Comparing 5.56 (.223 Rem) and the 7.62x51 (.308 Win). The .308 will most likely have the most energy during any part of the bullets flight.
7.62x39 is a Russian round that is used in the AK47, SKS, and Ruger Mini-30.
Looking at my Lyman Reloading Handbook:
<LI>.223 Rem has an average muzzle velocity of about 3000 fps (using data for a 55gn bullet and max powder charge). Since energy is velocity X weight: Energy(.223)=165,000 ft-gn per sec
<LI>.308 Win has an average muzzle velocity of about 2800 fps (data for 150gn bullet and max powder charge). Energy(.308)=420,000 ft-gn per sec.
<LI>7.62x39 has an average muzzle velocity of about 2400 fps (using 125gn - max powder charge). Energy(7.62x39)= 300,000 ft-gn per sec.
Ignoring bullet diameter, bullet tumbling, bullet revolutions due to twist rate, etc. I would surmise that the 7.62x39 has superior energy to the .223. I'm no ballistics expert, so take these "numbers" with a grain of salt.
As for which round is better in combat, that all depends on the weapon, engagement distances, obstructions, background friendlies, etc. Depending on the situation, a subsonic suppressed 22LR may be the best choice for combat.
"Ray guns don't vaporize Zorbonians, Zorbonians vaporize Zorbonians" The Far Side
[This message has been edited by jcoyoung (edited February 26, 2000).]