Build your own custom .458 Socom or .450 Bushmaster with an AR-15 Big Bore upper receiver from J-5 Tactical. Choose from stripped uppers, assembled upper receivers, barreled upper receivers and more. Check out J-5 Tactical Supply’s selection of Big Bore Upper Receivers and choose from products made by the world’s top manufacturers. All Big Bore Upper Receivers are made right here in the U.S.A.