The desert wars have shown the complete indaequacy of the M-16 and its various models when the terrain is flat and not wooded.
In those cases, a round with good range and ballistics is needed. Few are the equal of the 308 NATO round for known, predictable ballistics.
I'm pretty sure the soldiers WISH they all had FN-FALs or M-14s, whichever the individual soldier prefers, but the "powers that be" insist on small-caliber rifles in deserts. Who can say why?
A 50-caliber sniper rifle can tickle your fancy at 1000 yards and more. The 308 NATO can do the same out to 800 yards or slightly more.
The 5.56mm cartridge is pretty well done at 300 yards, despite being "officially" ranged at 450 m. Heck, at 450 m, if you hit someone, you might just piss them off instead of generating a kill.
Just MY opinion. I could be wrong, but I KNOW how hard a 308 NATO round hits comapred with the 5.56 mm cartridge.