The 300 Blackout AAC caliber round is one where the only part to change in the AR-15 Upper is changing to the 300 blackout barrel. All other parts are the same in the 300 blackout pistol and rifle. Along with, the standard 5.56 AR-15 Pistol and Rifle. This includes the bolt carrier group, upper, lower, magazine, stock, grip, etc. If purchasing a separate 300 Blackout upper it can be swapped with a 5.56 upper. Additionally, be careful not to mix up the ammo when changing from one upper to another. Due to one being a 30 caliber round and one a 22 round. The gas system on the 300 blackout is pistol length due to the short burn time. The pistol powder that makes up the 300 AAC round takes vs the 5.56 AR-15 Round.