Heckler and Koch SL8/USC Aluminum Picatinny Forearm Rail. Machined from quality aluminum, this forearm Picatinny rail section is designed for the HK USC and SL8 model rifles. This rail section installs onto the side of the receiver and provides roughly 4.5″ of Picatinny rail section to mount accessories. This factory-original rail section includes two mounting screws.