I think the natural cartridges that come to mind are either the .300 Winchester Magnum or the .338 Winchester Magnum. I would personally go with the .338 Winchester Magnum. If your primary rifle is .30-06 for deer and elk, then the .338 Winchester Magnum seems a good choice. You can use the .30-06 for deer and sheep to whatever range you can shoot to and score kills. On a bigger animal like the elk you may want more energy at ranges over 300 yards than the .30-06 will deliver, and the .338 Winchester Magnum fills that role admirably.