This barrel nut is designed to work with our COP monolithic upper receiver. It will not work with a standard upper receiver. Note: This product is already included with our M4E1 Enhanced Upper Receiver and COP Upper Receivers. It is not necessary to be purchased separately. This individual product is available for customers who have lost or damaged their existing barrel nut. Specifications for Aero Precision M4E1 / C.O.P. Barrel Nut: Manufacturer: Aero Precision Condition: New Color: Black Quantity: 1 Weight: 1.2 oz Country of Origin: USA Package Contents: Aero Precision M4E1 / C.O.P. Barrel Nut