I've never used a gun or pepper spray to defend myself from a human or animal (although humans are probably the most dangerous animal.)

However, I have read accounts of people killing grizzlies with a 357 mag. I think you could, but might be tough if the grizzly was charging. I believe you have to disable the bear, with a shoulder or hip shot. Otherwise it may not die before it kills you. Remember a bison can run a mile after a heart shot. Makes a CNS shot necessary to get the meat to the truck.

You might look in to a single shot rifle or pump shotgun for the car. Both can be fairly compact, and there are FAR fewer restrictions on long guns.

From what I have heard, Big Bend National Park ENCOURAGES people to have guns.