Excerpt from the "study" itself:
"Some people will look at this data and say "He's telling us all to carry .22s". That's not true. Although this study showed that the percentages of people stopped with one shot are similar between almost all handgun cartridges, there's more to the story. Take a look at two numbers: the percentage of people who did not stop (no matter how many rounds were fired into them) and the one-shot-stop percentage. The lower caliber rounds (.22, .25, .32) had a failure rate that was roughly double that of the higher caliber rounds."
"What I believe that my numbers show is that in the majority of shootings, the person shot merely gives up without being truly incapacitated by the bullet. In such an event, almost any bullet will perform admirably. If you want to be prepared to deal with someone who won't give up so easily, or you want to be able to have good performance even after shooting through an intermediate barrier, I would skip carrying the "mouse gun" .22s, .25s and .32s."
What the "study" doesn't sort out is who stopped because they physiologically lacked the ability to continue, and who stopped because they simply chose to. Those who choose to stop might be stopped by a realistic-looking squirt gun. We're told that often the mere presence/display of a gun (or a gun-like object) is often enough to diffuse a situation. So, this works sometimes. That's all fine and dandy, but not to be relied on when faced with a determined attacker.