It was also due to the tight tolerances. By way of comparison, I've read that you can drop an AK-47 into the mud and do just about anything short of jamming stuff in the barrel and it'll keep firing. Compared to the M-16, the AK-47 was crudely made and simple, but it was also very reliable and accurate enough up to a couple hundred meters, more than enough range in the jungle.

Some Vietnam vets I knew from my Army days said that they got so tired of the M-16 jamming on them that they got themselves an AK-47 instead. One problem with that was that, when you fired it, you tended to attract too much attention. Other Americans might confuse you with the enemy (or want your weapon for themselves) and shoot at you in addition to the enemy returning fire. Bummer.