The M249 5.56mm machine gun, better known as the Squad Automatic Weapon (SAW) is a fully-automatic, gas-operated, magazine or belt-fed, individual weapon. The weapon fires from the open bolt position. The M249 is an individually portable machine gun capable of delivering a large volume of effective fire to support infantry squad operations. The weapon has a gas regulator for selecting either a normal (750 rounds per minute) or maximum (1,000 rounds per minute) rate of fire. The maximum rate of fire is authorized only if the weapon's firing rate slows under adverse conditions. The weapon has a quick-change barrel. However, barrels must not be interchanged with those from other M249s unless their headspace has been set for that weapon by direct support personnel.
By 2010, a collapsible buttstock was available, allowing shoulder firing in the extended and collapsed positions and improving weapons control when fired in confined spaces. A short barrel was available, allowing for improved egress and maneuver in close quarter combat. An improved bipod was also available, providing Soldiers with increased reliability and weapon accuracy, including on uneven terrain.
The SAW is an infantry weapon capable of being used in 2 roles, either as an automatic rifle or a light machine gun. Weapons issued as light machine guns include, among other equipment, an adapter allowing the weapon to be mounted on ground tripods. The M249 can be fired from the shoulder, hand-held from the hip or underarm position, or from the integral bipod. When employed as a machine gun, it has a tripod with a T&E mechanism and a spare barrel. However, barrels must not be interchanged with those from other M249s unless the headspace has been set for that weapon by direct support personnel.
It can be equipped with MIL-STD-1913 rails to mount optics and target illuminators and has an optional short barrel for close-quarters operations. The weapon's official designation (Machine Gun, 5.56mm, M249), does not change when used as a light machine gun or fited with the short barrel. The M249 provides the squad with the accurate, effective and sustained fire required to suppress and destroy enemy soft targets.
The standard ammunition feed for the weapon is from 200 round disintegrating link belts. The belt is contained inside either a rigid plastic box or a fabric bag, clipped to the bottom of the weapon. The M249 is also capable of firing ammunition from standard M16 magazines inserted into a magazine well in the bottom of the weapon. However, doing so is known to damage the magazine's feed lips and is to be done generally only in emergency situations.
The M249 SAW is intended to be used to engage dismounted infantry, crew-served weapons, antitank guided missile teams, and thin-skinned vehicles. When adopted, the SAW became the standard automatic rifle of the US Army and US Marine Corps infantry squads and proved useful with the loss of fully-automatic fire capability in the M16A2. The purpose of the automatic rifleman billet is to provide a short-term automatic fire missions against point targets in the last 100 yards of the infantryman's assault.
The preferred combat ammunition mix for the M249 as of 1994 was a 4-1 mix of M855 ball ammunition and M856 tracer ammunition respectively. There were other variations of 5.56mm ammunition available. However, the 4-1 mix allowed the gunner to use the tracer-on-target method of adjusting fire to achieve target kill. M193 ball and M196 tracer ammunition could be substituted for training purposes. The M199 dummy and M200 blank were also used for training purposes. At tha time, the gunner's basic load was 600 rounds of linked ammunition.
Automatic rifles allow rifle squads to take a light automatic weapon with them in the assault. In the defense, they add the firepower of 10 or 20 riflemen without the addition of manpower. Characteristically, automatic rifles are light, fire rapidly, and have more ammunition than the rifles in the squad that they support. When adopted, each Marine Rifle Squad had 3 M249 SAWs. No additional equipment configuration was needed, because the automatic rifleman fired the M249 either from the bipod mode or from various hand-held positions. In either the offense or defense, automatic riflemen had to restrict themselves to firing 3-round bursts to maintain their effectiveness against enemy targets. The M249 in the bipod or hand-held mode moved too easily off its point of aim after 3 rounds and automatic riflemen had to readjust their aim. In the offense, the automatic rifleman was limited to what he could carry and fire on the move. Hence, while the automatic rifle afforded a high volume of fire, it also rapidly consumed ammunition. Conservation and careful logistic planning became important.
Although employed as an automatic rifle by the US Marine Corps, the M249 SAW was designed like a traditional light machine gun. As such it had design features that made it a more versatile weapon. It could be belt or magazine fed thus providing more continuous fire before reloading and it had a quick change barrel feature, which allows barrel changes during periods of continuous firing without taking the weapon out of action for more than a few seconds. The M249 SAW also had greater effective range and a higher rate of fire than any other weapon in the existing rifle squad. The M249 SAW could provide a heavy volume of continuous, accurate fire in support of offensive or defensive operations. Its presence in large numbers (e.g., 9 per rifle platoon) at the small unit level significantly increased the combat power of those units. Previously, medium machine guns were often attached to platoons or squads, more out of concern over the lack of fire power in those small units than for sound tactical reasons. The introduction of the SAW into those units changed that. The SAW provided the platoons with significant firepower against enemy personnel and light equipment. As a result, more times than not, the company's machine gun section could be employed as a section, in a general or direct support role, rather than deattached to smaller elements of the company. The M249 SAW's presence, in any type of unit, increased the available fire power and provided additional flexibility to the unit leader in terms of weapons employment options.
When used as a light machine gun, the M249 required a tripod, a T&E mechanism, and a spare barrel. These items increased the stability, the ability to make minute adjustments in aiming, and the ability to fire greater than 3-round bursts. As light machine guns were not as mobile as automatic rifles, they normally remained with and formed the key weapon of the base-of-fire element. It was possible to bring a light machine gun with the maneuver element for added firepower in the assault. Once it had been set up, however, it became another base of fire and was quickly left behind by the rest of the element as it would sweep across the objective. It would spend more time displacing than firing. Machine guns targeted enemy automatic weapons, key weapons, and command and control elements. Once the enemy deployed, machine guns engaged his supporting automatic weapons. As the enemy closed, if the machine guns had destroyed all of the enemy's supporting weapons, they could engage the assaulting troops with enfilading fires across the platoon front.