Please see this article on Bullet design and weight retention:
http://www.cabelas.com/story-123/carlson_huntbullet_bg/10201/Hunting%252520Bullet%252520Buyer%2527s%252520Guide .shtml
and in particular this statement by the author, which I agree with. The PB does NOT retain the weight, when it hits something hard or is pushed to fast.
Chap