M16 (more formally Rifle, Caliber 5.56 mm, M16) is the U.S. military designation for a family of rifles derived from the ArmaLite AR-15 and further developed by Colt starting in the mid-20th century.
The M16 rifle family including the M16/A1/A2/A3/A4 has been the primary infantry rifle of the United States military since the 1960s. With its variants, it has been in use by 15 NATO countries, and is the most produced firearm in its caliber. The M16 entered Army service in 1964.
In March 1970, the Pentagon shocked other NATO nations by stating all US forces assigned to NATO would be equipped with the M16A1. The British military was highly vocal in voicing its anger after adopting the 7.62 mm NATO round over their .280 caliber (7.1 mm) round nearly 20 years earlier. They were now being told the US recognized the need for such a caliber of firearm after all, and was willing to start the NATO standardization of a lighter round.
By the middle of the 1970s, other armies were also looking at an M16-style weapon. A NATO standardization effort soon started, and tests of various rounds were carried out starting in 1977. The U.S. offered their original design, the M193, with no modifications, but there were serious concerns about its penetration in the face of the wider introduction of body armor. The British offered a modified 5.56 mm round, using a longer and thinner bullet of 4.85 mm. The round had somewhat better ballistics, and considerably better penetration, able to reach 600 m and meet their requirements for a squad automatic weapon (light machine gun). The Germans proposed a new 4.7 mm caseless round, which was considerably lighter while offering similar ballistics to the original U.S. design. However, there was distrust in the caseless system due to the possibility of cook off. A final design was offered by the Belgians. Their SS109 round was based on the U.S. cartridge but included a new bullet design, with the same 5.56 mm caliber, but with a small steel tip added to improve penetration.
Testing soon showed that the British and Belgian designs were roughly equal, both outperforming the original US design. In order to get full performance from tracer versions of the SS109, however, barrels would have to use different rifling. Existing 12 in (305 mm) twist barrels reduced the effective range of the SS109 to 90 meters due to lack of stability. While the ideal twist rate for the SS109 projectile is 9 in (229 mm), a 7 in (178 mm) twist rate was chosen to stabilize the much longer L110 tracer. This tracer was designed to complement the SS109's ballistic performance. The M196 tracer (complement to the M193 ball round) had a burn-out range of 450 meters where the L110 tracer was bright to 800 meters. In the end the Belgian round was chosen. The U.S. Marine Corps was first to adopt the round with the M16A2, introduced in 1982. This was to become the standard U.S. military rifle. The NATO standard ammunition produced for US forces is designated M855 for the ball round using an SS109 type projectile and M856 for the tracer using the L110 type projectile.
Total worldwide production of M16-style weapons since the design's inception has been approximately 8 million
The M16's receivers are made of 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 lb (2.9 kg). This was significantly less than older 7.62 mm "battle rifles" of the 1950s and 1960s. It also compares with the 11 lb (5 kg) (loaded) AK-47. M16A2 and later variants 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.
One distinctive ergonomic feature is a plastic or metal stock directly behind the action, which contains a recoil spring. 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, user fatigue is reduced.
Another distinctive ergonomic feature is a carrying handle on top of the receiver where the rear sight is located. This design is a by-product of the original design where the carry handle served to protect the charging handle and mount a scope. In practice, the handle is rarely used to carry the weapon; holding the weapon by the pistol grip provides quicker response time while a shoulder sling provides a more convenient option when response time is not a concern. More importantly, with the sight plane 2.5 in (63.5 mm) over the bore, the M16 has an inherent parallax problem that can be confounding to shooters. At closer ranges (typically inside 15–20 meters), the shooter must aim high in order to place shots where desired.
Newer models have a "flattop" upper receiver with a Picatinny mounting rail, to which the user can attach either a conventional sighting system or numerous optical devices such as night vision scopes.
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.
A US Marine with an M16A4 equipped with an ACOG. 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, not unlike an aerosol can cool when depressurized. 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.