IMHO, (and I'm no expert) a "center mass" shot is a high probability of HIT shot, not a high probability of KILL - especially in these days of concealable, available body armor.

As mike 1100 said - even with HIGH power weapons, well placed lung/heart/vital shots are not one-shot-drop. The only shots guaranteed to drop the victim instantly every time are those that sever the spinal column - an inarguably low percentage shot from an accuracy standpoint.