The Lock, Stock & Barrel outfit (from your previous post)used to sell a Bonanza case trimmer that could be used with a drill bit to hollow point bullets. Maybe a dremel drilloradrill press could be substituted and done by hand?
TC made a kit with 4 die tipstomake round balls that would expand better.Thetips weresupposed to be place ona ramrodto notch the balls with different impressions after they were loaded into the barrel I believe, but a sharpspiked tip like theirs just might work on any bullet. You could probably substitute a nail and just punch a hole into the nose of a bullet. Anyhole placedinpure, soft lead should help it to open up and upsetin a slightlydifferent pattern than just a plain bullet would.
Another tip has a cross pattern, and a bullettop could also be lightly scored (or slit) that way with a knife.But the kitnever really gained muchpopularity for use with round balls.
