There is no question bear spray is, or at least can be effective. The question remains with the application methods.
There is a huge difference between cops spraying a bunch of harmless kids, sitting on their butts protesting, and a person who has just invaded the personal space of a hungry, and very angry large bear.
Bear spray has a range of just a few feet, and with limited accuracy. And it has to be applied to the eyes and lungs, (hopefully), in order to have any effect at all.
Pistols, rifles, and shotgun slugs increase that distance substantially. And can be effectively applied to the head, spine, as well as several other important organs and bones the bear cannot do without.
And as with any self defense scenario, be it man or beast, the more distance you are able to keep between yourself, and your attacker, the better.