You misunderstood me. I'm referring to a subsonic load...one that doesn't push the round to supersonic speed. A 5.56 needs velocity for accuracy and lethality. Slow it down and it's no better than a 22. As the other poster mentioned, the .300 blackout round was developed as a premium subsonic round (aka .300 WSPR) to fill this niche.
But to your point, yes, a 5.56/.223 running at 3,000 plus FPS delivers a devastating blow. It is a supremely lethal round out to 600 yd when running a full load (i.e. supersonic).