Why did the U.S. military adopt the AR-15 as the M16?

The U.S. military adopted the AR-15 as the M16 primarily because it offered a lightweight, portable alternative to the heavy, high-recoil 7.62-mm rifles of the era, such as the M14.12

Following the Korean War, military researchers sought a more effective infantry weapon, eventually testing a high-velocity 5.56-mm cartridge. Eugene Stoner’s AR-15 design was selected to fire this round because it featured a gas-operated system that reduced the number of working parts and produced manageable recoil, even during automatic fire.2

The U.S. Air Force first adopted the rifle in 1962, at which point the Department of Defense designated it the M16. Its effectiveness in the close-quarters conditions of the Vietnam War later led the U.S. Army to adopt an improved version, the M16A1, as its standard infantry weapon in 1967.12