No, No, No!
When shooting in a defensive scenario, the range is short . . . maybe very short. As a rule of thumb, the shot pattern spreads at a rate of about 1" for each yard of range. If you're shooting at 3 yards, your marksmanship with buckshot had better be as good as if it were a slug.
In a dangerous bear situation, a slug is the preferred shotgun projectile. Buckshot has its place, but that place is very specific. Those specific situations are when the shooting is in a built up or crowded area, such as in a campground, a cottage subdivision, or in a town. Choosing shot under those circumstances is prudent as the shot is less likely to injure anyone beyond the target. Should there be through and through penetration, the pellet(s) won't have much velocity left if someone is hit, so the injury is not likely to be life threatening. It was some years ago now, but I saw a local fellow shoot a bear in the face with a load of birdshot at contact close range, when the bear tried to push his way into the house. The bear rubber legged it off his deck, stumbled along for 100' or so, before dropping. The DNR guys shot it again when they arrived, but I doubt it was going anywhere. That doesn't mean I endorse the use of birdshot in a dangerous bear scenario, but it worked that day, and happily no pellets blew through the bear to injure us, or anyone else.
In an unpopulated area, if you have bear trouble, you wants slugs in that shotgun. If the bear is attacking a companion, would you risk the use of shot in that circumstance? Should that kind of situation unravel while you're 25 yards away with a buckshot loaded shotgun, you're status has just been reduced to that of an observer. With a slug, you can still influence the situation. A slug makes a deeper wound than pellets do and breaks the big bones that would stop pellets. A bear can't hurt you if he can't touch you, so the key is to stop his forward progression. Your best chance of doing that when you're armed with a shotgun is to load that shotgun with slugs.
