444, I agree with a lot of your points but as you said:
I hate to get into discussions like this because there are so many variables involved that there is really no right answer.
Totally agree, however, I think there are some points you may have missed or misread, whatever.
"Larger calibers improve the odds, but if you work on the assumption that all handguns are underpowered to a greater or lesser degree and you won't be far wrong." This has also been my experience.
Agreed.
The main thing I have to disagree about is the fact that you focus almost exclusively on bleeding. Agreed that a hit in the right spot with a .22 will cause you to bleed out as quickly as a larger caliber, however, the function of a handgun for self defense is not to kill, rather to stop the fight. Killing your attacker is a desirable end result but not the purpose. The purpose is to stop the fight. To stop the fight you must cause enough damage to your attacker to force (notice I said force) them to lose interest in continuing the attack.
I believe that energy transfer is the more important factor in stopping the attack. I do not have the energy figures in front of me but I do know that the .45 has considerably more energy than the .22. If the bullet does not exit then all the energy is transferred to your attacker, causing damage. I view this as similar to striking your attacker with a punch or kick. Take the difference between sparring in Karate with a lightweight or a heavyweight. All other factors being equal (experience, expertise) the heavyweight will win. Sparring with a lightweight, he will be faster and probably hit you more often but it simply does not hurt as much as a heavyweight.
In reference to bleeding, or stopping the heart, it takes 7 seconds to lose consciousness if blood flow to the brain is completely cut off. Try a sleeper hold sometime. 7 seconds is a lot of time for someone to do you a considerable amount of damage. You mentioned you could empty a Ruger into a quarter sized spot in 4 seconds. Again, 4 seconds is a lot of time in a fight. In 4 seconds, the average person could cover a distance of somewhere near 25 yards. Can you hit a moving target 10 times in 25 yards and still keep all shots in a quarter? I know I can't.
The only cartridge that I can think of that would reliably put someone down and out is the .50 BMG. I've heard of people surviving hits with that cartridge. There will always be people who can survive massive damage, but on average .38 special or larger is the self defense caliber of choice. When I don't know from personal experience, I have to go based on the consensus of other peoples' experience.
"A larger caliber like the .45 has enough power that even an off target hit will still be likely to stop your attacker."
Stop him when is the question. I saw a guy that got shot in the thigh with 230 grain hardball and he refused transport to the hospital. Nothing personal, but I don't agree with this statement at all.
Not taken personally. Perhaps I should have said "off center". However, too many people who have been there have agreed that the .45 is effective as a self defense caliber in most situations.
I apologize to the forum members for the length of this post but I felt that a well reasoned post such as 444's deserved the same in response.