I am attempting to take the rear site off of my TC Encore and am having problems with one of the screws that attach it to the barrel. The first one came out no problem. The other one might as well be welded in. I have messed up the screw head a little so it may be impossible now to get a screwdriver seated tight enough to break it loose. I have applied a little oil to it and let it set to no avail too. Any other ideas?