I have a buddy who is on a swat team in San Jose,he tells me if i ever have to shoot someone that I should aim for the lungs then aim for the heart or head,I've been practicing that way for awhile now on silhouette targets.I'm taking his word for it since he has put down a few bad guys.
A lot of silhouette targets put the vitals too low. Make sure yours are anatomically correct. It'll help you in the long run.