Can someone please tell me the difference between .223, 5.56 and 5.56 NATO? I just put my first AR together and I'm trying to learn as much as I can without moving to fast on this. I got everything (in pieces) but the lower receiver on trade from a buddy. So, I got to put it together which went great and knowing how the pieces work together is a bonus too. Anyway, thanks in advance for any help.