The result of combat experience in Vietnam made the United States Marine Corps to request the development of M16A2, and it was officially adopted by US Department of Defense by 1982 as US Rifle, 5.56mm, M16A2

M16A2 is the further improvement from M16A1 which were the first improvement from M16.

M16 along with M16A1 received many complaints about their reliabilties and malfunctions rate, which may be or may be not one of the reasons why America lost The Vietnam War.

Learning from the mistakes, M16A2 were improved extensively.

In addition to the then new STANAG 4172 5.56×45 mm NATO chambering and its accompanying rifling, the barrel was made with a greater thickness in front of the front sight post, to resist bending in the field and to allow a longer period of sustained fire without overheating. While the rest of the barrel was maintained at the original thickness to enable the M203 grenade launcher to be attached.

The M16A2’s new found reliability allowed it to be widely used around the Marine Corps’s Special Operations Divisions as well.

A new adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters to take full advantage of the ballistic characteristics of the new SS109 rounds and to allow windage adjustments without the need of a tool or cartridge.

The flash suppressor modified, to be closed on the bottom so it would not kick up dirt or snow when being fired from the prone position, and acting as a recoil compensator.

The front grip was modified from the original triangular shape to a round one, which better fit smaller hands and could be fitted to older models of the M16.

The new handguards were also symmetrical so armories need not separate left- and right-hand spares.

The handguard retention ring was tapered to make it easier to install and uninstall the handguards.

A notch for the middle finger was added to the pistol grip, as well as more texture to enhance the grip.

The buttstock was lengthened by 58 in (15.9 mm). The new buttstock became ten times stronger than the original due to advances in polymer technology since the early 1960s. Original M16 stocks were made from cellulose-impregnated phenolic resin; the newer stocks were engineered from DuPont Zytel glass-filled thermoset polymers. The new stock included a fully textured polymer buttplate for better grip on the shoulder, and retained a panel for accessing a small compartment inside the stock, often used for storing a basic cleaning kit.

The heavier bullet reduces muzzle velocity from 3,200 feet per second (980 m/s), to about 3,050 feet per second (930 m/s).

The A2 uses a faster 1:7 twist rifling to adequately stabilize 5.56×45 mm NATO L110/M856 tracer ammunition. A spent case deflector was incorporated into the upper receiver immediately behind the ejection port to prevent cases from striking left-handed users.

The action was also modified, replacing the fully automatic setting with a three-round burst setting. When using a fully automatic weapon, inexperienced troops often hold down the trigger and “spray” when under fire. The U.S. Army concluded that three-shot groups provide an optimum combination of ammunition conservation, accuracy, and firepower.

While USMC has retired the M16A2 in favor of the newer M16A4; a few M16A2s still remain in service with the U.S. Army Reserve and National Guard, Air Force, Navy, and Coast Guard.