I always went with the theory that leading in the forcing cone is caused by the bullet being too hard for the the velocity it is used at and leading at the muzzle is caused by the bullet being too soft for the velocity it is used at. Pretty simple but it has worked for me.

Missouri Bullets has bullets for either 38 sp. load velocities or .357 velocities

If God wanted you to walk and carry things on your back, He would not have invented stirrups and pack saddles