The extent of my knowledge of Vietnam-era weaponry extends only to the History channel, so take it as you will, but per a couple shows I've seen, the main gripe with the M16 wasn't the stopping power, it was its tendancy to jam easily. According to one of these shows, troops in the field often favored picking up AKs dropped by Charlie troops, but had to limit the practice due to suffering friendly fire, since the AK had a very distinctive sound.