[Sparticus]

What caliber is recommended?

Most people get way too hung up on caliber. What's most important is that it is something you can use well under stress, have practiced with, and will have with you and ready when you need it. Most Alaskan guides would rather see someone show up with a well worn .30-06 than a shiny new rifle in a mega magnum caliber that obviously hasn't been practiced with much if at all. Hits are what count. Some reasonable choices:

• A revolver in .357Mag or larger
• In semi-automatics I like the 10mm
• Your favorite deer rifle. Lots of bears have been taken with a .30-30, .270, or .308 even if the arm chair guides scoff at such things. A .243 might not be the best choice, but it would be better than nothing.
• A 12 gauge shotgun loaded with hardened slugs. Standard slugs are too soft and tend not to penetrate as well.

Perhaps what's most more important than caliber is loading your firearm with deep penetrating bullets. Heavy for caliber bullets that don't expand too much are best.

[CommerceComet]

A .243 might not be the best choice, but it would be better than nothing.

I watched a Vice documentary on a man who had been mauled by a polar bear, facing his fear to return to the site of the mauling. The site required an armed native to escort the group. A first-nations husband and wife team were pressed into service. The film shows the wife loading her rifle and you can clearly see that it's .243. I hope her husband had a better polar bear rifle - they didn't show his.