This is Colt AR-15 SP1 model that is a registered receiver. This was a semi auto at one point prior to 1986 and then registered as a transferable machine gun. The beauty of a registered received gun is that you can put any upper on it and also change calibers. As you can see by the pictures the gun is in fantastic shape and functions flawlessly. We put a PWS Mk109 300 BLK upper on there, but you can put any 5.56mm upper on it !