The only problem I see with store regular screwdrivers is that they are wedge-shaped and only a part of the wedge is gripping the slot. What I do is buy screwdrivers with chrome valadium shanks, under $2.00 a piece with 4" shanks, total length 6-7 inches. I grind them on the side of a grinding wheel, this way I can keep one side parallel to the other. I usually use a feeler guage to get the size of the slot and then use a vernier caliper to get the correct size of the tip. When done you will probably need to de-magnitize the tip because of the grinding.
Not only does this apply to gun screws, it applies to when I use slotted screws for appearance in woodworking.
