Larry and Jim are right. The weight soldiers had to carry was a main concern. Although, I watched an Americam Rifle type show (that was naming the top ten military rifles in the world) not long ago where they were shooting the M-16 at concrete blocks. It only took two or three rounds for the block to explode into pieces. Plus they were very accurate at long distances. If you remember the trial not long ago of the soldiers who were accused of shooting prisoners through the head and torturing them- they found out later that they were were shooting them from three hundred yards off. Very accurate at long distances. I heard they have recently made a larger round now for the M-16 in 3.16 caliber(or something like that- I can't remember). But next to the AR-15, the M-16 is considered by most to be the second best assault weapon ever. The only reason the AR-15 is considered the best is because it is almost indestructable, so many are made and they are easy to get parts for, parts easily interchange, they very seldom fail due to dirt and sand, etc.
