Before the internet 223 and 5.56 were interchangeable. While technically the 5.56 CAN be loaded slightly hotter, and has slightly looser tolerances in the chamber they are so close I don't see anyone ever having a serious issue. I've seen much greater variation in factory loads and rifles that were all chambered in 30-06. Heard of and seen more real problems with 30-06 rifles and ammo due to normal variations in manufacturing tolerances than I've seen between 223 and 5.56 rifles and ammo.
The most likely issue is that a semi auto with a 223 chamber might not function as reliable with hotter 5.56 ammo. Just like a Garand won't function with hotter 30-06 loads. But how many semi-auto rifles have 223 chambers.
You are kind of right.
In a bolt gun with factory ammo, you will never see the difference.
In an AR, if you load to max for a 5.56 chamber you will need to back off about a grain in a true .223 chamber, or you will be picking primers out of your trigger group.
