The idea is you use a cylinder slightly bigger than the barrel. it will clearance the side rails first, as you remove material it drops into the barrel channel, opening up the bottom last.Have you done this exact thing before? Curious as to what you used? Wood dowel into a drill? How do you attach the sand paper? How do your sand out the sides/walls of the barrel?
I havent done it before so someone else with experience might chime in but I would imagine it might produce a cleaner result to use a linear motion so you can maintain pressure evenly along the barrel channel. chucked in a drill might be hard to keep even pressure and/or might tend to jump out of the channel.
