Jennifer got it right!
There are very few places in this country where you have any reasonable risk of encountering a Grizzly; even less so of encountering a hostile one. For those areas, and assuming it's legal to do so AND that you know how to handle the weapon, carrying a heavy caliber rifle with a round in the chamber and the safety ON MAY be a smart move.
Otherwise, by the time you can recognize the danger, get the rifle in your hands, jack a round into the chamber, release the safety, and get ready to fire, the bear can be on top of you. Those of you who think that casually carrying some sort of firearm will magically grant you immunity need to remember the time all that preparation can take. Also, any weapon sufficient to drop a charging bear is going to be HEAVY and has to be carried where it is quickly accessible if it's to be of any use.
Those of you who have ever UNEXPECTEDLY (we're not hunting now, folks) walked up on any large wild animal in the woods can recall the momentary brain freeze as your mind asks your eyes if they are really seeing what they are reporting. Then recall how long it took your mind to send instructions to the rest of your body, be that to your bladder, legs, or quick-draw response for your camera or firearm. Remember the adrenalin rush?
Most unexpected encounters are at less than 50 meters, usually much less. B'rer Bear can cover that ground at 40 mph and he does it in bounds, which means any vital target is rising and falling in time. The one shot kill area on a bear's chest is the size of a softball. You have to be a very good (and/or lucky) shot to place a single round in there AND GIVE IT TIME TO TAKE EFFECT before the bear is on top of you. Your odds are poor at best. Think of this as a last-ditch effort on your part. You will be lucky to have enough time for a single shot so it has to score.
And once your firearm is in your hands ready to fire, you have to have the self-control to be able to disengage the safety, sight your target, and fire while being charged by something with 3 inch teeth and 5 inch claws, closing the distance between you at 40 mph.
The pucker factor is beyond the red zone.
Stick to the bear spray. Better yet, learn where the bears are apt to be at different times of the day and be somewhere else or know how to let them know you are coming - even if it makes you look or sound like a fool. Know when and how (and be willing) to react and retreat - even if it makes you look or sound like a fool.
Better a live fool than a dead one!