I have been getting the best cast bullet performance of my reloading life (30+yrs) by using the largest dia. cast bullet that fits into a fully fire-formed case mouth from each gun. Im not alone in this, check out The Cast Bullet Assoc., etc. You have to not use one too big or it expands the brass to the point it wont chamber up clean or, if forced in, want to extract.
If you are FL resizing you may need to get a lead bullet expander button to fit your bullet, but you will get better accuracy and no leading. Undersize lead bullets (ie, the same size as jacketed, usually) WILL lead the barrel.