Bell, from what I read, used a 7mm Mauser with fmj bullets. He made brain shots. Between the eyes or between the eye and ear from the side. Bullet didn't expand but penetrated the skull and killed them right there. Heart shots or lung shots mean they may run a distance before collapsing. He really didn't want this. He also left them to feed his caravan, then, came back later to easily pull out the ivory tusks.
