From the New York Times: "Early in my medical training, I learned that it is not the bullet that kills you, but the damage from the bullet. A handgun bullet enters the body in a straight line. Like a knife, it damages the organs and tissues directly in its path, and then it either exits the body or is stopped by bone, tissue or skin.
This is in contrast to bullets from an assault rifle. They are three times the speed of handgun bullets. Once they enter the body, they fragment and explode, pulverizing bones, tearing blood vessels and liquefying organs.