Fortis Manufacturing, Inc. Castle Nut and End Plate, Black, Anodized Finish QD END-CN-COMBO 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. Fortis QD End Plate features the quick detach swivel socket and is designed for carbine style buffer tubes for the AR15 & 308 platforms.