The M16 was an initial version first adopted in 1964 by the United States Air Force (USAF). The U.S. Army began to field the XM16E1
en masse in 1965 with most going to Vietnam. The US Marine Corps also
adopted the system during this period. The XM16E1 was standardized as
the M16A1 in 1967. This version remained the primary infantry
rifle of the United States military from 1967 until the 1980s, when it
was supplemented by the M16A2. During the early 1980s a roughly standardized load for this ammunition was adopted throughout NATO (see: 5.56 mm NATO).
The M16A3 is a fully-automatic variant of the M16A2, issued primarily within the United States Navy. The M16A2, in turn, is currently being supplemented by the M16A4, which incorporates the flattop receiver unit developed for the M4 Carbine, and Picatinny rail
System. M16A2 are still in stock with the U.S. Army and Marine Corps,
but are used primarily by reserve and National Guard units as well as
by the U.S. Air Force.
(Courtesy of Wikipedia)
