the 5.56mm Nato may not have the penetration that the 7.62mmX39 does but the bullet does tumble much earlier then the 7.62, thus causing much more tissue damage.Fighting Taliban in Afghan, who for the most part probably aren't wearing body armor, it'll incapacitate them much better then the AK round would.
Yes, and no. Due to the shortening of the M16 barrel - for variants, like the M-4 - the tumbling does not occur as it should. In addition, the velocity loss, also due to the shorten barrels, means the Afghans do not have to wear body armor, but just be far enough away. And, due to the velocity loss, the round does not fragment, as designed, at far ranges. Right now the major issue is snipers in Afghanistan.
An old bolt action rifle in a heavy caliber, like 7.62x54r, or 30.06, etc. out range the rifles the standard US Soldier would normally carry. This is why the US Mil is sending M-14 variant sniper rifles and (supposedly) issuing them at the platoon level.
Remember, the 5.56 Nato round is a very small bullet. Tumbling and fragmentation are the keys to getting stopping power in such a small round. It is a 55-62 grain bullet. Roughly 1/3 the size of the 7.62Nato.
Even the Wiki points to stopping power concerns:
http://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO In comparison, the AK-47 shoots a 7.62x39 round, 123-154 grain bullet a x2 to x3 weight difference. It transfers 2x the amount of energy.
However, the 7.62x39 round does not tumble nor fragment well, which can lead to stopping power issues. Newer ammo corrects this a bit by design.
http://en.wikipedia.org/wiki/7.62%C3%9739mm