From the link -
After the Korean War (1950–53), U.S. military researchers, dissatisfied with rifle-power ammunition, began to test a .22-inch (5.56-mm) cartridge that propelled a lighter projectile at a much higher muzzle velocity of 3,000 feet (910 metres) per second. To fire this small-calibre high-velocity round, in 1958 they chose the AR-15 rifle, designed by Eugene M. Stoner for the ArmaLite Division of Fairchild Engine and Airplane Corporation. The AR-15 was gas-operated, but it eliminated the piston in favour of a tube that directed propellant gases directly into an expansion chamber between the bolt and bolt carrier. By reducing the number of working parts and chambering the rifle for a smaller cartridge, Stoner had come up with a lightweight weapon that, even on automatic fire, produced a manageable recoil and yet was capable of inflicting fatal wounds at 300 yards (270 metres) and beyond. In 1962 the U.S. Air Force adopted the AR-15, and the Department of Defense designated it the M16. Five years later, with units engaged in the Vietnam War finding the weapon very effective under the close conditions of jungle warfare, the U.S. Army adopted it as the M16A1. Early complaints about the tendency of the M16 to jam were addressed with improved education about weapon maintenance and a change in the chemical composition of the powder in the cartridge that it fired.