Colt M16A2

The M16A2 was largely the brainchild of the United States Marine Corps, who drove the requirement for a heavier barrel, improved sights, and the three-round burst mechanism. The Marines adopted it a full three years before the Army in 1983. The burst mechanism was controversial from the start: unlike the M16A1's simple full-auto trigger group, the A2's burst cam does not reset between trigger pulls, meaning a shooter could get a 1 or 2-round burst instead of the expected 3 if the cam was mid-cycle from a previous pull. The Army later reverted to full-auto with the M4A1, essentially admitting the burst feature was a mistake. The M16A2's development was heavily influenced by NATO standardization efforts that led to the adoption of the Belgian SS109 cartridge as the 5.56mm NATO standard in 1980, chosen over the American M193 in competitive trials.

The M16A2 replaced the full-automatic fire selector with a three-round burst mechanism to improve ammunition conservation and hit probability in combat. The heavier barrel profile forward of the handguards resists bending from rough handling and is slower to overheat during sustained fire, while the barrel under the handguards retained the M16A1 profile for M203 grenade launcher compatibility. The 1:7 twist rifling was specifically adopted to stabilize the longer, heavier NATO SS109/M855 62-grain bullet and the even longer M856 tracer round at 948 m/s muzzle velocity, a reduction from the M16A1's 991 m/s with the lighter M193 round. The rear sight was upgraded from the M16A1's simple L-type flip to a fully adjustable windage and elevation aperture. Production was shared between Colt and FN Herstal (FN Manufacturing). The M16A2 continues to see service with reserve and training units in the US military and remains the primary service rifle in many countries worldwide.