What Mt. Hunter says above. Plus, most lead bullets are going to shed a bunch of their mass when they hit something, as bits of lead come out. So the bullet that started as a 165 grain, is now a 125 grain bullet in terms of its ability to penetrate the rest of the way through the elk. The 150 copper bullet is going to retain nearly all of it's mass, so it has 149 grains driving penetration. This is why you really don't need bullets that are as heavy when you're shooting monometals. My kids are now shooting 130 grain TTSX's out of their .308.