The AR-15, now under Colt's control, underwent extensive testing by the U.S. military. Its lightweight design and high rate of fire appealed to military planners seeking a modern infantry weapon.

The AR-15 was officially adopted by the U.S. Air Force as the M16 rifle. This marked the beginning of its widespread use in the military.

The M16 faced challenges in the harsh conditions of the Vietnam War, including reliability issues related to ammunition and maintenance. These problems led to various modifications and improvements, resulting in the M16A1.

Over the decades, the M16 underwent further refinements, resulting in models like the M16A2 and M16A4. The shorter, carbine version, known as the M4, became increasingly popular for its maneuverability in urban environments.