BLACK CREEK LABS MRX BISON MAGAZINE - .223 / 5.56, 20-ROUNDBlack Creek Labs MRX Bison Magazine . 223 5. 56, 20 Round The MRX MAG is a double stacked, straight magazine made from a proprietary blend of polymer. The MRX MAG has extended length feed lips and a no tilt follower design that allows ultra smooth and reliable feeding. The MRX MAG uses follower stop ledges which keeps the follower from topping out, which allows the MRX Supershort bolt to be manually pushed forward when the magazine is empty and still