I've noticed having to rezero optics, slightly, between the two. The real reason to use .223 over 5.56 is cost and types of loadings. 223 has many different bullet types for hunting and precision. 5.56 not so much. Shooting one over the other probably won't wear out the gun in any noticeable way. I've never seen any proof that would indicate otherwise. Accuracy is negligible when using standard brass cased ammo.