This is really strange. Just a couple weeks agoI got a reply from Ruger over this same question.I had written Ruger a letter asking if the 5.56mm was safe to shoot in a gun chambered for the 223 Rem.I askedabout using it in the Mini 14's and the M-77.They said that their rifles are chambered for the 5.56mm and either could safely be fired it these rifles.A couple years ago I bought two cases of Lake City 5.56mm Ball.That'show this question came about. Some said it's safe other said don't shoot it. Ruger tells me it's safe to fire in their rifles, I assume it's safe in all the one's chambered for 223.I have a Winchester Coyote in 223 and I will run the 5.56 through it.
Before hearing back from Ruger I asked a old friend who was a gunsmith before retiring. He was also a rifle builder. I asked him this same question and he said they were the same cartridge and interchangeable.The 5.56mm was a military designation for that round.
That'sproof enoughfor me.