I've heard that myth for ever that you can only fire a .223 out of a .556 but you cannot fire a .556 out of a .223 or such.
But this is bogus, they are the exact same round.
So people say that well it's the pressure, the pressure is higher in the .556 round than the .223 because of the smaller interior space, but this won't matter at all when firing your .556 in a .223 chamber.
There is absolutely no difference in the dimensions of the brass so unless you load a .223 quantity in a .556 casing you'll have no problem and even then at most you'll just risk blowing the casing which isn't usually a significant problem, persay.
But what buffoon is going to load that much powder in the .556 round? Come on.
Thread: Shooting .223 and .556 rounds