Your short barrel is fine for slug shooting, just be sure to choose those intended for use in a smooth bore rather than those designed for use in a rifled bore. If shooting becomes necessary, the additional bulk of a longer barrel, particularly one longer than 18" does not compensate for any gain in velocity which is minimal, and accuracy at reach out and touch ranges need not even enter the equation. Your gun should have sights that are easy to pick up the instant your cheek hits the stock, and you should practice on both stationary and moving targets if you can arrange it.
Brenneke classic slugs are the ones I trust with Challengers in second place. In both cases the penetration and wound volume exceeds the more common foster slugs. If your magazine holds less than 5 rounds, be sure to choose 23/4" slugs rather than 3" as the longer shells will cost you a round. Now I've said it myself, that if you haven't solved the problem with one or two shots, you probably won't be able to, and that is a good rule to keep in mind, but more ammo is comforting, and the importance of confidence cannot be overstated.
IMHO, buckshot has very limited and very specific use in the bear defense scenario. A bear can't hurt you if he can't touch you, so the purpose of shooting is to stop the bears forward progression, and this is usually done at very close range. If we allow an inch of increase in pattern diameter per yard of range, we see that there is no marksmanship advantage to buckshot; as a miss with a slug at 3 or 4 yards will also be a miss with buckshot. Stopping a bear's forward progression is best done by breaking a major supporting bone, the spine, or destroying the brain cavity. Buckshot ain't up to that. Where buckshot should be considered is if any shooting becomes necessary in populated areas like crowded camp grounds. The idea being that even if a pellet exits, it will have such little velocity left that is poses a much diminished danger to bystanders compared to a slug that exits. Should it become necessary to use buckshot, the head should be the aiming point on a face on bear, keeping in mind that the brain lies behind the eyes, ahead of the ears and is only the width of the snout.
Given the massively wide head typical of a bear, quickly acquiring an effective aiming point on the head somewhat confusing, but keeping the width of the snout and the ears and eyes as benchmarks helps. You should preprogram yourself to what you will do if you have to shoot. This will effectively reduce your reaction time.
If and when to shoot will have to be your decision. IMHO, shooting a bear at 50 or even 25 yards doesn't give the bear much of a chance to just go about his business. But every situation is different, and if you've been around bears enough to read them, you shouldn't hesitate to shoot a bear that makes you afraid, particularly if you are on foot and unable to get to safety.
