"Center Mass" is not a helpful construct. To be sure of effecting a handgun stop, as many of us have mentioned up-thread and as FredBart and flop-shank point out, the bullet has to hit something that either results in an immediate failure of the upper body to receive oxygenated blood or else the upper central nervous system.

You can shoot someone in the stomach with a .500 S&W Magnum and still have a failure to stop. You shoot perforate his aorta with a .25 ACP, he's not going to be coming at you any more.

Aim for specific vitals - as I always preach - study Gray's Anatomy and learn to target the heart/aorta and the brain.

Gray, Henry. 1918. Anatomy of the Human Body