I recently decided my chokes were too tight on my 20 bore stogie and managed to find a dowel with a close fit. I slathered it in clover emory paste and ran it in till tight on the lathe.
I used a wood clamp as a spindle clamp and a Arthur Leonard, (brother of bolt gun collector Lou Leonard) cigar box as a sort of steady rest, and let it go for a few hours. Every ten minutes or so I would run it in until it squealed like a distressed maiden. After half a day I opened it up from 600 to 605. I took it down to bigsky sporting clays to test it out, and it seems to be much improved.