"The human heart does not sit under 12" of tissue."
IF, and I mean IF, every shooting involved your target standing still, facing you, arms at sides, then 12 inches of penetration would be overkill.
But, oddly enough, humans tend to move around. In a firefight situation, they tend to move around a LOT, which includes ducking, turning side to side, leaning over, putting their arms up, etc...
When you start adding in all of the possible angles and the such, you quickly get to, and can even surpass, 12".
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.