Awesome idea. Heres one suggestion for you. Use the end of the barrel to hang on the wall but cut a piece of plywood to attach in the end that would hang on the wall. Then cut the top off about 3" and use that as a door. Inside on that plywood you could hang a dartboard and inside the top you could put a small chalk board and two small block of wood for the darts. Maybe at the bottom put a tray for the chalk.

Jeff Vandenberg aka "Woodsconsin"