I remember guys arguing about this back in the late 70's and early 80's, when I owned a Colt HBAR. Back then, everybody "knew" that you could shoot either round in barrels marked with either designation. Only a few professional worriers were banging the drum about how dangerous this could be; the rest of us just shot whichever ammo we wanted in those guns.
Here we are, almost 50 years later, and those worriers...or maybe their grown-up kids...are still telling us how dangerous it is. But...where are all the disasters? Has any factory-made rifle shooting factory loads or milsurp actually blown up? Based upon the preponderance of people doing this, we should have graveyards full of horror stories; where are they?
I'd bet that today, many or most of the rifles that are marked .223 are done that way only as a workaround for legislation that limits the sale or use of "military" ammo in certain areas or countries. Frankly, I don't believe for a moment that Ruger (as a random example) chambers the barrels on a .223 Ruger Predator any differently than the ones on 5.56 Ranch Rifles. Can't say for sure, and I'm happy to be proved wrong...key word there is proved, as opposed to just being yelled at by fear-mongers.
I wonder if any of those fearful people also blindly obey the warning that is seen in many new firearm manuals...you know, the one that says you should never use reloaded ammo in their guns...?