This is a replacement bolt extension that comes in the package with out Gen 3 ARH300 Folding Stock Adapters. These are built to fit a standard .625 BCG opening and fits AR15's as well as most AR9's. To ensure a proper fit for your AR9, please measure the opening to your BCG. If you find you need a smaller bolt extension we have a .531 available as well! If you have any questions please contact us before ordering!