The 12 gague will be a heck of a lot better that the 30-30. The 30-30 would never even enter my mind as an elk rifle.

A 12 gague slug will essentially be like a muzzle loader shooting full bore bullets. You will be limited to 100 yards or less. It will also depend on the exact type and weight of the slug that you would be using. Don' t bring a 30-30 elk hunting.

Do you have any other rifles of your own or that you can borrow?