I have two Son's who fought in both of those theaters, Marine and ANG!
First of all, their are some stringent engagement and arm's guidelines that specify what weaponry, and ammunition can be used or carried by a US soldier. So no, you are mistaken, it doesn't work like that at all. In fact, both of my Son's told me about soldier's who got CM'ed just for modifying their ammunition, drilling a HP into it. Unless they have an AK slung over their shoulder for any reason, other than having confiscated it from the enemy, would get them into serious, no, make that very serious trouble. And had better be made safe, not locked and cocked.
While in a very hot zone, one of my boys fellow soldier's managed to drop his M4, it actually got ripped out of his hands by a big branch or something while driving down a very narrow and heavily brushed in levy, almost pulled him out of the HV, I got to see the video. Of course, several other soldier's had to go with that soldier to retrieve it after it had been grabbed up by a local, they saw him pick it up and run with it. So my Son gave him his M4, and everyone left him in the HV alone with only his 9mm, and the 50 cal. which was useless, due to the heavy brush all over, and around the top of the HV. A group of the thought to be "good guy locals" were helping to defend his position, being that he was left all alone and all, and offered him an AK, which he had to decline the offer by rules of engagement stuff. BTW, they got that M4 back real quick and in a hurry.
As for reliability of the M4's and the other weapons, our guys are given the best of tools to do their jobs. My boys told me that before a mission, they would go out and run a few magazines through their weapons, just to make sure everything is running good, 50 cal. included, and rarely do they have problems, unless it's their own fault, in which case they would get a butt chewing from the other brother's, they take it very serious, of course.
GS