If you use a screwdriver-pattern driver, ILO an angle wrench, you can bear down into the screw while turning the driver with another tool.
You need to use a good/hardened driver bit - not cheap, Chinese junk.
I mount the rifle securely in a padded vise, and use a wrench on a square-shafted driver, or visegrip pliers on a round-shaft driver - and bear down with all my weight/strength while turning s-l-o-w-l-y.
Also, with especially stubborn specimens, I've found it helpful to heat soak the screw head just prior to the attempt, with the tip of a very hot soldering iron.
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