A 150 grain bullet in a 30-06 will be plenty. I have harvested elk with this set up no problem myself, and my father has killed elk year after year with the same setup for over 30 years. My uncles have killed just as many elk using only a 270 winchester and 140 grain bullets. I know several people who have killed elk with no probelm using even smaller firearms (243's, 7mm-08's). Just make sure you shoot them in the right spot and it won't matter how big a bullet you use.