The AR57 Upper Receiver Assembly converts an AR-15 to shoot 5.7x28 FN ammunition. The unique upper design loads on top and accepts FN PS90 magazine (not included) and ejects downward through the AR-15 magwell. This upper is precision machined from a one piece aluminum forging. Forged from one 7075 T6 Mono bloc Aluminum, called a Kryptonite Mono bloc with a Black Melanite Treatment Finish.