M16 5.56mm Rifle
The M16A2 variant of the M16 series is a select-fire rifle that fires either a 3-round burst or single shots in semi-automatic operation. The system incorporates an adjustable rear sight that corrects for both wind and elevation, a heavier barrel that previous standard M16 and M16A1 rifles with 1-in-7" rifling, and a redesigned muzzle device to prevent muzzle climb during semi-automatic operation. The M16A2 was capable of firing all NATO standard 5.56mm ammunition at the time of its introduction and could fire 40mm grenades when equipped with the M203 grenade launcher.
The M16A2 replaced the M16A1 in US Army service in 1983. First referred to as the M16 Product Improvement Project and initially designated as the M16A1E1, the M16A2 was essentially an improved M16A1. Major changes included: a switch from fully automatic to 3-round burst capability; a heavier barrel; improved sights; and new, stronger plastic buttstock, hand guard, and pistol grip.
More specifically the improvements were:
- A heavier, stiffer barrel than the barrel of the M16A1;
- A redesigned hand guard, using 2 identical halves, with a round contour, which was sturdier and provided a better grip when holding the rifle;
- A new buttstock and pistol grip made of a tougher injection moldable plastic that provided much greater resistance to breakage;
- An improved rear sight, which could be easily adjusted for windage and range;
- A modified upper receiver design to deflect ejected cartridges, and preclude the possibility of the ejected cartridges hitting the face of a left-handed firer;
- A burst control device, that limited the number of rounds fired in the automatic mode to 3 per trigger pull, which increased accuracy while reducing ammunition expenditure;
- A muzzle compensator, designed to reduce position disclosure and improve controllability and accuracy in both burst and rapid semi-automatic fire;
- A heavier barrel with a 1 in 7 inch rifling twist to fire NATO standard SS109 type (M855) ammunition, which was also fired from the M249 Squad Automatic Weapon. This further increased the effective range and penetration of the rifle cartridge. The M16A2 was also able to shoot the older M193 ammunition designed for a 1 in 12 inch twist.
As base, the M16A2 5.56mm rifle was a lightweight, air-cooled, gas-operated, magazine-fed, shoulder- or hip-fired weapon designed for either automatic fire (3-round bursts) or semi-automatic fire (single shot) through the use of a selector lever. The weapon had a fully adjustable rear sight. The bottom of the trigger guard opened to provide access to the trigger while wearing winter mittens. The upper receiver and barrel assembly had a fully adjustable rear sight and a compensator, which helped keep the muzzle down during firing. The steel bolt group and barrel extension were designed with locking lugs, which lock the bolt group to the barrel extension allowing the rifle to have a lightweight aluminum receiver.
While referred to as a burst feature, the "limiter" function on the M16 differed significantly from other burst capable weapons. In other weapons, holding the trigger when set to the burst option fired the prescribed number of rounds, or a portion if the trigger was released before the burst was finished. If the weapon was then fired in again, the same would be true. On the M16A2 (and the M4 carbine, which also used the same mechanism), if the firer released the trigger before the full burst was fired and then fired again, only the remaining shots in the burst would be fired. That is to say if the shooter fired 1 round and then fired again, the weapon would fire only 2 shots before stopping.
Also, some existing M16A1s were upgraded after the introduction of the M16A2. These upgrades only replaced the weapon's "furniture," to include the hand guards, pistol grips, and stocks. These weapons, however, were often redesignated M16A2 and remarked as such. A firm called Balimoy also provided lower recievers built to M16A2 specifications for the upgrade of existing M16A1s. These recievers were originally marked M16A1, but were generally over-stamped with the new nomenclature.
In response to a requirement from the US Navy SEALs in the early 1990s, a variant of the M16A2 was developed that retained the fully-automatic fire capability of the M16A1, but incorporated the various improvements to the design. The US Navy had previously upgraded some of its stock of M16A1s to retain the fully-automatic capability. Unlike the more basic upgrades to other M16A1s, the US Navy replaced the entire upper assembly, including the barrel. These weapons were referred to as M16A1 Heavy Barrels (M16A1HB). The new production weapon based on the M16A2 was initially designated M16A2E3, but was eventually type classified standard as the M16A3. These weapons remained in US Navy inventory even after the US Navy SEALs had begun to investigate other rifle types, being issued to Naval Construction Battalions and other units.
Thought it is commonly suggested that the M16A3 shares upper receiver with the MIL-STD-1913 accessory rail with the M16A4, the M16A3's MIL-STD, MIL-R-71135(AR), does not make any of the references found in that for the M16A4. Colt applied the prefix A3 to any rifle with the accessory rail upper receiver, which was subsequently adopted as a standard frame of reference in other commercial nomenclature, causing the confusion. Colt marketed weapons fitting the M16A3 and M16A4 descriptions both under the nomenclature "M16A3."
Experimentation with various optics and the possibility of a "flat top" upper receiver with an integral sight mount of some type had begun at the same time as the adoption of the M16A2. In 1985, Colt had delivered M16A2 Enhanced rifles, designated M16A2E1, which featured a folding front sight and the carry handle replaced with a mount for optics. The original mounts were eventually replaced with the MIL-STD-1913 accessory rail developed in large part at the Picatinny Arsenal. In the late 1990s, the weapon fitted with the upper receiver featuring the accessory rail became the M16A2E4.
The M16A2E4 rifle was effectively a standard M16A2 Rifle modified by changing the upper receiver to a flat top upper receiver with a detachable carrying handle. The flat top upper receiver had the integral rail, which was utilized (when the carrying handle was removed) to mount optical devices to the weapon. The integral rail was designed to conform to MIL-STD 1913 and allowed common mounting of many different items. The 300-meter mark on the M16A4's rear sight was 6/3, not 8/3 like it was for the M16A2. After soldiers reached the mechanical zero when zeroing the sights, they were also required to move the elevation knob clockwise up 2 clicks past the 300-meter setting, not one as they had with the M16A2.
The M16A2E4 was subsequently type classified standard as the M16A4 rifle. The replacement of the weapon's standard hand guards with the Knights Armament Systems (KAC) M5 Rail Adapter System (RAS) formed the M16A4 Modular Weapon System (as known as the MWS Rifle Version), which provided soldiers the flexibility to configure their weapons with those accessories required to fulfill an assigned mission. There were no differences between the internal dimensions of the M16A2 rifle and the M16A4 rifle, even when fitted with the KAC M5 RAS.
In 2009, a detail specification for the M16A4 was issued that removed the carry handle from the weapon's specifications. The Carry handle would no longer be issue equipment for soldiers equipped with the M16A4. In its place, a back-up rear iron sight (simply known as a back-up iron sight or BUIS), which could be attached to the accessory rail even when optics were fitted, was to be issued.
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