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.
all guns lost in a tragic smelting accident.
I never said half the crap people said I did-Albert Einstein
You can't believe everything you read on the internet-Benjamin Franklin
Bean counters told me I couldn't fire a man for being in a wheelchair, did it anyway. Ramps are expensive.-Cave Johnson.