M16 (more formally United States Rifle II, Caliber 5.56 mm, M16) is the U.S. Military designation for a family of rifle derived from the ArmaLite AR - 15 and further developed by 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 countries, and is the most produced firearm in its caliber. The M16 entered Army service in 1964.

source: M16