I thought it might be interesting to share all the different methods that have been developed for removing a stuck screw. There must be a ton of them, from penetrating fluids to easy-outs.
One of the very best I ever found was to use a drill press with a screwdriver in the chuck. Fix the rifle (or whatever) in something soldily, such as a stockmakers vice. Or if a pistol, in a real vise with padding. The basic idea is to immobilize the gun, even to the application of considerable force. Because that's what you're about to do.
Set the gun and fixture under the chuck, and lower the chuck until the screwdriver is dropping into the place where the screw slot used to be. Using the height adjustment, apply plenty of force to the screwdriver and rotate the chuck BY HAND to remove the screw. You must grind the screwdriver to the proper diameter, or you will scrape the screwhole. You also need to use a screwdriver of decent quality steel, or you may just twist the screwdriver tip.
You can also lift the drill press cover and hand turn the largest pully for greater force / torque. I've used this technique to remove screws so badly mangled by wal-mart screwdrivers that there wasn't even a screwdriver slot visible any more: Just a sort of wide valley where the slot used to be. But with a drill press and plenty of downward pressure, the tightest, rustiest mangled screw will just back out like its momma told it to behave or get spanked.
Of course, a little penetrating fluid and / or a little heat to encourage it won't hurt anything, but I have not found them to be necessary.
OK, who's next?