Apples to Apples, tacdriver.
Let's avoid blanket statements about .223
What kind of round (55gr? 60gr? 69gr?)
What's the barrel length? Is the twist rate appropriate for the ammo? (not a terminal ballistics issue, but ammo that tumbles before it hits the target doesn't help you stop badguys)
What's the range?
Terminal ballistics is not as simple as "what caliber is it?"
Yes, the US military has made some changes recently to improve "badguy-stopping" performance.
Do a bit of looking around before you decide that you MUST go to 7.62X39 instead of 5.56 in order to stop badguys. Keep in mind that NATO is not the only military alliance that switched to a small caliber after using a 7.62mm
Most of the issues with decreased stopping power in 5.56mm are related to FMJ rounds dropping below the 2700FPS speed level...which is how fast they need to be going in order to reliably yaw and then fragment at the ferrules on impact. From what I've read, there seem to be two fixes to this:
#1. Make sure that your bullet is going more than 2700 FPS when it impacts (don't combine heavy bullets and short barrels and long ranges)
or
#2. Don't fire FMJ...use a TAP/JHP/Nosler/WYHD round (WYHD=whatever your heart desires)
A quick look at the sites belonging to Federal, Hornady, and Black Hills will show a variety of bullets that will work well out of an 5.56mm AR. No, they're not going to be as cheap as surplus 7.62x39. But they don't have corrosive primers either. For my money corrosive primers + direct gas impingement system = not such a great idea.
I'm not slamming 7.62X39. It's a perfectly serviceable round. I'm simply saying that 5.56mm will also work just fine if you do your part when it comes to selecting ammo and rifle.