The M885 62 gr 5.56x45mm is an adequate battlefield perfomer. The new all copper M885A1 is better, and performance reports of the Mk 262 72 gr round (used by certain SOCOM units) are even more promising.
Is the 5.56x45mm the best round possible? Probably not - but does hypothetical advantage of a new caliber outweigh the costs of fielding it? So far, no.
The bottom line real-world performance of a rifle cartridge in combat is more a function of the weapon system (sighting system, barrell length, etc.) and of the training of the operator than it is of any particular characteristic of the round itself.
If you put enough 62 gr 5.56x45 center mass, people will fall down and stop bothering you. The only place where anyone should care about 1 shot performance of a particular round is in the sniper/DM world, and in most cases they are using a 7.62x51mm/.300 Win Mag/.338 Lapua Magnum etc. where one round is usually plenty.
The whole "one-shot, one-kill" mantra is BS for a typical rifle
man. The key to success is rapid, well aimed shots into a target until it falls down. Particulalry in a CQB enviornment, I'm not going to fire one shot and see what happens. I'm going to bang out controlled pairs (or triples...) until I get the desired effect.
Sure there are circumstances where 7.62mm or 6.8mm or 6.5mm might give you an edge - better range, better penetration, etc. But there are always trade offs. 30 rounds of 5.56mm weighs 1.07lbs. I typically carried a basic load of 9 30 round magazines (8 on me, 1 in the weapon) - thats a 270 round combat load. 9 mags x 1.07 lb = 9.6 lbs of ammo. A magazine of 20 rounds 7.62x51mm (for an M-14) weighs 1.6 pounds. So a 10 pound load of 7.62x51mm only gets me 6 magazines - 120 rounds. I'd rather have more bullets. To carry aproximately the same load out I would have to carry 13 magazines at 20.8 lbs. Twice the weight.
I don't think US Army is going to change rifle caliber until some more significant change in technology matures - plastic cased telescoping ammo, etc. that deals with the weight issue.
As far as full-auto - I think in a rifle it is rarely necessary, nor particulary useful most of the time. That being said, I think the 3-round burst setting on the M16A/A4 & M4 is pointless. Too much for when semi will do, and not enough when semi isn't enough. Good training will determine that Soldiers employ full auto properly and in the correct circumstances. The US Army is moving that direction by converting more M4's into the M4A1 "special ops" configuration, which has full-auto.
Adrian