.300 AAC Blackout, or 300BLK, is the hot new 30 caliber round for the AR-15. It allows users of .223/5.56 AR-15s to utilize their existing mags, AR lowers and bolts when switching over to 300BLK. All you need is a new barrel, or to make things even easier, an entire new 300BLK upper. Another great benefit of the new 300BLK round is that you can use existing .223 brass you may already have. A member of r/guns posted this really cool picture of the process of reforming .223 brass to 300 AAC Blackout. You can also read up on the 300BLK round here.