Besides the leading issue, this is what you may run into if the diameter is too small and the bullet is too hard.



This is a keyhole example of the same bullet (Oregon Trails .357" SWC) shot in two different guns and they were undersized for the gun producing the keyholes.

As far as leading with a plain based bullet, I shoot Lyman 358429s at 1750 fps without leading and the hardness is only 12 BHN.