I'm not sure what the OP is really trying to say. Are you saying that 223 is more accurate, or slower velocity, or both?
If I am going hunting for deer lets say, then ok, maybe I'm picky enough to get a good 223, wooden stocked rifle. If I'm worried about defense, I'm still going 556. The distances are within 100Yds and the 1/4 MOA difference is immaterial. With a 556 barrel you can use both. Win win...