Shop › CMMG AR15 BUFFER TUBE CASTLE NUT CMMG AR15 BUFFER TUBE CASTLE NUT $8.95 CMMG Receiver Extension Nut, Castle Nut for Buffer Tube, Fits AR-15, Matte Finish, Black 55CA9CB Quantity: Add To Cart Added! CMMG Receiver Extension Nut, Castle Nut for Buffer Tube, Fits AR-15, Matte Finish, Black 55CA9CB