Hunter Customs, you must be thinking of a different gun than I'm talking about. The crackshot and little scout that I have use coil mainsprings not flat mainsprings. Kinda wish that they did. Making flat mainsprings are easy. Bend them to the shape that you want and temper them back in and your done.
I have used old hacksaw and saw-saw blades to make flat springs but have never tried a feeler gauge. May have to borrow that idea in the future...lol