Check out the link in the FAQ Sticky at the top of this forum that covers it, but this is something best left to either professional gunsmiths or someone with a high degree of mechanical knowledge, lots of guts, and about a thousand bucks or more in special tooling or access to them. Many good rifles have been destroyed by kitchen table-hacksaw gunsmiths. I'm not saying it is impossible, just try to fully grasp all possibilities before starting such an undertaking, including all possible safety risks as well as monetary losses.
Also type "barrel stub" into Google and check out the first site about stubbing a handi and chambering into the .308 Be||m. And if you search through the old posts here, Trotterlg has made at least 3 or 4 very well illustrated posts on this subject that I have been studying.
Hope this helps, and above all else, BE SAFE, AND BE CAREFUL!