I bought an old Marlin 1893 lever action in nice shape, but found that the tang screw and one of the tang sight filler screws was stuck and could no be backed out. Of course I was using gunsmith screwdriver bits which fit the slots. I tried soaking the areas around the screws with all the Kroil I could get on them without soaking the wood, heating the metal as hot as I dared without burning wood, and carefully using an impact driver and a hammer with the rifle solidly held in a bench vise. Nothing moved either screw, and I took the rifle to my brother, who is a hobby gunsmith, to have him drill out the screws on his small mill.
Instead he put the rifle in a well padded vise and used a dental burr in a small rotary grinder to deepen both screw slots. He then used a screwdriver bit in a battery powered impact drill from Lowes, set at the lowest torque setting and after "tat-tat-tat" forward and reverse the screw backed right out. Jim said torque did not move the screw, but the repeated forward a reverse impacts broke the grip of the rifle on the screw. I had tried my impact driver, but I had no way to deepen the slot and apparently the impact needed to be applied to a larger area. As it turned out, the screws were not frozen in by rust or crud on threads. The stock wood had swollen around the screws and both holes had to be drilled out to get things put back together.