These 30 round mags for the Mac-10 9mm Single feed bolt, have been limited to 10 rounds with a rivet stop. Magazine has an external mag stop, to prevent over insertion of the magazine into the mag well. 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. 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.