Luth-AR Lock Ring/Castle Nut Carbine MBA-3 & 4 Car Buff Kits Only Black. 5 oz Features of Luth-AR Carbine Lock Ring/Castle Nut Threaded Steel ring used to retain the carbine buffer tube to the lower receiver Package Contents: Luth-AR Carbine Lock Ring/Castle Nut What firearm models does the manufacturer say Luth-AR Carbine Lock Ring/Castle Nut is compatible with? The manufacturer says that Luth-AR Carbine Lock Ring/Castle Nut is compatible with AR-15 AR-10Luth-AR CS-02: Lock Ring/Castle Nut Carbine MBA-3 & 4 Car Buff Kits Only Black