Knowing where to shoot OTHER then center mass I believe is as important to know as where in center mass to aim ..

If you shoot center mass and are sure of hits but they have no effect the Perp could be either wearing armor or is high on a drug of some sort .. then being able to go either up to head shots or down to the pelvic region which is slightly larger area then the head and will cause a person to be unable to stand or use of their legs if hit there ..

Also in todays world they could be wearing an explosive vest center mass that you might not want to fire at either !! So other areas need to be practiced also !