The development of the M16A2 rifle was originally requested by the United States Marine Corps as a result of the USMC's combat experience in Vietnam with the XM16E1 and M16A1.[11] The Marines were the first branch of the U.S. Armed Forces to adopt the M16A2 in the early/mid-1980s, with the United States Army following suit in the late 1980s. Modifications to the M16A2 were extensive. In addition to the new 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. The rest of the barrel was maintained at the original thickness to enable the M203 grenade launcher to be attached.[11] 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.[11] The flash suppressor was again modified, this time 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.[74] 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.[11] The new handguards were also symmetrical so that armories need not separate left and right spares. The handguard retention ring was tapered to make it easier to install and uninstall the handguards.[11] 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 5â„8 in (15.9 mm).[11] 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 fiberglass-impregnated 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).[citation needed] The A2 also uses a faster twist rifling to allow the use of a trajectory-matched tracer round. A spent case deflector was incorporated into the upper receiver immediately behind the ejection port to prevent cases from striking left-handed users.[11]
The action was also modified, replacing the fully automatic setting with a three-round burst setting.[11] 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. The USMC has retired the M16A2 in favor of the newer M16A4. However, many M16A2s remain in US Army, Air Force, Navy and Coast Guard service.[citation needed]
