While it can be done, their is considerable debate about potential accuracy impacts. By turning it you could variously introduce stresses, or reduces stresses, that changes the interior bore diameter and hurt the accuracy. Same argument that people have about fluting barrels after they have been bored. No one will know for sure, and on a hunting rifle you might never be able to see a difference.
You need to find a machinist who will slowly turn it down in a bunch of passes.
Given you will need to have it polished and reblued, you might be money ahead just installing a new stainless barrel in the contour you want.
