Evidently not adequate penetration.
I worked peripherally on a case in which a guy shot two guys from a different social club with a 21a. Both DRT, dropped like rocks.
We could do this all day.
You need to hit the vitals to effect a stop with a handgun, which requires shot placement and adequate penetration. Choose your ammo to best be able to get this done, practice, and study up on where you need to shoot. Ultraman specifically requested that we avoid comment on his caliber choice - presumably he's aware of its limitations.