Don't get me started on M16s and Vietnam. For a guy who wasn't even alive, I'm full of opinions on this subject! For one, I don't believe the M14 should have been phased out so quickly. The M16 was clearly rushed into service, and the combination of misinformation about the rifle's maintenace needs and poor ammunition resulted in many unnecessary American deaths. Vets, am I on target here or just spouting off where I shouldn't?
Back to the original subject, I've got more experience with the 20-rounders than 30-rounders, because that's what we have at the Academy. There are always lots of problems with them (many are probably too worn) and we never load them to full capacity. Alibis during shooting strings are a very regular occurrence. I wouldn't trust my life with the metal 20-rounders we have - on the other hand I find the 30 round Thermolds to be great.
As for the semi auto thing, my best friend was a Marine infantryman and to this day he shudders when we talk of M16s on full auto. They absolutely DRILL semi auto into them. No one, and I mean no one dares fire an M16A2 on burst mode in the Corps. Machine guns are for full auto and rifles are for aimed single shots (the way it should be IMO).
In Vietnam the infantry often traded aimed fire for saturating the bush ahead with spray & pray, full auto, rock & roll mayhem. I can't remember the exact statistics but the doctrine meant that tens of thousands of rounds were fired to inflict a single casualty. I'd like to see similar statistics for rounds fired vs. casualties inflicted for WWII and Korea; I imagine we were a little more efficient then.