I'm curious; why all the talk of extreme penetration being needed for a defensive shot (full frontal)? I'm assuming we're not going to be taking raking or quartering shots. If I hit a bear in the chest at close range, what would I care if it ends up in it's rear end? That's not where I want the energy dumped.
Under normal(?) circumstances I would prefer 45-70 type rounds or other smaller cals throwing heavy bullets. If I was sleeping in a tent or anticipating a 10 yard or under type encounter I would be tempted to lean towards large buckshot. Not for any perceived energy advantage (it won't beat a properly loaded 45-70) but for the imediate damage within the first few inches of penetration. Primarily head and neck with a slightly greater chance of putting a lethal ball in the brain. Also considering the off chance that even a poor shot may give it unusable jaws (or less paw) if it does end up on top of me.
For the largest bear which the vast majority aren't going to be dealing with, back to the rifles, preferably .358 and up.
Thoughts on my choices?
Nobody told the bear that being shot in the chest is fatal, so until he figures it out, you could be in for a bad time. It all depends on how long it takes for his brain to run out of oxygen and die. A bear has to touch you in order to hurt you, so the purpose of shooting then is to stop his forward progression towards you. The primary difference between hunting and bear defense is time, hunting provides a comparatively leisurely time frame to make the shot and for the damage caused by that shot to take effect. If it takes several minutes for the bear to expire that doesn't have much effect on us, but it could have a huge effect on us when we are forced to shoot defensively from very short range.
Energy dump doesn't have much to do with killing an animal. The bullet can only destroy that which it passes through, so the deeper it penetrates the more complicated and less survivable the wound becomes; the volume of blood is lost more rapidly, and the more rapid the blood loss, sooner the brain dies. If the bullet breaks large supporting bones on its trajectory through the bear, all the better, as this will interfere with the bear's mobility and give you time for a follow-up. But clearly it is better if after smashing though that dense heavy bone, the bullet continues to pass through any soft tissue that lies beyond it. While the bullet is supersonic, soft tissue is pushed aside by the shock-wave that comes off the expanded nose section, that tissue is crushed and pulped, which again increases the volume of blood loss. But the shock-wave has little effect on bone other than to scatter fragments that were already created by the impact of the bullet. If that scattering of bone fragments contributes to additional bleeding, so much the better.
As for buckshot for use in a contact close shooting situation as may occur in a tent, there are a few considerations to keep in mind. Buckshot will not penetrate more than a few inches, seldom more than a foot, and this is good if your tent is located in a crowded camp ground. Because the shot column is still tightly packed on impact, the pattern does not give you any margin of error over a solid slug or a bullet, but with a solid hit, it will work under these circumstances.
When a bear comes straight in, the head shot can be viable if the bears head has little lateral movement and if the shooter remembers where the brain is. A big bear has a massive head and it can be difficult for the inexperienced to understand that the brain is only the width of the snout when the bear is nearly 2' between the ears. Just shooting him anywhere in the head might not produce the desired results. If the bear comes in a rush, spineing him might be an easier and surer shot.
In terms of cartridges that are suitable for bear defense, all things being equal, the larger the frontal area of the bullet, the more damage it does as it passes through living tissue. Thus .30 is better than .20, .40 is better than 30, and .50 is better than .40. But as bullet weight and diameter increases, one approaches the point of diminishing returns in terms of a gun's portability and handiness and in one's ability to use if effectively. Thus, in some cases a .30/30 lever gun might be a better choice than a .460 Weatherby.