A fatal bullet through the heart can leave a guy up and shooting for over ten seconds — obviously an eternity for a law enforcement officer in a point-blank shootout.
A guy can get shot and not know it. Police officers are trained to check themselves carefully when any shots are fired, because it has happened that officers thought they were unharmed, went home, laid down to rest, and bled to death.
A guy can be in a shootout and THINK he got shot, and it turns out there’s not a mark on him. They get knocked over, feel intense pain, cry out, etc. There were enough of these cases that the researchers were able to find some involving officers who were previously decorated for valor and had in fact been shot before.
A guy can get shot multiple times with a powerful weapon and get lucky, or he can get shot with a tiny weapon and get unlucky. I still remember the story used to illustrate this: a fight between roommates got ugly, and one guy shot the other six times in the chest with a .45. The second guy went to his attic, found his great-grandfather’s Civil War-era relic small caliber revolver (something like a .22), went back downstairs, and shot his roommate through the heart, killing him. Forty-eight hours later, his desire for medical care overcame his aversion to getting caught, and he walked into an emergency room. He recovered completely.