The M16 is a 5.56mm automatic rifle based off of the ArmaLite AR-15. Deployed in 1964 for jungle warfare during the Vietnam War, the M16A1 later replaced the M14 rifle in 1969 as the standard service rifle for the U.S military. The development of the M16A2 was requested by the U.S Marine Corps as a result of the combat experience the M16A1 had in Vietnam. The M16A2 was later adopted by the Department of Defense in 1982.
Modifications made included new rifling, a thicker barrel at the front to prevent overheating with sustained fire, and a new adjustable rear sight. There were also changes to the handguards, buttstock length, and buttstock material as well as a three-round burst setting that replaced the fully automatic setting. The reliability of the M16A2 allowed it to be widely used, especially by the Marine Corps’ special operations divisions with a few still remaining in service throughout the U.S military today.