Patience....solvent....shock....heat....patience.
Having never removed a Sterlingworth trigger plate, I did not know the screw shank was that substantial as compared to the head diameter. I would have to agree that it is unlikely that this screw head would shear off before either deforming the slot or breaking the bit. But neither of these scenarios is desirable either, and a broken bit can leave some nasty scars around the screw.
A good gunsmith or advanced tinkerer generally has a pretty good notion of how far and how hard to push things before backing off and trying another approach. Many many folks do not, and the evidence of that inexperience and impatience is everywhere. That is why I always emphasize the same things Mike Campbell reiterates here... especially the patience part. And I always consider the fact that probably not more than one in twenty people who decide to disassemble a gun have a decent set of hollow ground screwdrivers, let alone a drill press and bit #340-2. Sometimes the best advice we can give is the address of a good gunsmith.
