Unless you know someone that has a router bit contoured to match the radius of the barrel. I think the tablesaw idea is a good one. It will take a few passes and trial and error (cut close and raise the blade a hair at a time) but it should work pretty well.
I would trace the barrel on the end of your board where you want the channel to go (make sure it is centered on the board) and that will give you the dims from the fence and the blade height. Measure from the edge of the board to the start of your channel, this will give you the fence setting.
Like I said, lower your blade lower than you think it should be (for some reason it always cuts higher than you think it will) make a pass and raise the blade in small increments until the fence edge of the kerf of the blade is right next to the line.
Then spin the board and do the other side of the channel since the blade height is already set. Then just move the fence away from the blade 1/8th of an inch and do the same thing, repeat until you're done.
Then as 45-70 said sandpaper on a dowel rod.
