Jimbo, large mag capacity is a bonus in theory, but I seriously doubt that having 10-14 round is any better than 5 rounds for one reason:
I have never heard any storys of anyone ever being able to shoot a bear of any kind more then 3 times before that bear either got them, or got away with only 1 exception. That one was not long ago when a man in Alaska killed one with an AK-74 and a whole swarm of 5.45X39 bullets. (21 cal, 62 grain,FMJs at 3000 FPS) But in that case the first 2-3 rounds caused the bear to stumble and not be able to charge at full speed. The man doing the shooting did the wise thing and just kept shooting until the threat was vanquished.
Yes there are several stories of mountain men and trappers having shot grizzlies a LOT of times, but in every case I can remember reading about, it was several men shooting at one bear.
So if you and only you have a bears angry attention, I doubt that having even 30 rounds in a gun is much better than having four or five, because if you don't turn, cripple or drop a bear with 3 or less, you either get mauled or you have the bear get away. I am sure, as with the Alaska shooting I just mentioned, there are exceptions, but the common stories are about men who shoot 1-3 times and make good hits, and drop the bear. I just think going with a good powerful arm with good bullets made for the job is a better option. That doesn't mean you need some new gun. It means you need to be able to use what you have well, and you need bullets that are going to break bone and keep going. How deep? My best guess is that 30" is fine, 60" inches is finer and 100 inches is extra fine.
I believe this, but I can't say "thus saith the Lord".
But having seen a lot of grizzlies in the wild, and having seen 2 of them kill other animals, I am HIGHLY impressed with their power and SPEED.
A grizzly can run down a race horse in the first 50 yards. You will not have time to fire 8-16 rounds at any bear that is not already stopped or at least not able to charge anymore after your first few hits.
Now, again I am not being dogmatic here, but I can't be personally convinced that you could shoot any grizzle more then 3 times with anything if the first or second shot didn't work to turn it or drop it.
And if I am wrong, and you could shoot it 8 times, so what? If you are doing that it means the bear is still in the fight. If you shoot better and keep the bullets where they need to be, 1-2 are going to kill better then 8 that are shot "pretty well".
Anyway...on to the question of the shotgun slugs.
I can't honestly say.
I do know that the old 7/8 OZ Foster slugs have been used for many years by Wyoming Game and Fish, as well as the US Park Service on bears. I don't know what they used today, but in the 60 70s and 80s they used the standard 7/8 OZ slugs. They work quite well.
Again I say if you are shooting a bear in an attack you are facing him, and he is facing you. If that is so and you can make a devastatingly large wound in the bear and break bones, the slug may only go in 14 inches, but that isthe most important important 14".
So if the old Foster 7/8 slugs are good I am certain your heavier slugs are going to do it least as good in dropping a bear, and probably much better.
At least if you turn a bear and you have such penetration you can still fire killing shots shooting from mid body forward. A shorter wound channel would not be as good in that case.