I wouldn't recommend shooting 5.56 in .223. 5.56 is loaded to a higher pressure and could cause problems. I hate hearing supposed gun experts telling others it's ok that they are the same. Example. My younger cousin and I went into a gun store in his home town on a lunch break from deer hunting.He's been wanting an ar since he shot mine. He asked the dealer if the one he had was 5.56 or .223 since he has a bunch of 5.56 that was given to him. The dealer stated that they we're the same. I told him he was wrong and he started rambling on about military and other stuff. Asked if I was in the service. I said no I just like my ar's and didn't want to see my cousin get hurt or tear up his rifle. We didn't end up buying anything from him. He did order a nice bushmaster the next week though.