If you have access to a grinder, you might be able to 'sharpen' a screwdriver for your own purposes. This is a practice that nearly all gunsmiths must use, and it ensures that you will have a screwdriver that fits your screws exactly!
If you wish to undertake this on your own - take your time. If there is a color change in the metal as you go, you've lost the temper of that piece, and it will not work like you want it to. Especially if you are cinching down scope or sight screws. Another thing to keep watch of is a cheap, chrome plated screwdriver. Usually when you get past the plating, you'll find some really soft metal which will distort at the first application of torque.
Personally, I'd practice getting the shape right with the cheapie, and then go for the finished product on the better quality tool. I'd also consider using drill rod for my customs jobs, a dicarded old screwdriver handle epoxied on, and then your never need to look back.
Good Luck,
Unkel Gilbey