I hunt bears [ black ] every year, and have been successful most years. Last year I shot 3, one in the Spring, one in the Fall, and another during the Summer. The Summer bear had broken into a neighbour's home down the road, and she called me to deal with him. As I suspected, he was a 2 year old male. No bear is a push over. They are unpredictable. Most will run away, or at least try to when shot, but others won't. I have had them stalk me, and that will turn your blood cold because they are absolutely silent. I just caught the movement of one from the corner of my eye while sitting against a tree on a deer trail. For the last number of years I have used my smoothbores, usually the 10 but sometimes the 20. I've used rifles in .50 , 54 and .62 cal and they worked ie the bears died.
That said, I absolutely want penetration enough to have an exit wound. Bear bones are tough, and you need to get through the fur, hide and fat to reach the vitals. My .50 liked /preferred a charge of 80 gr of 2 F and would shoot into an inch to inch and a half at 50 yards with that load. 3 F opened the group to around 2 inches and that is what I used for bear. More is better IMO. I would not and did not hesitate to shoot the 2 bears I got with that load. But.....the largest bear I ever shot was taken with the 10 bore. I was sitting on a rock about 7 feet up and it walked right under/ beside me.
When I shot, the entire load with wad went through upper back, both lungs and clipped the heart. The paper cartridge remnants and ball were deep in the dirt and smoke was coming from the hole. If you figure that the Chambers N E fowler has a 46 in barrel, you'll see how close he was to the muzzle. That bear went a good 30 yards into the bush and had turned around on the path facing back. Luckily he died while I reloaded. Good reason not to follow up too soon.
So, figure out the accuracy you need, and shoot the largest charge that will give that. I wouldn't go less that 70 gr 3F I've posted some photos of a few of the bears I've taken with my flintlocks , just to show I'm not talking through my hat