Obturation is Not Wanted Or Needed
Lee wants the pressure below where Obturation would happen. http://www.realguns.com/archives/118.htmCompare the pressure listed here for Obturation to take place.http://en.wikipedia.org/wiki/Obturate If a cast lead alloy bullet Obturates, it will deform and break down, leading the barrel. The structure of the bullet will be changed when jumping for the cylinder to the forcing come. Not so much change will take place in a auto fixed chamber firearm. Bullet's BHN x 1422 = Pounds per square inch.
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According to the chart, a very popular #2 alloy carries a 16 BHN, has a strength indictor of 22,703 PSI and should be limited to 20,000 PSI as maximum pressure. Wheel weight alloy with a BHN of 9 carries a strength of 12,748 PSI and a MAX pressure rating of 11,473 PSI.