supposedly it's mostly the pressure of the rounds with a very slight chamber difference but there are strong arguments that interchangeability is a myth and that it is not. I just either buy rifles chambered in 5.56 or 223 wylde and don't worry about it.
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
I never said half the stuff people said I did-Albert Einstein
You can't believe everything you read on the internet-Benjamin Franklin