22 with 100 round box of ammo. Since the chances of being attacked by any animal is so low in Colorado protection, except maybe from a rabid raccoon, it should almost not even be a consideration.
Colorado's small black bears are usually only problem is you haven't prepared your camp/clothing/cooking to keep bears away. A small can of OC (pepper spray) does wonders to drive them off.

Food to keep up your spirits while lost will be the primary use of a gun. Food to sustain you is not needed either unless you are lost for almost 2 weeks. Water is probably a better thing to pack rather than a gun... but I carry both.

Now to mix things up. Should you carry something bigger to protect yourself from big varmints? You know the kind you hear about of the news that are just crazy? There once was a FBI investigation that showed that 90% of hunting accidents where people were shot while hunting were not accidents at all. It's not the bears you should worry about.