Hoo-boy...where to begin on this one.
First, to the OP. I think your question was answered a post or two above this one.
Second, I'd second (alliteration is a wonderful thing) what Alaskan-RV'r said.
Third, and we are securely in to my not very humble opinion territory:
- Lots of agencies in AK use shotguns for bear protection when loaded with slugs, some level of buckshot, or both. If one were to carry birdshot, save one shell to put oneself out of one's misery. Birdshot would only antagonize a grizzly and might make a blackie just mad or scared enough to fight back. I have a friend who routinely is bear guard for a couple of wildland fire crews. He is issued a Remington Pump 870....and he typically finds a way to carry a modified 45-70 levergun as well.
- IMNSHO don't bother with the hand cannons. It takes 5 (FIVE!) times the practice with a side arm to reach the same proficiency as with a long gun...and except for bear guides and the occasional trooper, proficiency with a long gun is in short enough supply with a charging bear.
- Unless you're determined to wade into the middle of a salmon stream, or in a village dump, or on top of a pile of fish or moose guts, you're not likely to see a bear, particularly on an ATV. As Alaskan-RV'r mentioned, the risks attendant to ATV'ng in Alaska have very little to do with cousin Ursus and much more to do with the dangers inherent in the activity. A good GPS and the knowledge to use it, good raingear, good firestarting materials, good hiking boots and the health to use them, and possibly some sort of emergency contact device (a SPOT) or Sat Phone....are all orders of magnitude more beneficial than any firearm, MANY orders of magnitude more beneficial than a hand cannon unless you can shoot expert with it when 800#'s of mean is chuffing and grunting as it runs toward you in the 15 -30 seconds you'll have to react.
