No wonder your name is 2+2=5 you don't add things up right in your head. Those monkeys up there are not trained very well. You don't even give your backing for your BS. The 223 is not a particular deadly round. No more than many cartridges. The 7.62x39 has much more penetration, the 3006, is daughter the 308 are much more devastating than a 223. A 22lr can be very deadly that does not make it "ideal" for killing humans as you put it. Why is the 223/5.56 Ideal for killing? Its not. That is an arbitrary statement you can not back up because I doubt you know much about guns.
When the 5.56 was adopted by the US it was due to the idea that a lighter bullet could then allow a soldier to carry more. The most deadly caliber is useless in a fire fight when you are out of ammo.The Battle rifles of the day, the 303 British, the US 3006/308, the Russian 7.62 Russian were all large full on battle rifle ctgs. The guns that shot them were heavy, the M14, the Enfield, and the Mod 91 Russian were all long range battle rifles. The lessons learned in the seocnd world war were that a lighter less powerful rifle could be better in that most fire fights were done at shorter distances and the fire superiority theory. Enter the M16 the end result of the Armalite design. AR does not stand for assault rifle it stands for Armalite.
You don't know much about guns ammo or ballistics. There is no such thing and an "ideal gun" to kill people. If you want to shoot someone in a closet any long gun is almost a club you can not swing, A hand gun would be ideal, at 1500 yards the 223 would be like getting hit with a pellet gun. Now go look up coefficient and muzzle energy and how it drops.