I find it interesting folks often refer to "center of mas" when they don't mean center of mass. As noted in the OP, COM would be a gut shot. I can't think of any firearms instructors or schools that I have attended or read their books that advocate gut shots for stopping a threat, though nearly all refer to making COM shots. The terminology is being misused by folks who don't actually understand what they are saying, in part because most have not studied anatomy or physics to any great extent or because they are just repeating common defensive shooting terminology without actual consideration for what it means, sort of like when reporters call almost any long gun an assault rifle.
Here is an excellent explanation for human center of mass...
http://hypertextbook.com/facts/2006/centerofmass.shtml
Actual COM shots are not supposed to necessarily take out vital organs. They are to give the shooter the best location for aiming that will provide for the greatest amount of error while still hitting the target.
Some folks refer to COM when they are actually meaning the center of the body itself (from the bottom of the pelvis to the top of the shoulders) and not taking into account the arms, legs, and head. This raises the shot to the level of the lower sternum. This is the point on the body that yields the greatest amount of room for error while still hitting the body.
Some folks refer to COM and they actually mean center of the chest which raises the shot up higher to about nipple/armpit level. This location gives the greatest amount of room for error while still hitting the chest and it also gives the greatest amount of error for being able to hit vital cardiopulmonary structures, but it isn't the actual center of mass of a human.
Area descriptors like "thoracic triangle" or "center of chest" tend to be more accurate for intended representation and inclusion of the primary vital organs to be targeted.