Since when did the U.S. Military start using AR-15s?
Since day one, during the Kennedy adminstation. The M-16 was orginally the AR-15 when it was designed by Stoner and purchased by the Air Force and later the Army. A difference between the factory designation and the military designation. However the military mostly doesn't use/buy Colts anymore, although they do for the spec ops community, and some others, such as military police. Mostly they buy from Fabrique Nationale Manufacturing.