if I had to use a shotgun against a bear I would one of the following 2 with my prefrance being the latter.
Brenekke black magic (3" 1500fps of mollywopp)
http://www.cheaperthandirt.com/37896-5.html
remingtion accu tip bonded sabot slug (i would use the 3" 385gr. 1900fps variety myself)
http://www.remington.com/products/am...sabot_slug.asp