grouse guy,
Here is a simple method to free slotted screws used by English gunsmiths.Grind a turnscrew to fit the slot in the screw head, make sure that the driver blade bottoms out in the screw slot.With the driver located in the screw slot,using small mechanics hammer give the top of the handle of the turnscrew several sharp downward blows.This will in most situations free a rusted in screw, prior to normal turnscrew removal.

Roy Hebbes