Mike, the barrel is stamped 5.56 which is loaded slightly hotter than .223 which both can be fired just fine. A very informative video of the differences of each is here : youtube.com/watch?v=VCS4fXFmCyA

its chambered and proof tested for the "hotter" 5.56 NATO round so it is safe with the .223 .The only way to see which is more accurate is to try them both in as many different brands and bullet weight as you can

Mike, Every firearm is a little different in its ammunition appetite. Get some 223 rounds and 5.56 rounds. Different weights and manufacturers and you will find out. Popular brands like Federal, Fiocchi, Hornady, Winchester, or Remington would be a good start. Most ARs are super accurate and eat everything no matter what you put through them. Chambering is basically the same for 5.56 and 223 if its stamped 5.56 or 5.56/223. If barrel says 223 only then chamber is not designed to handle additional pressures of a 5.56 round.