I would submit one more argument for a 12 ga. shotgun. Seems to me that, at the end of the day, a large part of the question should resolve around the damage a weapon does. Damage from a .45 long colt is acceptable and the same can be said for a .410. A shotgun, on the other hand, is capable of horrific damage to flesh.
Ballistic gelatin (see below) is used as a substitute for flesh to show us what a round is likely to do when it hits a person. There are failings in the system of course - gelatin doesn't mimic the effect of rounds on bones, but it's a handy model and ... it's fun to play with. ^_^
Here's a picture of ballistic gelatin that was hit with a 2 3/4" load of 00 buckshot, 9 pellets.
The path of the shot can be seen as darker lines moving horizontally through the gelatin, from right to left. The lighter clouding around the lines demonstrates a transference of kinetic energy which often results in additional damage to the flesh.
The ruler at the bottom of the gelatin shows the penetration of the pellets measured in inches.
From the outside, this is a picture of a nameless mid-sized animal shot with a similar load from 15 yards away.
This is the entrance wound.
And here is the exit wound
This does give rise to the question "How much is too much?" I was never sure that I knew how much was "too much," but I do know that a shotgun qualifies as enough.