5.56 is the nato designation for .223 its the same or at least i know no diff. i own several ar's and have shot truck loads of nato and .223 ive never had a single issue. ALTHOUGH AS ANY AUTO LOADER they can be picky eaters! were you using reloads? swollen cases? or cheap ammo brown bear wolf etc? metal case not brass id try some remington or federal {federal green tips are my fav.] hope these ideas help?