I just got around to asking my co-worker some questions today, which is why I'm resurrecting this thread.
Hawkmoon: "I was there in 1968, with the very first generation of M16s. You might ask the guy if he knows (or remembers) that the reason the M16 was changed from full-auto to 3-round burst fire was that it was basically uncontrollable in full-auto mode. If you didn't hit what you were aiming at with the first round, the following rounds mostly went cloud hunting."
I asked him about the three-burst fire, and he said that his M16 was full-auto, that his last real training with guns was in Viet Nam, and that any change must have come after 1972, when he left. He said that, in his training, he was taught to fire three to five round bursts, and that his instructor would give hell to anyone who fired more. He then said that the new recruits would often empty a full magazine at a target, and then have to reload, and that maybe that's why the military changed the rifle.
He also said that he kept all the rounds he was issued in magazines, rather than have any loose, that the army didn't issue 30-round magazines until some time after he'd been over there, and that he didn't trust their reliability.
He also said that, depending upon what he was doing, he would either carry his M16 only, or would also carry a shotgun, which he said was the best for going into the tall "elephant" grass, where an encounter might just be a matter of feet away.
Another thing he mentioned was sometimes carrying a Thompson, which he said he had purchased himself. He thought the Thompson was a good weapon, but felt that the M16 was more controllable under full-auto fire (my own experience runs contrary to that, but I shot my Thompson many times, and an M16 only once).
He also trained with the M60, the M207 grenade launcher (don't hold me to that numerical designation), and the M14. He said that the tree snipers favored the M14. He also described the various types of grenades: concussion, grenades loaded with buckshot, smoke, etc.
He's my age and, if he was drafted right out of high school, he would have gone in sometime in late 1969 or early 1970. As mentioned, he said he was there until 1972, which leaves another four years unaccounted for.
I'll gently press him on other points, but thought someone might like to respond to what he'd said today.