The M-16 is just the Military Designator for the AR. In reality, the real Colt designation for the M-16 is AR-15 Model 604. The Colt Disignation for the Civilian AR-15 is either the AR-15 Model 750 or the Colt model LE6920. All use all but 3 parts from the same parts bin. There are some LE690s (AR-15) that are currently in the US Army that are single shot only. What is stamped on the side? It isn't LE6920 or AR-15, it's M-16. All Military Rifles will have an M designator. Even when they were using the Remington Model 700 as a sniper rifle for awhile.
So, in effect, the AR was adopted in 1959 by the Malaysians and the US Air Force in 1962 by the United States Air Force. The Air Force didn't have an M desginator requirement at the time so they called theirs by the Colt AR-15 Model 601. Later, in 1969, the M designator was adopted, the Air Force restamped theirs to read AR-15 Model 601 (M-16). The Air Force had over 14,000 of these weapons and didn't retire them until 1992. These were full auto even AFTER the Army went to the 3 shot burst. If you want a surplus Army M-16 that's been rode hard and put away wet, you can buy one starting at $15,000. But if you want a good quality Air Force AR-15 full auto Model 601 (M-16) you can purchase one for about $3000. Of course, FFL laws do apply.