The classic trick for removing serial numbers and foil attempts at recovery is to weld over. For guns this gets complicated because you're potentially altering heat treated parts that would need to be re-treated but that's doable. The kind of people who tend to own heat treat ovens tend to be the kind to ask questions but that would cost $$. There are many common and popular guns where the serialized parts are not mechanically significant enough to warrant special heat treating in which case weld away.