I'm assuming the screw that is giving you problems is a machine screw threaded into the action, and not a wood screw. As you have been previously advised, patience is the single most important tool you have for removing a stuck screw without damage.
All of the advice about using a perfectly fitted screwdriver bit, soldering iron for heat, use of a drill press or mill for perfectly straight downward pressure, and the impact driver as a last resort before drilling out, etc. is all correct. But I've said it before and will say it again... there are much better penetrating oils than Kroil out there. Kroil has let me down many times, and I have found that even with prolonged multi-day soaks on screws that aren't severely rusted, it doesn't penetrate very far down into the threads. The people who think Kroil is the best are living very sheltered lives. I still use Kroil because I have over a gallon of it, but only for stuff that isn't bad, or as an assembly lube. I have no less than 20 different brands of penetrating oils in my shop right now. I went a little overboard attempting to find a good replacement for Cabot's Tasgon, which is no longer produced because it contained creosote. I still haven't found anything to equal it, and I have about a quart that has to last the rest of my life. I found a full unopened pint of Tasgon at a car parts swap meet a couple years ago, and it made my whole day. I've had good results with Burlite, Zep 45, and Mouse Milk. I also like the home brew of 50% ATF and 50% acetone, but it has to be frequently reapplied because the acetone evaporates quickly. Another one that has worked well for me on small fine thread screws is pure Oil of Wintergreen.
But that patience thing is the most important if you are not a gunsmith who needs to get a gun repaired and returned to the customer in a week. Even with my last resort Tasgon, I have soaked and reapplied for as much as 6 months, and then the stubborn screw suddenly broke free and came right out. Wrapping a part with Saran Wrap often helps to keep your penetrating oil from evaporating during prolonged soaks. It helps to have a lot of other projects to work on so that you aren't tempted to apply too much force or start drilling. And if you ever find a can of this stuff at a flea market or garage sale, buy it!
