I don't see a huge difference between a heart/lung shot from a bow or rifle and you can't compare the energy differences between rifles and bows. However, with a bow, you don't have the option for a) a fast second shot; b) imobilising shots (i.e. leg, hip, spine); and c) brain shots. There is also a reason that most elephant hunters opt for brain shots - its quick, clean and instantaneous. No, its not always possible, but ethically most people think its the best/correct/sporting way to hunt them. JMHO