IN MARCH OF 1965, the first U.S. troops landed in Vietnam. They were carrying the M14 rifle, chambered for the 7.62×51mm NATO (M80 Ball) cartridge, which had a detachable 20-round magazine and was capable of semi- and full-automatic fire. The military soon learned the M14 on full auto was extremely difficult to control; most burst fire was ineffective. As a result, many M14 rifles were issued with the selector levers removed, making the rifle effectively, an M1 Garand with a 20-round magazine. The M14 was accurate but heavy, weighing nearly nine pounds, empty. As U.S. involvement in the Vietnam War...