Rusan thermal / night vision clip-on adapters are made for a litany of rifle scope models and for thermal clip on devices. It is the outside diameter of your scope objective lens housing that determines the adapter size needed, NOT your actual glass lens size diameter.
For example, Swarovski scopes with 50mm objective lenses have an external housing diameter of 56mm. For a Swarovski 50mm scope you would need Rusan Adapter Model: RUSARM52-56. It is the last two numbers in the product SKU that indicate adapter size. Please use the fitment size chart below to determine your correct size or call before ordering if you have questions. You can determine the correct adapter from the manufacturer's adapter size chart.