If you mount it on a Picatinny rail mount, you can see the FO sights, but you cannot line them up. Since the FastFire mounted on a rail sits too high to look right down the barrel and line the FO sights up. Burris does make a different model mount called the SpeedBead that sits low enough to use the FO sights. It is a gun specific mount that consists of a plate that mounts between the stock and receiver. It is not available for all models though. For instance it isn't available for Mossbergs because of intererence with the top tang safety.