The Wylde chamber is essentially a NATO chamber with the cylindrical portion of the throat resized to .224" . It will digest either 223 Rem or 5.56 NATO ammunition with equal aplomb. For whatever reason, it will shoot very well with about any load. I have chambered heavy barreled bolt guns to 223 Wylde and they are amazingly accurate. To confuse the issue a bit, there are at least two official "5.56mm NATO" chamber print variations. Very minor differences between them, but they do exist. Shoot it and enjoy, you have a very fine rifle.