Big bears are not dangerous until they come at you. When they come at you the parts you need to hit are in front. Brain, jaws, shoulders, spine/neck and the heart and lungs. So if you can get a bullet to penetrate 2-1/2 to 3 feet, going deeper only gets more guts, and if you are lucky, may hit the spine farther back or even a hip joint or pelvis.
Good bear guns are mostly about good bear bullets.
A bullet that holds together and breaks big bones and still goes 36" or more is going to do what you need.
Bigger is "better" as long as you can shoot it well, but a man with a 458 who doesn't shoot it well and fast is going to be worse off then a man with a 270 with good bullets who does shoot well and fast. My experience with men who get big guns to come out west to hunt is not all good. Many get a gun a year or less before their hunt, and don't shoot it near as well as the rifle they have used on deer for the last 10-30 years.
Any gun you may have to shoot in defense should have at least 500 round through it before you can call it familiar. Many men get a new rifle and "fumble the ball" on a hunt just because the rifle is not familiar enough to them, and when they feel pressure they don't do so well.
Now just imagine how much more pressure you will feel when you are not trying to hit a big horned bull elk, but instead trying to hit something coming at you as fast as a charging horse, and it's full of teeth and claws.
I like big and I like powerful, but I like accuracy a lot better. if I have to choose.
A man with a 30-30 he has used for 30 years if going to be a lot better off using that against a mad bear than the same man with a rifle he doesn't really know.
Best guns/loads for bear defense is a subject that is hashed over all the time, and it may be fun, but the truth is that any big powerful gun with a bullet that holds together is fine, if it's used well.
No gun and no load is fine if it is not used well.
I hunt around grizzlies every year. I have never been charged. I have had a few hair raising times, but so far I have not shot one.
I carry a 44 magnum or a 454 Casull on my body at all times when I am around them, and I hunt with a 375 H&H or a 9.3X72R unless I am carrying my muzzleloader.
When I use my flintlock I still carry the 454 Casull, and my flintlock is 62 caliber and loaded with a hard ball and 140 grains of powder. I only have one shot, but that first shot is a real killer. I dropped a large moose with it one time, and he hit the ground like it's front legs disappeared.
I am pretty confident I would be fine if I needed to shoot and I had my 270, or a 30-06, 308 or 8MM Mauser, but if I know I am going into their back-yard I have a tendency to take my biggest guns. Just makes me feel better, and I shoot those rifles well enough to hit running rabbits with them more time than I miss.
I am not paranoid about grizzlies, but I am not foolish enough to trust them either.