The range of the M-16 is indeed a valid concern, ROMT, at least under certain combat conditions. In Vietnam, the vast majority of firefights took place at relatively close range, less than 100 yards, often
MUCH less than that, thus it was rarely a concern. I did have a few times though, where I could have used a bit more effective range. Charlie liked to get in close when he could because he knew from bitter experience that if the range was fairly long, more than likely he'd get pulverized when we called in the artillery or gun ships on him.
One thing I didn't like about the M-16 though, was its relative lack of barrier penetration as compared to the 7.62 NATO.