If you mean rebore, call Bobby Hoyt in fairfield pa. He did two for me in years past. Great job.
If you mean to actually freshen the rifling, like the old timers did, that is a different story. It is something that can be done by a hobbyist, but is an invloved process and not easy to describe without photos or diagrams. I did it once years ago using a dowel, some lead and a piece of file for a cutter. It was a time consuming process.