Black Rain Ordnance Upper Receiver 5.56 Black. 56 Specifications for Black Rain Ordnance Upper Receiver 5.56: Manufacturer: Black Rain Ordnance Color: Black Finish: Anodized Caliber: 5.56x45mm NATO Gun Model: AR-15 Package Contents: Black Rain Ordnance Upper Receiver 5.56 What firearm models does the manufacturer say Black Rain Ordnance Upper Receiver 5.56 is compatible with? The manufacturer says that Black Rain Ordnance Upper Receiver 5.56 is compatible with AR-15Black Rain Ordnance BRO-SPEC15-URC: Upper Receiver 5.56 Black