You can't chamber 300 BLK in a 5.56 barrel. You can chamber 5.56 in a .300BLK barrel.How about the 300 Blackout?
I can't bring myself to own something I could accidentally chamber in my 5.56 rifle and blow myself up with...I'm not that dumb, nor am I that absentminded, but I just don't even want the chance...