I thought the M16 in 7.62 was an M-10, or something like that. I know the civilian version is an AR10.
The M-16 was never issued in 7.62 NATO.
There was a similar-looking predecessor in 7.62 NATO, called the AR-10 (not to be confused with the civilian 7.62 rifle of the same name) that was made in Holland and fielded by a few forces prior to adoption of the AR-15 as the M-16. I *think* Sudan used the AR-10; for certain the Portuguese paratroopers that fought in the Colonial War in Africa used the AR-10.