Actually I thing stopping power comes from gunwriters who invented a subject suitable for endless debate. In the real world we encounter states of being such as 1.Unhurt 2. Wounded 3. Incapacitated 4. Dead. The faster someone transitions from the first state to the third or fourth the more apparent "stopping power" seems to be but there is no formula which relaibly predicts the speed of transition with any given weapon or even wound location. Some people are incapacitated by a word. Others endure multiple grievious wounds for a considerable time without being incapacitated or dying.
"Let us speak courteously, deal fairly, and keep ourselves armed and ready." Theodore Roosevelt
"If only God has magic, how does Santa get down the chimney?" Natalie Peters age 4