the muzzle energy on a .223/5.56 is around 1300 pounds max, the .308 is 2600, hell, even the .22-250 is only 1700 max, which is also why you cant hunt large game with it...

its not effective enough...

and as i said, its not the gun doing the damage, its the round, its fired from a barrel, that happens to be attached to a gun...

a .308 will cause far more damage than a .223 ever will, but that is because of speed and weight, not just speed....