The Custom AR9 Bolt Extension is for our Folding Stock Adapter customers that have a smaller diameter BCG. The Bolt Extension diameter is .531, commonly used to fit FX9 and other AR9 builds that have a smaller diameter bolt carrier group. To ensure proper fitment, please measure the inside diameter of your BCG. Not all AR9s require a smaller bolt extension and can work with the standard .625 bolt extension that comes with our Folding Stock Adapter. Will not fit Gen 4 Folder Product Details Weight: 2.925 oz. Diameter: .531 in. Most commonly needed for FX-9s If you have any questions, please contact us before ordering.