I’d use the shotgun with those same Brenneke slugs again though it isn’t my first choice. For the record, that 870 is specifically set up to shoot slugs though with a 20” rifled barrel, stout factory sights, and a stock intended for rifled slug barrel use which puts your eye in line with those sights.
But perhaps the important lesson here is not to be fooled by massive size which the shotgun slug has - and the Brenneke type slug is a good one- but that placement is the most critical factor. A deep, deep penetrating bullet, not necessarily a solid, will have more chances to destroy something that will quickly end things.
The advantage a shotgun has is that there are many options where deterrence projectile are concerned which aren’t available for rifles. (I’ve deterred many potential bear problems by shooting various projectiles toward them.) Shell-crackers/popper shells are very effective (assuming you don’t launch them beyond the animal.
) Rubber slugs and buckshot would probably work also though I’ve never tried them on bears. Noise alone hasn’t been nearly as effective, especially on bears that aren’t already wary, as something that disrupts their immediate space. Bears with attitudes or showing territorial aggression seem to react best to rifles when rocks in their vicinity begin ‘talking’ to them. (Bullet holes in the tundra in their vicinity seem to have almost no effect.)
One of the biggest reasons I prefer rifles is that they offer more options around other people (family). While I would hate to be put into such a situation, I would much rather have a rifle than either a shotgun or a handgun if a bear got ahold of a family member who might be some distance off. I shoot a rifle much more precisely if it comes to hitting something while missing something else.
