If the screw head is accessible from above, find someone who has the Forster sight base/scope base drilling jig. They can set up perfectly centered over it and drill the head off the screw. Then lift the sight off and turn the exposed screw out. I've had to do it many times and it works like a champ. If you would like, P/M me your email address and I can send you a picture of the setup. Lots of other ways to do it, but this one is the safest that I've found.
If a drilling jig is not available, a verticle milling machine can do it, or in a pinch, if you are very very careful, a drill press will work.
Good luck, Tom
