All good advice above. I recently traded my M&P 340 away that I had for many years and got a 649 instead. The M&P340 was just not that much fun to shoot even with .38+P and was terrible with .357's. I would get something a little larger with longer barrel length like a 66, 686, 669 or a 10mm.I have all sorts of choices but when I go up on the Grand Mesa here in CO which is right up the hill from my house, I take something bigger than an M&P340.
Keep in mind; if your first, second or third shot doesn't stop that bear, bears can run faster than you AND can climb trees! A 357 might only piss off a big bear!