Never heard of a bear who decided not to attack because you had a gun. No deterrent there.
Actually, bears know what gunshots are!
My family took a dream vacation to Alaska when I was about 12. We were dropped off at a lake by floatplane...dad had a bolt-gun 30-06 that I'm now pretty sure was a sporterized Springfield. Being former Brit army and trained on Enfields, a boltgun made sense.
Anyways. We came out of the cabin one morning to find the absolute cutest little pair of GRIZ CUBS!!! playing on the porch. Oops.
Pop handled it right. He herded us hoomans inside, fired a couple rounds into the lake, slammed and bolted the door and crouched in mid-room quietly reloading...while telling all of us to shut up.
Sure enough, mama grabbed the babies and headed out of there pronto.
Because she knew what gunshots meant, and her fear of same would have been transmitted to the cubs in that instant.
So yeah, IF I spotted a bear at long enough range charging in that I couldn't yet take the headshot, and I had a 357, I'd consider a warning shot.
The other key thing is, you have to know how a bear's skull is laid out. The "forehead" is a lump of bone that anchors the jaw muscles, and can deflect a .44Mag or stronger. In some cases bears have been knocked out with a rifle shot to the forehead and then woke up and killed somebody. A bear's brain is low and rearward, you get to it through the eye sockets, cheekbones or snout, or upper portion of the open mouth. Basically aim dead on his nose, and be accurate to the next 3" or so either side or 2" top or bottom on a big one, a 6" by 4" oval more or less, and you can indeed kill him with a 357.
The problem is, if you miss by more than that, esp. windage, you will NOT screw up his shoulder the way a semi-miss with very hot hardcast 44Mag+P (Garrett or Buffalo Bore, with 45-70 or a slug-gun preferred!) on up can. That's the thing you're lacking with the 357: not killing power exactly, but the ability to knock him around with a semi-miss to help you stall for a better killing shot.
If it's a 200 to 300lb black bear, then the hottest 357 hardcasts DO have the ability to "knock him around" that way.