A couple things I keep handy at my work bench for barrel prep, prior to bluing. I buy schedule 40 PVC in various diameters, and cut it lengthwise to make two halves. I cut these about 11" long, so a sheet of sandpaper fits and just overhangs each end. Depending on the diameter of the barrel, I use an appropriate size piece with sandpaper inside, and work the barrel length in strokes a little over half the barrel length. Then turn the barrel around, and work the other half, overlapping the strokes.
For octagon barrels I have a piece of 1" thick aluminum bar stock, about 2" wide. It is also cut the same length, and I wrap sandpaper around it to work down the finish and prep for rust bluing.
Half octagon barrels I work both halves up to the transition, but when it comes to final polish, I finish the octagon first, and then do the round up to the octagon, so I remove any polishing marks on the octagon from running over on the round portion.
