Ken, what you are talking about requires more energy, i.e., two bullets having the same destructive effect on tissue per inch of travel, but one remaining inside, while the other exiting. Can't be done unless the exiting round had more energy to start with. If we assume identical foot pounds energy, either the energy is used up in deep penetration and flying out the back or in making a big ugly wound channel (ideally penetrating far enough to damage organs). There is no free lunch. In order for it to fly out the back at all, some of the potentially destructive energy is being wasted in travelling through the air behind the target. Best possible scenario would be for the fully expanded bullet to pop out the back and the then fall straight to the ground, having used up all of its energy in destroying tissue and bone. It's possible I'm missing something here, but I don't think so.
