First of all, the M-16 Colt Arms number is AR-15 Model 604. And at one time, it did use the 20 round but has long since, gone to the 30 round mags. The argument that the M-16 isn't an AR-15 sailed a long time ago.
Military Assassins don't use the M-16. It has too short of range and the bullet doesn't have the kill power. The two rounds for snipers is the 7.62 and the 50 cal. If you have to go up to 800 yds, the M-16 loses it at about 400 yds. And the 7.62 has the knockout power at 800yds and the 50 cal (Barrett) goes out beyond that and has done some really fantastic distance shots.
The M-60 doesn't use a box of ammo. It uses a belt fed magazine. It's a 7.62 at about 600 rounds a minute. There is a reason it's normally mounted on something. When I was in the Military, I wasn't big enough to fire that monster for a very long time. But, then again, the old M-60 used 30.06 ammo which has more power than the 3.08. Still, the M-60 isn't normally a single person rifle.