This is what I did to one that stripped on me. (I was in industrial maintenance for over 20 years and did this all the time.) Get a drill bit the size of the screw, drill the top of the screw head off. It will pop off when you hit the bottom of the taper on the screw head. Then you can remove the sight, and take a vice grip and unscrew the screw stud. It will come out easy because it has no pressure of the screw head on the sight.