Double Naught Spy
Sus Venator
I think that meaning of center of mass or center mass in the context of armed self-defense is well established and well understood as standard self-defense jargon, so the point that it is not the actual center of mass, while worth noting, is somewhat beside the point.
See, another person who wants to call a location that is other than center of mass, center of mass. Well understood? I think you are confused. A big part of the problem is that it is not well understood, hence the multiple incorrect revisionist usurps of the name, and that is why there is confusion.
So: some folks have advocated avery precise aiming point (like the aortic valve), and have been criticized because such a precise aiming aiming point won't be hit under combat conditions. While that is likely true (and attempts to be overly precise during a gunfight would probably make one's response too slow), there is nothing wrong with declaring such a spot your "aiming point" if rapid shooting "around there" is the goal.
I always find this interesting. How do folks aim at that which they cannot even see? At best, you can aim at some external location where you hope is the proper location for the bullet's trajectory to pass through and then hit the desired internal structure should it have sufficient penetration to reach it. People like to note that shot placement is king. What they are talking about are those external locations. However, shot placement is nothing without proper trajectory and penetration.
As loneviking noted, once the bullet enters the body, it can do some unexpected things and doesn't necessarily fly true. So even if you hit the desired external location that you are using as a landmark guide to hit some structure behind it, that doesn't mean you will hit that structure. With a bit of searching, we can find several examples of people shot in the head where the bullet missed the brain or brain stem, shot center of the chest and didn't have the heart or lungs hit, shot in the hip without hitting or breaking the pelvis.
So until which time we develop x-ray optics for firearms that will allow us to peer inside the body and see the internal organs we want to impact, you cannot actually aim at a particular internal structure that you cannot see. You can only aim at some external landmarks...which as it turns out, are often cover by clothing.