Seriously. The human heart does not sit under 12" of tissue. A center mass shot in to the heart with just about any load is a thug buster. On the other hand, an angled shot through a thick coat, the lower lats up through the ribs, and the heart might be more than 12" deep.
I think 12" is a generalization. Heck, I know a psycho relative-in-law that killed a deer with birdshot at 20 yards... 7 shots in fact. Yeah, that guy scares me.