"The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of military rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-round magazine.
In 1964, the M16 entered US military service and the following year was deployed for jungle warfare operations during the Vietnam War.
The M16 has also been widely adopted by other armed forces around the world. Total worldwide production of M16s is approximately 8 million, making it the most-produced firearm of its 5.56 mm caliber.
The US military has largely replaced the M16 in frontline combat units with a shorter and lighter version, the M4 carbine."