Fortis Manufacturing, Inc. Billet M4 Upper, Stripped, Fits AR15, 223 Remington, 556NATO, Black Color, Anodized Finish U-Billet-M4 The Fortis Billet Upper is a precision piece" machined from Billet 7075 T6 aluminum. Like Fortis' other products" they designed this upper with function as well as form in mind. Fortis shaved off the ounces where they could without sacrificing rigidity or strength. The end result is increased function and performance with the design aesthetics that Fortis is known for.