Oburtate (Bump Up) is not wanted or needed.
Cast bullets go thru a swaging process when entering the barrel grooves, same as a jacketed bullet. This is why bullet diameter is the same or larger then the barrel groove diameter when using cast bullets. Bullets are sized to the correct diameter. They do not need to oburtate (Bump Up). If you study Lees BHN chart and the Obturation link page, you will see that Lee keeps the pressure UNDER the pressure where obturation would happen. Go to Hodgdons site , check the pressures for given cast bullets loads. Compare there BHN to the max. load pressures. You will see that Lee and Hodgdons max pressures are under when a bullets obturation would happen. http://en.wikipedia.org/wiki/Obturate Also check out the Lee hardness tester chart for maximum pressures used for different alloy hardness. Bullet's BHN x 1422 = Pounds per square inch, flat base bullets. You never want hot gas getting past the bullet as it goes down the barrel, lead alloy or jacketed bullets.