Excuse me for pointing out that giving a shotguns model doesn't tell much of anything. 12 gauge shells come in three 3 length and velocitys. The slugs come in various weights 1 oz, 1 1/4, 1 1/2.; --- 1 oz = 436 grains.
Using a 1 0z 12 gauge rifled slug at 1600 fps produces half the bullet energy of a 30-06 with a 220 grain bullet traveling at 2,800 fps.
The 12 gauge has poor ballistics, sectional density and 4-5 inches drop at 100 yards.
Actually, knowing the model of shotgun does matter in terms of certain specifics, at least to me. We still keep an 870 around sometimes. It isn’t our #1 choice but some of the deterrent ammo available is still a better option under certain circumstances. The 870 is a proven design. Like any other option, how it’s set up matters. Suggesting that it’s a shotgun so you only have a bead sight is silly. I would not use a “bird gun” for a bear gun unless I was in a bird blind. The 870 is a very useable platform with the factory-sighted, rifled slug barrel, and it shoots better than probably 99.?% of people can with those sights at 50 yards under the typical bear defense scenario (whatever that is; assume a rapidly moving rottweiler type deal). I can’t imagine calling it a defense situation if the bear’s farther than that except if you’re trying to peel an animal off from a third party.
And since you question the importance, or lack of it, of the shotgun model being named, I would add that a standard 2 3/4 - 3” type 870 both use the same receiver ejection port which is basically designed around the shorter shell. While 3” hulls do eject from it (due to the use of a different ejector), they are also more prone to hanging up. For that reason, I have gotten away from the use of 3” shells in mine in order to reduce the chances of an issue at a bad time.
As for penetration and energy concerns, give me a hole, a long 1/2” hole. Great placement is fine as an ideal to strive for if shooting bears in defense is your cup of tea. But assuming head shots are the only way to stop a bear in its tracks is silly. And assuming you can hit that fine a rapidly moving target with any sighting device is even sillier. Breaking lots of important stuff can also work (and a long hole seems to increase the odds.) But I can tell you that energy can be singularly unimpressive.
