An electric vibrating pencil is the best tool for stuck small screws, replace the sharp engraving point with an 1/8 inch hardened dowel pin, get a friend to twist the screw driver as you hold the pin on the screw driver flat, it works just like an impact tool on a small scale.
If a friend is unavailable, you can do this yourself if you have a rifle vise to hold the rifle tight while you twist and vibrate.