Another trick that can make a difference is to get a piece of steel or iron roughly the diameter of the screw head you want to remove, long enough to hold with pliers of vise grips. Heat it red hot or hotter if you have a torch, and hold it to the head. This causes the head and shank of the screw to expand slightly. Soak in penetrating oil first and the combination of this gentle heating and oil can shock the threads loose.
Be careful of any wood or delicate metal bits that the heat can damage. I've used this on old military guns with a pretty good success rate.
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