...is a myth.stopping power
PERIOD.
I have not shot people with handgun rounds, however, I HAVE shot people with rifle rounds in the 5.56 variety, and have seen the results of 7.62 / .308.
Humans "stop" as a result of gun fire due to 1 of 2 reasons...
1. Mental / psychological reasons.
2. Physical reasons.
For number 1, they "stop" / fall down because they realize they have been shot, are still alive, and stop / fall in hopes of NOT getting shot anymore. They are still capable of resisting, but lack the motivation / mindset to continue to fight.
For number 2, they "stop" / fall down because some part of their structure has been damaged enough that they are no longer physically capable of either standing up or maintaining the fight. This is caused by either structural bone damage, circulatory damage, or CNS damage. CNS damage is the ONLY guaranteed instant physical show stopper. When you cut off the communication to the brain through damage to the brain or spine, the body does not function. Circulatory damage, even a heart shot, still gives time to resist until oxygen is depleted from the brain. Seconds may not seem like much time....until you are getting shot at. Structural damage to bone mass, like femurs, pelvis, fib / tibs, can put an individual down, however, they may still have the will to resist.
Round selection or shot placement has little to do with number 1, however, round selection and shot placement DOES influence number 2.
A defensive gun SHOULD be geared toward achieving number 2 imo. I want a round that has good potential to stop a threat physically if it doesn't mentally. A round that can punch through bone and tissue without having a significant effect on trajectory stands a better chance of penetrating key areas within the body that can incapacitate and stop a potential threat. This means punching through arms, ribs, and whatever else may need to be penetrated, and still have enough mass and velocity to damage CNS or key circulatory systems within the body.
NOBODY wants to get shot...by ANY firearm. A .22 short can stop an attack just as easily as a .45 long colt if the threat lacks the will to resist.
However...
To STOP a determined threat, you must have a round capable of reaching and significantly damaging the systems within the body that will immediately incapacitate the threat. This favors the larger and more powerful rounds as they tend to have the mass and velocity to both penetrate and maintain their trajectory. That's why I carry .45.
Defensive approaches have pros and cons just like pretty much everything. Smaller arms are more convenient, however, they lack some potential effectiveness. Can they do the job? Sure.
Larger arms are not as convenient, however, they provide a greater deal of POTENTIAL effectiveness. Would you ever actually NEED this effectiveness? Who knows...
Learn to fight with what you have and maximize the effectiveness of what you DO carry, and understand the limitations of yourself and your firearm of choice. That's about the best you can do.