Just out of curiosity, what's your opinion about the M-16 rifle and the 5.56mm round?
I have absolutely no reservations about the 5.56mm AR15/M16 system - it has flaws, but it's the best rifle of its type available today.
As to 556mm, there are a host of issues associated with lethality and the reduced velocities from the popular sub-16" barrels in use today, especially when combined with the steel core 62gr M855 ammo used by NATO armies. With the right ammo, from a rifle length barrel, 556mm is tremendously lethal. Anybody that believes otherwise should closely review the 2002 DC sniper case.
Ultimately, shot placement and SPEED are what wins gunfights- two critical capabilities that the 556mm M16 system excels at. These elements also lead militaries and law enforcement agencies to choose 556mm over larger calibers.
Finally, the rifles popularity speaks for itself. While the "poodle shooter" crowd wants you to believe the M16 is somewhow forced on unwilling troops, the truth is that skilled warriors, given the choice of any weapon in any caliber, will more often than not choose an M16 variant.