Let's see, so far only one guy who carried the AK-47 in preference to an issue weapon, and that was 37+ years ago. So where are all the guys who ditched their weapons and picked up enemy AKs? According to the internet commandos and mall ninjas, entire divisions have put their M16s/ M4s away in the arms rooms and grabbed enemy weapons and ammo that they picked up. Here's a little inside info for those with no experience in small-unit level tactics: You don't always hold your fire until you have a clean shot. If I am engaging someone who is barricaded behind something that a .50 cal is not going to go through, I'm still going to lay down suppressive fire on the barricade, probably about one round per second. Why? So the BG keeps his head down and does not engage my wingman while he bounds forward to a closer firing position. I expect him to do the same when I bound past him closer to the BG. This will continue until we get close enough to flank around the barrier. So why should I carry heavier ammo? I won't be able to carry as much 7.62 as I carry in 5.56 without overexerting myself. As a matter of fact, eight M16 magazines, two M9 magazines, a GPS, an infrared beacon, and first aid gear completely filled up the front of my Interceptor body armor, so I didn't have any extra room for AK mags. If I had mounted them lower on my vest than the M16 mags to make up for the space, they would have interfered with my legs. So, by accepting lighter and smaller rounds, I can carry more ammo, allowing me to perform my tactics longer than I would with less, but possibly (nothing is a definite) more effective ammo.
Jonathon, if you don't want to carry a plastic gun or 5.56 ammo, don't join the military. It's an all-volunteer force, nobody is going to make you join.