The very best bear stoppers at touching ranges, are 12 bore Pump guns loaded with solid round balls.
: A properly trained shooter can shoot much faster with a pump than with an autoloader. Working the slide brings the barrel back to bear, without having to muscle it down. It happens automatically - with training, 3 aimed shots at 25 feet in less than 2 seconds are easily done - You need only practise! Start with trap loads. Model 870 20" bl. riot guns are my favourite (and specialty). They're what I trained our local riot squad boys on. I used to slam 6 B27 silouettes at 25 yards in 3 seconds, using buckshot loads, as a demo, of course. This is difficult if not impossible to do with an auto. You should have 3 empties in the air at once.
: A properly loaded pump, will have those round balls almost or at bore size (non-chokes only), and at a safe velocity of 1,400fps to 1,500fps. This is easily ackomplished with SR4756 powder or Unique or Herco if you wish. Safe load development is necessary. Lyman shotshell handbook has some data for reference.
: Factory slugs will work OK, but I'd much prefer the 1-1/4 ounce solid round ball at the African BP equivalent velocity of 1,500fps. 545gr. of solid lead is better than 437 to 500gr. of hollowbased soft lead from the factory Foster slugs. Any big bore rifle (like a .375 or .458) is better than the undersized 12 bore sabots.
: The above is about the most slam you can get in a rapid firing gun. My smoothbore made 3" to 4" groups at 50 yards, plenty good enough for close range hunting or protection.
: Stay away from black powder loads for these fast firing, defense loads. Due to the amount of powder required (as in the African BP loads) recoil is extremely high in an 8 pound gun and fast shooting is next to impossible. Too, the smoke hides the quarry.
: Shooting round balls in 12 bore is fun, as is the load development involved. They can be accurate, much more so than hollow based factory ammo. Smaller balls, like .615" RB's fit inside 12 bore trap wads, and can be an easy way to develope lighter round ball loads. They are very light, though, only running 320gr. or so. So- 2, .615" round balls weighs about 640 gr. which is almost 1-1/2 ounces, a most lethal impact at close range. A 12 bore will drive 1-1/2 ounces up to about 1,400fps with safe breech pressure. There are no real limits for someone with imagination and ability to develop his own data - tread softly here. This is only for those who understand what they're doing.