[smokingfrog]

.223 is basically a hopped up varmint cartridge.

I don’t know how much truth there is to it, but have been told more than once that when you wound someone on the battlefield, it often takes a couple more soldiers out of the fight trying to save their wounded comrade. So a cartridge that would be nearly 100% fatal would not necessarily be such a huge advantage.

[Hillarys Gate Cult]

That is true but only if that particular side cares about their wounded. In the Korean war, the Chinese didn’t.

[bill1952]

That depends on the mission profile.
A more lethal weap will tap you farther out, preventing you from coming close enough to be a threat.
In anything other than close up fighting think about range

[Farmer Dean]

The .223 is a mid-range varmint cartridge,serious varmint hunters use a .22-250 or something similar.

[GingisK]

... it often takes a couple more soldiers out of the fight trying to save their wounded comrade...

It depends on who you are fighting. Some foes care little for fallen comrades.