From what I have read and from my experience with my Lancasters, the bullet needs to be sized for the minimum bore size and soft enough to upset to fill the bore under BP push. I got very good results with PP soft bullets in my 450 BPE. I also got excellent results with my little 360 EX using naked cast bullets for a 38 Special but horrible results with hard cast, GC bullets (keyholing @ 50 yds and side of barn at 100yds.)