BearschlayerX2,
My .338 Winchester Magnum was put together many years ago on a .30-06 sized FN Mauser action. I had a local gunsmith open up the bolt-face and fit a slightly used Sako barrel picked up for $30 at a gun show, but I did the work on the feed rails myself, which really isn't too much of a mystery if you do a little reading on the process beforehand, and go slowly. As it turned out I didn't have to change the magazine box. The rifle shoots great and feeds very slickly.
You'd have to ask a reborer about the possibility of your particular barrel being rebored and rechambered. I've had very good luck with both Classic Barrels & Gunworks (
www.cutrifles.com), and also JES (
www.35caliber.com) which despite their name also does .338's.
