The MPA magazine extenders allow you to convert a 10 round mag into a 14 round mag by simply replacing the floorplate of the magazine with the MPA magazine extender. These are magazine specific due to differences in the width at the base of the magazine. Accurate Mags have a slightly larger base and require the Accurate Mag specific extender. The American Rifle Company magazines will also require the Accurate Mag extender because of the wider base. These magazine extenders can also be used on 5 round magazines, turning them into 9 round magazines.