No handgun is very good for bear defense. Sure, you can effectively hunt bears with handguns, but hunting is a lot easier than stopping on in the middle of HIS/HER attack on YOU.
Bullet placement of a bullet heavy enough and with enough momentum to penetrate deep and break bone. Even a bear with a shredded heart has enough time to eat your lunch before losing consciousness and dying. Unless your shooting is accurate enough to hit the central nervous system (CNS) or powerful enough to break a shoulder bone, which might slow a bear down enough to let you take a more aimed shot.
Yes, it can be done. Near Soldotna, Alaska in August 2, 2009 Greg Brush took down a charging 900 lb brown bear. Look it up. Greg admits freely he was lucky.
A fast-handling 45-70 is considered adequate. 12 Gauge with hard cast slugs (Brenneke Black Magic slugs are good) or anything with more than 3,000 ft lbs of energy and the heavier the bullet, the better.
However, bear spray has a better track record of keeping humans from injury than any firearm.
Read this thread, especially post #18 The 44s and 45s used by Murphy on Black Bears in Maine and North Carolina all had considerably more energy, momentum and mass than any .357 Magnum.
http://forums.outdoorsdirectory.com/showthread.php?t=54852
or if the link does not work, paste this into your web browser
forums.outdoorsdirectory.com/showthread.php?t=54852
Then read this newspaper story from 4/18/08, Anchorage Daily News,
http://www.adn.com/bearattacks/story/147318.html
or if the link does not work, paste this into your web browser
adn.com/bearattacks/story/147318.html
"Bear spray stops charging sow .. SAVED: Couple hiking Peters Creek Trail used Counter Assault."
This was not an advertisement. Craig Medred was an outdoor writer on staff at the Anchorage Daily News.
A followup story ran on 4/20/08
http://www.adn.com/bearattacks/story/381252.html
or
adn.com/bearattacks/story/381252.html
Good luck. Thanks for asking our advice
Lost Sheep