I used a tool similar to this for removing corroded-in-place phillips screws from outboard motors that have seen use in salt water.
http://www.grainger.com/Grainger/PROTO-Hand-Impact-Driver-Set-4JW91?Pid=search The thing to keep in mind is that at the exact same time the tool is attempting to turn the screw, the same impact is also driving the blade deep into the screw slot and jarring the corroded threads. It worked well on outboard motors, so may be useful as a last resort to remove stuck blade type screws from a gun.
I do remember using it on phillips screws really saved the screw slots from becoming rounded out and useless.
Steve
