Really? I wouldn't have thought of it as such. Actually: (from Wikipedia)
The M16A3 was a fully-automatic variant of the M16A2 adopted in small numbers around the time of the introduction of the M16A2, primarily by the U.S. Navy for use by SEAL, Seabee, and Security units. It features the M16A1 trigger group providing "safe", "semi-automatic", and "fully-automatic" modes.
The most recent type is the M16A4, but it says nothing about fire modes, though, so I'm assuming it's like this.