First I would try a penetrating oil, when/if that doesn't work remove the barreled action from the stock, place the barrel in a vice , heat with a hair drier until uncomfortably hot and the try the screw. If that does not work and you have a drill press, place the barrel in the drill press vice, put the proper size screw driver bit in the chuck, carefully align the bit and screw, and use the drill press chuck and twist like you would a screw driver handle while maintaining pressure on the bit in the screw slot. Be careful you dont twist the screw head off.
