2-dimensional echocardiograms were performed on 150 standing adults in the parasternal, apical, and subcostal views. From each view, the shortest linear skin-to-heart distance was measured.
parasternal. : adjacent to the sternum.
Average skin-to-heart distances were: parasternal 32.1 +/- 7.9 mm (1.26378 inches).
So it`s less than 2 inches from the surface of the skin to the outer surface of the heart.
1 mm = 0.0393701 inches
"The gun must be fired at a so-called “vital area” of the attacker. This usually means the heart or the brain/spine. Hits to the lungs and other organs can also be effective, but results may be slower.
The bullet must have the ability to penetrate whatever is between the muzzle and those vital organs in order to disrupt their function — for non-uniformed civilians, this typically includes clothing, tissue, and bone.
No matter what else may happen during the course of a lethal confrontation, if you shoot the guy in the right spot with a bullet that penetrates deep enough, the threat will go down." -By Chris Baker
Handgun Self-Defense Ammunition Ballistics Test