In the words of an old CO pal of mine, "It won't do em any good!" Typically this type of shooting occurs within 10 yards, but you never know exactly what shot angle you'll have, the bear could be above you, or you could be above him, or you could be eye to eye.
Brenneke and Challenger type slugs have the advantage in penetration because they don't expand much, where the fosters can pancake if the velocity is high enough (Federal) and the target dense enough. Under some circumstances, fosters are chosen due to their limited penetration when the shooting is close top a populated area. Actually I carry a shotgun rather a rifle when I'm close to town, and have buckshot available when I'm in town. I saw a bear killed one morning with a shotgun loaded with birdshot as the bear attempted to follow a dog into a house, and the home owner took exception, and fired contact close, hitting it in the face. The bear jumped off the porch and rubber legged it about 100' before falling down. Any of the bears killed near town by the DNR boys were shot with foster slugs, and they all died.