This talk about theory is all fine but the tests I have done and the videos that I have seen are pretty convincing. The 1-1/4 ounce of #8 shot at 1400 fps will easily penetrate through the heart and destroy it from a frontal shot at 12 feet.
HERE is a video that shows less powerful loads going through layers of fabric, a pork steak, pork ribs, a beef heart and the drywall behind the target. (look at 7 minutes on if you want to skip most of the preliminary wall tests.)
The results shown in that video are very close to the results that I got using 1-1/4 oz of #8 shot at 1400 fps from 12 feet. I got full penetration to 6 inches and pulverized the heart in the process. You have to aim a shotgun in order to hit a "kill zone" at these ranges just like a pistol or rifle. The advantage to the load I used is that it causes a much bigger wound and is stopped by an exterior wall even if you completely miss the target. That makes it safer than a magnum pistol or a rifle and it still does more damage to the target.