I'm sayin that you're not supposed to fire 5.56 in a rifle chambered in .223 but you can fire .223 in one chambered in 5.56. The reason being that the 5.56 has more powder and runs at higher pressures.
He is saying that the 5.56 has a thicker case and therefore should hold less powder.
Pressure difference is insignificant.
NATO brass may be thicker but you have to measure the volume of water each case holds to know if it holds more or less powder.
It's about the leade, or the amount of free bore from the tip of the bullet to the beginning of the rifling. "Theoretically" a longer/heavier 5.56 round could possible be too long for a .223 chamber. If the bullet is jammed into the rifling when chambered it may result in a over pressure when fired.
People have been shooting 5.56 in .223's for years and I have not found any documented cases of a failure from this.
Isn't it true that you can fire a 7.62x51 in a .308, but you shouldn't fire a .308 in a 7.62x51?
NO. There is a very small difference in max head space between the two chambers, .002-.004". Either chamber well fire either round. Pressure differences are insignificant.
Read FALphil's excellent paper on the difference, it's a sticky here someplace.