Just need to add my $0.02
Someone said "The GI's simply liked the guns more than the M-14 they were using"
No, no they didn't. The GI's in Vietnam HATED their M16s, as Bobby MacNamara's Whiz Kids sent the thing into the field before it was developed completely, inlcuding the lack of a crome lined bore/chamber and cleaning kits.
The M16 has been improved, to some extent, although the A2 is heavier than the A1 and I see NO reason for this. It doesn't NEED a heavier barrel. And they STILL haven't figured out that they ought to put a knob to adjust the freaking front sight.
As for the weight of the weapon, this isn't really an issue today. If they actually tried, I'll bet they could make a state of the art .308 rifle that weighed about as much as an M16. And the M16 could be a lot lighter, too. Ever fired a Carbon-15? It's got some flaws, but the damn thing weighs just a hair under four pounds. That's impressive. I would venture to say that seven pounds, maybe eight, would be as high as you'd want to go for an individually issued rifle.
On the other hand, they DO issue 15 pound SAWs to individual troops, and expect them to walk just as far, so I have to wonder if the weight argument is really valid at all.
The United States never signed the Hague Treaty banning "dum dum bullets". But, like the land mine treaty, we follow it anyway, which is really stupid, because the effectiveness of .223 could really be improved with the use of hollow point rounds. (Not to mention 9x19mm...)
Not mentioning whether I agree or not, but Colonel Jeff Cooper seems to not like the .223 cartridge at all, referring to it as a poodle-shooter. If nothing else, he does have quite a bit of combat experience and it is food for thought.
Seems to me, that if we are, in fact, going to stick with .223 as a service cartridge, then perhaps a more modern rifle ought to be adopted. Sure, sure, if properly maintained and in good condition, and AR can be good enough, but I maintain that our troops deserve the BEST, not "good enough". Besides, we don't all have the benifit of a new out of the box M16. My unit still has old A1s. REALLY old. Hell, my rifle was DEADLINED last drill. But I digress.