Fostech Ultra-Lite Castle Nut Black. Fostech machined these AR15 Lower Parts from their proprietary lightweight material. Swapping in this Fostech 2505 Lightweight Castle Nut will reduce your build by 0.34oz compared to a standard castle nut. Save weight and keep the strength with the Fostech Ultra-Light Castle Nut. Specifications for Fostech Ultra-Lite Castle Nut: Manufacturer: Fostech Gun Model: AR-15 Color: Black Fabric/Material: Aluminum Features of Fostech Ultra-Lite Castle Nut Lightweight Machined Staking Notches Package Contents: Fostech Ultra-Lite Castle Nut What firearm models does the manufacturer say Fostech Ultra-Lite Castle Nut is compatible with? The manufacturer suggests that Fostech Ultra-Lite Castle Nut is compatible with AR-15Fostech 2505: Ultra-Lite Castle Nut Black