I came across this project on the Hobby-Machinist site. Looks fairly cheap and easy to build. When/if I get caught up on my current projects I may try to make one and see how it works for barrel surface finishing.
http://www.hobby-machinist.com/threads/pipe-sander-plans.57255/

http://www.hobby-machinist.com/threads/pipe-sander-plans.57255/
