As for further follow-up on bullet expansion and designexperience and theories, please see this gentleman's experience. I have his ML hunting book:

http://www.amazon.com/Modern-Muzzleloading-Todays-Whitetails-McMurchy/dp/0873419510/ref=sr_1_1/102-4416274-2032936?ie=UTF8&s=books&qid=1194698291&amp ;sr=8-1

and he is an excellent hunter and practical hunter, with tons of experience, but here is what he says about the Noslers onbear:

http://www.cabelas.com/story-123/mcmurchy_bearbullets/82/Bullets+For+Bears.shtml

and in particular about slow moving bullets with large frontal areas. I believe you will find that PB which mushroom very quickly will release their energy too quickly too quickly in some cases causing no penetration and no kill. That is my theory and supporting documentaiton. Thanks for listening.
Chap