Like someone mentioned earlier, the constraints under which the DoD must work are different that what a hunter would need. If one could use hunting type ammunition in the military, then the whole "lethality" issue would probably not even exist. That being said, to make the 5.56 round as lethal as possible, while still remaining within the confines of the 'agreements' (Hague, Geneva, whichever...), things like high velocities and fragmentation need to be used. This gives rise to required barrel lengths for certain 'fragmentation ranges', etc. The MK262 round fired within its fragmentation range is likely to be more 'destructive', or 'lethal', than a non-expanding, non-fragmenting round under the same conditions.
Pistols are less effective stoppers than rifles, regardless of the whole .45 vs 9mm debate. With a pistol, shot placement is even more important. If limited to ball ammo in a pistol, the advantage of the higher firepower of the 9mm vs the higher energy of the .45 will have to be weighted against one another. More rounds, or bigger bullets? Both will penetrate, without expanding, since we are only using ball ammo. So, what's better? A greater number of .38 inch diameter (9mm) non-expanding bullets, or a lower number of .45 caliber non-expanding bullets? 'Tis the great question.
Bullet expansion was used to great effect before the advent of smokeless powder, when the bullets were made of lead. They expanded quite effectively. Then, when higher velocities required copper jackets over the lead bullets, they had to be redesigned to expand. Right around this time was when the whole "Hague / Geneva" happened, and, here we are! Weeeee