They start out life as two magazines and are welded together to create a monster 50 round magazine. They are not the cheap imitations but high-quality mags, matched to fit only the Mac-10 9mm Single Feed Bolt. Each mag is disassembled, feed lips are machined evenly, front of the feed lips are cut back similar to a zytel for fit, deburred on the inside, the followers are checked for smooth movement, each spring is inspected and oiled. They are parkerized steel mags that outlast the zytle plastic mags. They last longer without wearing down the feedlips and they are notched for the magazine catch. They perform the same if not better than plastic mags and most competitive shooters prefer steel. Will not work with regular double feed bolts. FOR SINGLE FEED BOLTS ONLY. Also has a mag stop welded on front to prevent over insertion of the magazine into the mag well.