A belt fed AR seems to be as utilitarian to me as a dog with a kickstand.
I'd leave these things to Hollywood.
Carbines and "assault rifles" are particularly ill-suited to be used as crew-served weapons, and if you have a belt, you've really got a crew-served weapon. The barrels are too light and not easily swapped out. You NEED spare barrels on a belt fed weapon unless you want your barrel to glow white hot, melt and have rounds blow through the top of the barrel. Even on a squad automatic, you should be able to change barrels (my one complaint about the BAR).
You might also cook off rounds from the chamber getting too hot. That's bad news! The AR fires from a closed bolt, whereas belt-fed MGs (and even most SMGs) as a rule fire from an open bolt to allow for cooling.
And if you're not firing enough rounds to require you to swap barrels, then you're not firing enough rounds to need a belt. Just change magazines. No belts to get full of mud and crud. Makes the weapon lighter, too, to not have a 7 pound belt hanging on there.
Just my personal opinion.
Semper fi.
Bruegger out.
