Wear some depends your going to need them anything shy of a cannon and your not going to like the outcome.

I would recommend at the minimum a .375 H&H for an attacking bear. Understand a bear you are hunting versus one that wants to kill you is not even close to the same thing. On average (Once again this is an average) from the time you see the bear attacking, You will have 2.5 seconds to stop it. The brain can function for up to 15 seconds with out oxygen. Even if your the fastest gun in the west, you draw fire and hit the bear in the heart he still has up to 13 seconds to tear you apart.

Get some bear spray it is 10 times more effective than a gun. I would also carry a gun no harm in having back up.

This is also CO we only have black bears they are very docile and as others mentioned they will only attack if you get between a sow and her cubs.

Just my .02