I'm halfway through season one, and so far I've seen two US soldiers carrying AK 47's. Am I missing something? Or was someone not getting their guns straight? They looked like spec ops soldiers so there may be some leeway, but even then from what I read, it was never a good idea to rely on enemy weapons, mainly because you couldn't get ammo or parts.
A handful of US servicemen (Army, Marines, etc.) did on occasion used captured AK-47 automatic rifles taken from dead Viet Cong/North Vietnamese troops because Ak-47s were to be much better use then the standard military type M-1 or M-16 automatic rifles. AK-47's almost never jammed and they could function in hot and humid climates of Vietnam. In contrast to other Russian and Chinese made automatic and semi-automatic weapons, AK-47s were the weapon that could still function in the worst climate environments. Despite the fact that they were heavier and had a less rate of fire and less accurate range then M-16s, AK-47s were in favor of being the weapon needed for all cases in combat. AK-47s and their affiliate copies will continue to serve into the 21st century, and beyond.
AK47 ammo shouldn't have been hard to find in a country full of Commies...
I believe another reason for carrying AK47s was that the sound was easily distinguishable from that of an M16, so it wouldn't give them away so easily in hostile areas.