Secure the gun in a padded vice and use an impact screw driver, carefully. That, following a good soaking with penetrating oil. If all fails, you may have to drill the head off the screw, using a drill bit slightly larger than the screw shank. With the head off, you can remove the bottom metal, grip the remaining shank with Vice Grips and unscrew it from the action. Of course, you'll need a new screw.
DF
