XTS Castle Nut Black. A simple press of the Vipper in combination with the steel Castle Nut forms a fully functional muzzle brake. Vippers have been used in military sniper rifles for decades. The combination of the muzzle brake and a properly fitted castle nut keeps the muzzle brake and barrel in place making it difficult to remove. XTS machined the castle nut from heat treated steel alloy and it is MIL-STD Manganese Plown. It can be re-installed in seconds using the provided heat wrenches. Specifications for XTS Castle Nut: Manufacturer: XTS Color: Black Features of XTS Viper Muzzle Brake for AR-15 Vipper Muzzle Brake Package Contents: XTS Castle Nut XTS CN: Castle Nut Black