I'd hoped my explanation made that clear, lol
I guess it should also be noted that the 5.56 we buy as "civilians" is not military ammunition. We stopped using m193 a very long time ago and m855 was replaced with a steel core variant.
You can use .223 in a 5.56 barrel for the same reasoning above. Its not ideal from an accuracy standpoint, but the chamber pressures will be lowest in that situation. This is why intermediate chamber sizes were created such as the wylde. Military chambers are notorious for being bigger than needed. Same goes for .308 and 7.62.