Cast Bullet Leading

A clue to what is causing the leading is where the leading first begins to appear.  If it appears near the chamber, chances are that bullet diameter or hardness are the cause. 

A diameter too small and/or too hard an alloy will allow high pressure gas to leak past the bullet, which erodes the bullet and leaves leading near the chamber. 

If the leading first appears on the leading edge of the rifling (if you imagine the bullet being pushed through the barrel, you will note that one edge of the rifling does most of the work of imparting a spin to the bullet.  This is the edge you see when you look through the barrel from the breech end) the bullet might be too soft, and/or the velocity too high. 

If the leading appears in the second half of the barrel, the bullet is running out of lube.  You should see a star shaped pattern of lube accumulate on the muzzle.  This is an indication that there is a little excess lube.