Roger the lighter weight, but I recall reading long, long ago that the 5.56 caliber and the M-16 itself were designed around the concept of jungle warfare and the containment mission, with recent experience in the Philippines and Malaya in mind. You might say the M-16 was designed for "one, two, many Vietnams", and not for the European or Korean theaters at all.
Well, it's not doing so well in the desert or urban environments.
It seems to me that a small caliber round is good for a squad weapon that's used in the full auto role often. Smaller equals less weight per round, so the SAW operator can carry more rounds. Meanwhile, the average infantryman should be carrying a larger round for more precise, and more lethal, shooting.
If the truth be known about the origin of the M16, I think you'd have to take a close look at the SOB that forced that POS on our combat troops in Vietnam. Robert Mcnamara pushed that Mattel toy and it was responsible, directly, for alot of troops being wounded or worse when it jammed up.
Indirectly, Mcnamara should be held responsible for the injuries and deaths of those young men.