I'm definitely no expert, but from what I've gleaned a cast hunting bullet (for larger game) functions best when it has a large meplat (flat area on the nose of the bullet). This allows it to penetrate deeply in a straight line with a large wound channel. If the bullet is too soft and it will deform and lose effectiveness. Too hard and it will shatter on heavy bone. One answer is to heat treat the projectiles. Good article on heat treating bullet lead is http://www.lasc.us/HeatTreat.htm Also see views of Veral Smith of Lead Bullet Technologies.

Personally, my casting of premium bullets hasn't been all that flash and I prefer to buy quality jacketed for when it matters.