Fortis Manufacturing, Inc. Castle Nut, Black, Anodized Finish, Mil Spec CN-Stand The Fortis Standard Castle Nut is manufactured from a solid piece of billet aircraft grade aluminum that offers a significant weight reduction to your castle nut. If you're trying to save ounces" this is a great way" but without sacrificing durability. The castle nut will fit onto mil-spec buffer tubes and quickly upgrade the form and function of your build.