From wikipedia:
“The select-fire AR-15 entered the US military system as the M16 rifle. Colt then marketed the Colt AR-15 as a semi-automatic version of the M16 rifle for civilian sales in 1963.[8] The name “AR-15” is a Colt registered trademark, which refers only to the semi-automatic rifle.”
http://en.wikipedia.org/wiki/AR-15
The “AR” designates automatic rifle. Don’t believe everything you read on wiki.
I qualified with AR15 in 1964. Now this was the Air Force but it was a full auto and not known as the M16.