The only method I have for swamping a parallel barrel is with files. It works but is a great deal of work.
As for lining...I have an English rook rifle that has a tapered barrel and a hook-breech. It was .38 cal. with a fast twist but in desperate condition inside. So I bought a new 9/16" drill bit, joined it to a length of 1/2" drill rod, and using a lathe, bored the original barrel out. I then took a brand new .36 cal. GM parallel barrel, and turned it down to just a hair under 9/16", slathered it with epoxy, and slipped it into the original shell. After dressing off the muzzle, and sighting it in, I have now a wonderfully accurate small game rifle that is a pleasure to shoot. It is almost impossible to see the joint.