Frankly, I think a lot of people who seem to know what they're talking about ... haven't a clue.
Speaking specifically about the AR16 and the M14: I was there.
In 1969 in III corps (RSVN), First Infantry Division, we actively searched for M14 rifles to perform a very important mission: arm "Point Men" to defend themselves.
Sure, the M16 was great for laying down a volume-of-fire, but for the guys who stalked the jungle one step at a time, trying to watch for trip wires, bunkers, snipers, spider holes, and ambushes ... it wasn't much fun to multi-task when the .556 cartridge was not worth a damn when you're shooting through dense foliage at an L-shaped ambush. This was the duty of our point men; all volunteers, and among the bravest of us.
True fact, the M16 rounds were great for wounding the enemy, but not so great trying to beat a path through the cover and concealment (foliage) which protected the ambushers.
We looked for M14's so our guys out on the bleeding edge might have a small edge of their own; the enemy was protected against M16, which couldn't reliably break through the cover they used to protect themselves. The NVA and VC didn't need a lot of cover when first-reaction rounds from American troops came their way.
And our point men knew it. So we gave them M14s; 50 pounds of hardball that would go through leaves, trees and vietnamese. (I exaggerate the weight of the M14; but if you carried one, all day and every day ... when your day began it was lighter than a feather; by the end of the day, it was heavier than a mountain.)
Maybe the M14 didn't have QUITE the penetrating power that we told them (our point men) it did, but nobody had any doubts that it was better than trying to back out of an ambush zone with nothing better than an M16 to make Charley duck and cover. And we willingly lied to our point men; whatever it took to convince someone to lead us through the jungle.
The thing is ... our brave men who knew they were expendable booby-trap trippers also knew that we equipped them with the finest infantry rifle ever invented: the M14.
Nobody else in the company/battalion/division had The Hog. It weighed too much, it carried awkwardly (you couldn't grip one by the 'center of balance' because it didn't have one!), and it was nearly impossible to get magazines, Level one maintenance, or even ammunition. But these men were the eyes and ears of the squad/platoon/company/battalion. They were BY GOD deserving of the best weapon available!
Charley opens up on you? Shred the jungle with 20 rounds of 7.62 and let their mothers weep.
Think Charley is thinking about opening up on you? Recon by fire with heavy metal which has been designed to push through leaves, trees and vietnamese. Don't worry about the Ammo, Private; there's lots more where that came from
(Truth is, it was almost impossible to get M14 rifles, magazines or ammunition. Every round the point-men fired meant that the company had to take up a collect to bribe some REMF supply officer to order in a shipment. But we didn't tell them that. Until the Patrol Leader could bribe a case of 7.62 from Saigon with six bottles of Jack Daniels and introduction to a new Saigon whore, whatever those guys carried on their web-belts and Alice packs was all the 7.62 ammo in the world.)
YOU CAN MAKE your own decision whether Point Men in Vietnam were the most naive, or most ignorant, or the bravest men in that hideous war. What you cannot deny is that many more of us would have come home in a box except for their willingness to learn a trade which no man should become 'good at': willfully walking into a trap, for the good of his brothers.
We gave him the best weapon we had ... the M14 7.62mm select fire "Assault Rifle" (Ptui the term! but my understanding is that it replaced the B.A.R. --- Browning Automatic Rifle) and wished them luck, and then cowered behind them in hopes that their MUCH MORE POWERFUL weapon could confuse and thin the enemy when they attacked; at least long enough that the rest of us could find cover and concealment to return a 'volume of fire' from our Poodle Shooters (which worked as well as you can expect from the lowest bidder).
Yes, this is nostalgia. The thing is, nostalgia is the luxury of someone whose life, and the lives of his friends, have been saved by a gullible infantryman who busted up an ambush which could have / would have killed all, or most of them.
So AC97: I recognize that you are speaking as a person who looks at the question objectively.
Also, I perceive that you haven't lived your long life because someone who was armed with an M14 stood his ground for his brothers.
But if you have never had the luxury of a long life because of a brave man with an M14 who broke up an ambush purely because of his more-effective arms, I respectfully request you to STFU!