M16 Rifle - Design

Design

The M16's receivers are made of 7075 aluminum alloy, its barrel, bolt, and bolt carrier of steel, and its handguards, pistol grip, and buttstock of plastics. Early models were especially lightweight at 6.5 pounds (2.9 kg) without magazine and sling. This was significantly less than older 7.62 mm "battle rifles" of the 1950s and 1960s. It also compares with the 6.5 pounds (2.9 kg) AKM without magazine. M16A2 and later variants (A3 & A4) weigh more (8.5 lb (3.9 kg) loaded) because of the adoption of a thicker barrel profile. The thicker barrel is more resistant to damage when handled roughly and is also slower to overheat during sustained fire. Unlike a traditional "bull" barrel that is thick its entire length, the M16A2's barrel is only thick forward of the handguards. The barrel profile under the handguards remained the same as the M16A1 for compatibility with the M203 grenade launcher. The rifle is the same length as the M16A2. The rifling twist of early model M16 barrels have 4 grooves, right hand twist, 1 turn in 14 inches (355.6 mm); later models have 6 grooves, right hand twist, 1 turn in 12 inches (304.8 mm); current models have 6 grooves, right hand twist, 1 turn in 7 in (177.8 mm).

The M16's most distinctive ergonomic feature is the carrying handle and rear sight assembly on top of the receiver. This is a by-product of the original design where the carry handle served to protect the charging handle. As the line of sight is 2.5 in (63.5 mm) over the bore, the M16 has an inherent parallax problem. At closer ranges (typically inside 15–20 meters), the shooter must aim high in order to place shots where desired. The M16 has a 500mm (19.75 inches) sight radius. The M16 uses an L-type flip, aperture rear sight and it is adjustable with two setting, 0 to 300 meters and 300 to 400 meters. The front sight is a post adjustable for elevation in the field. The rear sight can be adjusted in the field for windage. The sights can be adjusted with a bullet tip and soldiers are trained to zero their own rifles. The sight picture is the same as the M14, M1 Garand, M1 Carbine and the M1917 Enfield. The M16 also has a "Low Light Level Sight System", which includes a front sight post with a small glass vial of (glow-in-the-dark) radioactive Tritium H3 and a larger aperture rear sight. The M16 can mount a scope on the carrying handle. With the advent of the M16A2, a new fully adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters and to allow windage adjustments without the need of a tool or cartridge. Modern versions of the M16 use a Picatinny rail which allow for the use of various scopes and sighting devices. The current United States Army and Air Force issue M4 Carbine comes with the M68 Close Combat Optic and Back-up Iron Sight. The United States Marine Corps uses the ACOG Rifle Combat Optic and the United States Navy uses EOTech Holographic Weapon Sight.

Another ergonomic feature is its straight-line recoil design, where the recoil spring is located in the stock directly behind the action. This serves the dual function of operating spring and recoil buffer. The stock being in line with the bore reduces muzzle rise, especially during automatic fire. Because recoil does not significantly shift the point of aim, faster follow-up shots are possible and user fatigue is reduced.

The M16 utilizes direct impingement gas operation; energy from high-pressure gas tapped from a non-adjustable port built into the front sight assembly actuates the moving parts in the weapon. Combustion gases travel via a gas tube above the barrel directly into a chamber in the bolt carrier behind the bolt itself, pushing the carrier away from the bolt. This reduces the number of moving parts by eliminating the need for a separate piston and cylinder and it provides better performance in rapid fire by keeping reciprocating masses on the same axis as the bore.

The primary criticism of direct impingement is that fouling and debris from expended gunpowder is blown directly into the breech. As the superheated combustion gas travels down the tube, it expands and cools. This cooling causes vaporized matter to condense as it cools depositing a much greater volume of solids into the operating components of the action. The increased fouling can cause malfunctions if the rifle is not cleaned as frequently as should be. The amount of sooting deposits tends to vary with powder specification, caliber, and gas port design.

The M16 rifle fires the 5.56×45mm NATO cartridge and can produce massive wounding effects when the bullet impacts at high velocity and yaws in tissue leading to fragmentation and rapid transfer of energy. This produces wounds that were so devastating that the International Committee of the Red Cross (ICRC) and many countries considered the M16 to be an inhumane weapon.