ~18.5lb 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. Along with the Mk 46 Mod 0, the Mod 0 has been discontinued in favor of the Mod 1.