Those of you who have hunted dangerous game will undoubtedly have noticed that your guide or professional hunter usually is carrying some sort of big-bore long gun. The reason is that the professional knows that anything can happen when dealing with dangerous game and he has to be able to stop the attack right now, before someone gets hurt. In short, He doesn't plan for when everything goes well; he plans for when everything goes bad. The defensive shooter is in the same situation. The best defensive rule of thumb is to carry the most powerful handgun that you can shoot quickly and accurately. It should not be so powerful that you can't manage the recoil in order to get off a quick second shot. And you must be able to shoot it accurately enough to reliably hit the vital zone of your attacker with practiced regularity.
Those who are new to defensive shooting may find that this equates to a .32, .380, or even one of the .22 calibers when they are first getting started. However, with practice and professional training, a person will soon find that they can handle a quality pistol in a much larger caliber with ease. That is the time to trade up and to carry the most powerful handgun that will get the job done for you. Another mistake that leads to the packing of small defensive handguns is that the neophyte chooses a gun to match the way that he chooses to dress. The experienced shooter changes the way he dresses in order to accommodate concealing the more substantial and effective defense gun. To those with experience, being alive is better than being in fashion.
To those who insist on the effectiveness of their pop guns, I would ask why the police and military don't use such tiny, small-caliber guns. The rules of engagement are certainly different for police and military but they are still dealing with same kind of human beings that the armed citizen may have to deal with. In truth, we see them often having to make multiple hits in order to stop the threat in spite of the fact that they are using substantial pistol calibers.
The smart attitude for the defensive shooter is to carry the most powerful handgun that he/she can shoot quickly and accurately. Plan and train for the worst-case scenario and hope it never happens.