They do make them but you are going to have to be more specific as to what make and model, you are refering to. I do know that TC use to offer one for their Hawken and Renegade models. You may not have any choice about drilling and tapping. An alternative would be a peep sight, tang mounted and perhaps a fiber optic front beed. Again, it somewhat depends on the make an model. I am not aware of any magnetic bases to flat mounting. ....
Be Safe !!!