You will need a roller guide to keep the file in position when making new screws. Brownells sells one:
http://www.brownells.com/gunsmith-t...sories/screw-slot-fixture-prod18694.aspx Freehand cutting a new slot is not going to work unless you have the skills of a watchmaker and even they use file guides. You cannot buy too high quality for such precision work. The best files are made in Europe and are expensive.
If you have a drill press, this might be of some interest:
http://www.shootingtimes.com/gunsmithing/gunsmithing_a_better_way_to_slot_screws_011211/RAN
RAN
