Many manufacturers have begun making lower receivers built to accept pistol-caliber magazines for AR pistol builds. For an AR 9 you have 2 options for a lower, either a dedicated lower that accepts GLOCK or Colt mags, or a standard AR lower with a magazine adapter to hold GLOCK or Colt mags. all the lower parts can be the same as a regular AR in this case. Compatibility, Yipee!! Part of the enjoyment of owning an AR 15 for many people comes from purchasing a stripped lower, parts, and the required tools to build the rifle for themselves. Don't be afraid to dive in and begin a build! There are a variety of resources online to help you through the process, including on our Primary Arms Blog, and our Primary Arms customer service team and product specialists are always glad to be of assistance.