Good question! I have looked for two years for a .308 barrel. Liked the rifle and bought one in .243 (new from a gun shop) and thought I would pick up a .308 or .270 barrel, but they have disappeared. Mossberg factory still had a few .12ga slug barrels left last fall. I took my .243 into a Gander Mountation to have a scope mounted and the gunsmith said "Oh, so they did really make some of these".
If you find any be sure and pass the word along.