Hey Cam,
It's easy, lay out your design. For the barrel channel you can use scrapers & chisels, OR you can rough it out with a router, and a 1/4 " router bit, rigt up the center of the barrel channel to the breech position, and to the depth of the barrel. You then can off set to the left and right of the center line for the barrel walls, but only 1/8" deep. You can then use a rarius bit to finish, or use chisels for the same effect.
Good luck!
Old Ford