A friend of mine asked me for some help the other day concerning cartridges that were hard to extract and showing other pressure problems like flattened primers, even a pierced primer. He was shooting new factory ammo out of an older bolt rifle that he'd just bought. I told him to bring me a bunch of the brass, even the ones that showed no pressure signs. When I got them I knew right away what it was.
"You're shooting 5.56 out of a rifle chambered for 223."
He looked at me like I was stupid. "They're the same, one's just metric because it's a NATO round."
"Who told you that?"
"Everybody knows that" he said, apparently still wondering if I was fucking with him or not.
"Look, the bullet is the same but the cases are slightly different. The case thickness at the neck of a 5.56 is .002" thicker than a 223 and the headspacing as well as the throat on a 223 rifle is different than on a 5.56 rifle. And on top of all that, the 5.56 is loaded to just a little higher pressure that really ain't that big a deal until you figure in the headspacing and shorter throat in the 223. That's what's giving you your pressure problems. That's really only a big deal in older rifles - most of the newer ones accept both, but not the older ones."
He wandered off and I still think he doesn't believe me.
Now, after all that, I have never in my entire life heard of a 223 rifle blowing up or even being damaged by firing 5.56 out of it but as a reloader, if I were to see those signs on my loads, I'd be backing that fucking powder charge down. That shit ain't healthy.