Hi all, I'm putting together my first AR and came across this conundrum, which I'm assuming is only a mystery to me since I can't find hardly anything on it. So the .223 and the 5.56 NATO seem to be the same round, and they look pretty much identical, but I read on this AR-15 upper receivers site that the 5.56 can take a .223, but not the other way around. Do I need to buy components that are made for 5.56 if I want my rifle to be more versatile, or is this a misconception? Are the two completely interchangeable? Thanks.