The M16 is the United States military designation for the AR-15

AR-15

The AR-15 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed semi-automatic rifle, with a rotating-lock bolt, actuated by direct impingement gas operation. It is manufactured with the extensive use of aluminum alloys and synthetic materials....

 rifle adapted for both semi-automatic and full-automatic fire. Colt

Colt's Manufacturing Company

Colt's Manufacturing Company is a United States firearms manufacturer, whose first predecessor corporation was founded in 1836 by Sam Colt. Colt is best known for the engineering, production, and marketing of firearms over the later half of the 19th and the 20th century...

 purchased the rights to the AR-15 from ArmaLite

ArmaLite

ArmaLite is the name of a small arms engineering facility founded in the early 1950s, and once associated with the Fairchild Engine and Airplane Corporation. ArmaLite was formally incorporated as a subdivision of Fairchild on October 1, 1954...

, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO cartridge. The rifle entered United States Army

United States Army

The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

 service and was deployed for jungle warfare

Jungle warfare

Jungle warfare is a term used to cover the special techniques needed for military units to survive and fight in jungle terrain.It has been the topic of extensive study by military strategists, and was an important part of the planning for both sides in many conflicts, including World War II and the...

 operations in South Vietnam

South Vietnam

South Vietnam was a state which governed southern Vietnam until 1975. It received international recognition in 1950 as the "State of Vietnam" and later as the "Republic of Vietnam" . Its capital was Saigon...

 in 1963, becoming the U.S.