I carried a Mossberg 500 with an 18.5” barrel, very light (the lightest you are going to find) and decent gun, will carry six slugs in 2 ¾ “. As mentioned stay away from pistol grips for bear defense, they are ok for lighter power ammo, but hard on you firing full charge slugs and painful with 3” . my normal load is 2 ¾” slug, 3”x3 slugs and one final 2 ¾” slug
I just bought a 590 to replace it, the 20” barrel is a bit long, but I wanted the ghost ring sights, will be experimenting with new loads next week and will be looking at the copper jacketed sabot rounds as I have been told that they offer better penetration, although the effective range out of a smoothbore seems to be under 50m.
I also carry bearspray as a backup.