These 30 round mags are not the cheap imitations sold at the gun shows but high quality mags, with magazine stops, matched to fit the Mac-10 9mm Single Feed Bolts. The feed lips are machined evenly, front of the feedlips 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 Zytel plastic mags. They last longer without wearing down the feed lips and they are notched for the magazine catch. These magazines perform the same if not better than plastic mags and most competitive shooters prefer steel. Also, if you are looking for the perfect pouches to carry these for competition or just shooting, our SMG pouches are a perfect fit for these magazines. e do a bit of work to these mags as well to get them to run. you can see a quick video on them at the range. go to u tube.com , search USMACHINEGUN.COM and you will see a few videos. If your gun uses the OEM Zytel mags, you have a single feed bolt/gun. **If you have questions about double feed and single feed magazines, Please call us.