I’m sure everyone has run across a stuck or stripped screw in a gun. They can be really hard to remove. Always try a good penetrant oil like Kroil and let it soak for a week or so. If that doesn’t work try heating the metal around the screw to make it expand away from the screw threads and the last resort is to drill it and use an easy out.
Watch this video and learn how to remove a stuck or stripped screw.