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Another reason that spray might be a better option than a firearm except if the person is an expert marksman. There is just a small box to aim for if you want to put one down with a handgun.

I think even a person that's armed with a suitable handgun would be better advised using the Bear Spray first and resorting to the handgun as your last resort.
Killing something as large and powerful as a  Grizzly Bear is much different than stopping one dead in it's tracks once it's full of adrenaline and looking for blood. You might get away with it with a Black Bear unless it was one of those huge Alaskan varieties.
I've read stories of the Inuit hunters killing unsuspecting bears with .22 at close range with a single shot thru eye/ear into brain.
No hunting guide in his right mind would allow a client to attempt that.
Most guided want there clients to be able to hit a bear accurately and do the same with a back-up shot.
It's all about shot placement. Most guides carry guns that far exceed what it actually takes to kill a big bear. Some carry guns that would stop an elephant.
The reason is simple. If the client muffs the shot or panics they might only get 1 shot to stop the charge. You literally want a caliber that will knock a 500lb + animal over backward and allow for follow up shots.
Very few people have the experience and skills necessary to stop a charging Grizzly with a handgun.
A .44 magnum is more than most people can handle and even the venerable 30-30 Winchester has more foot lbs of energy than it does. Not many people would consider a 30-30 adequate protection in big bear country.
I'd spray them first, and resort to the 12 gauge/Magnum handgun if that failed to deter them for long.
The spray might disorientate them long enough for you to get a good shot to the head and maybe a follow up one?
About 2 shots and if the bear isn't down, your in for a world of hurt. Even a dying bear can kill you. That's where that double action that Wally mentioned comes in.
You try and fend their jaws off with you off hand and with the other ( gun hand) jam the muzzle in it's jaws and empty the cylinder.
If you don't see any obvious effect, you might want to save the last round for yourself :'(