BC-10 .308 Upper 18" Parkerized Heavy Barrel 1: 10 Twist Mid-Length Gas System 15" MLOK Split Rail - $261.50 This BCA AR-10 complete .308 rifle length upper has a 18" parkerized Heavy barrel, and features a 1: 10 twist rate, with a mid-length gas system. It includes the new 15" MLOK split rail, an