I probably should stay out of this but the problem with the two rounds are that the .223 Rem. has a shorter throat or lead to the riflings. The .223 can be safely shot thru a rifle with a 5.56 NATO chamber. The 5.56 NATO round has a longer throat or lead, the rifles chamber in the neck area is actually longer than a .223 chamber. Thus when a 5.56 NATO round is chambered into a .223 Rem. chamber the bullet can engage the riflings before the bolt is closed, then in order to completely close the bolt, the bullet can be pushed into the case neck a little farther. When shoved in farther the bullet is solidly in the riflings and the bullet will take up more space inside the case and the pressure of the firing round could be dangerously elevated causing a rifle that could blow out thru the bolt. Hopefully the pic below works, it shows that the throat on the 5.556 NATO is .164 long and the throat on the .223 Rem. is .045. Without burying the bullet into the riflings the pressure of the 5.56 can be 15,000 psi higher than .223. A lot of folks have shot 5.56 NATO in .223 chambers without incident but they are at least sending out the invitation for it. Hope this clears up at least a little bit of the actual difference.