At our police range we have a 1/2" plate steel 5 foot dummy on a spring stand that is supposed to simulate a 185 pound man standing erect. This device is used for a variety of target shooting exercises and for training in order to demonstrate what it take to put down a "person" with a handgun round.
Stopping power vs Knock down power: Shot placement from anything from a .22LR to a 44 mag will stop most anyone with a fatal shot to the vital organs, namely brain and heart. Having said that, during an ambush of FBI agents in Florida some years ago one of the criminals was fataly shot in the heart, lungs, liver, and head but still was able to kill two FBI agents and wound another before he fell with fatal wounds already inflicted from the agents 38 specials.
Although most attackers will drop or cease their attack when shot and wounded either because of phsycological reasons, self preservation, or a programmed response learned from TV shows and movies but those attacker who are highly motivated mentally or hopped up on drugs often continue their pursuit and attack.
So, again we come to stopping power vs knock down power. The 1/2" steel plate 5 foot dummy demonstrates to police trainees and other CCW self defense shooters knock down power needed to stop an average size male attacker. The 9mm wobbles it a bit, the 40 S&W wobbles it a bit more, and the 45acp knocks it over. Down and out is where you want an attacker who is trying to take your life or the life of a loved one.