I know for a fact a 22LR will kill, a friend of mine was shot in the back of the head when he was camping with "friends" while we were in junior high school. He bled to death over several hours while the punks ran in circles trying to figure out how to get out of their mess. All they got was a wrist slap after the kid expired.
Pros use the 22 on a regular basis, MOSSAD trains for the cranio-ocular (thru an eyeball) quick kill shot, but then that's MOSSAD. I guess the question becomes can you make that aspirin sized shot when it's life or death and you've got the Indians attacking while screaming like a banshee from the left while the marauding wolves are trying to eviscerate you from the right? Just for kicks, gets a string of firecrackers and instruct a buddy to light them off when he thinks you least expect it while you're target shooting sometime. Probably have to get range permission and maybe even have an RO light them off, but I think you'll find the results instructive.
The place to do this is off some lonely country road, probably. Just be safe. Better yet wear an iPod under your muffs and let your friend crank up the volume whenever he wants while you shoot for score and accuracy on a target. Yeah, that'd do the job fine. You'll find even a "normal" distractor will pull your shots way off the line. Now imagine the adrenalin dump of chemicals into your bodies system because you're right at the edge of life and death staring into eternity if you don't make the exact shot you need the first time.
That's why an increase in caliber can slightly offset the need for precision accuracy. With a 9mm, a shot to the cranial vault of the forehead will turn the lights out like you flipped a switch whereas for the 22 you'll be needing more precision for that softer tissue to insure penetration for the round moving at a slower speed with much less mass.
But the same parameters for the nine also hold for all the other calibers, too. I don't care if you use a nine, forty five, 10mm or a 44 magnum, to simply drop the bad guy requires a shot into the cranial vault. You can't take the big 44 and say because it's so much bigger I can take a lesser shot to the torso and count on a one shot stop.