A great tool is a lathe. Put that on your list of things to get.
You can turn brass laps to fit inside the sear hole, charge it with diamond compound, and bore out the sear hole pretty quickly. Split the lap with a jeweler's saw, so that you can spread it slightly, so it will maintain good contact with the hole. Use a light oil while honing.
Brass is soft enuff so that the grit embeds itself in the brass. The sear is hard enuff so that the grit won't embed in the steel. Bore the hole with coarse grit, then polish with fine. Wash the abrasives out, then make the screw to fit.
A lathe will help tremendously for making screws and getting threads started straight.