A dowel rod and sandpaper will get you so far but you could run into the bedding block depending on your barrel contour. If you get down to the aluminum a dremel tool will work but go slow it's real easy to let it get away from you. If sanding doesn't do it for you, have your smith open it back up with his mill. A couple of passes and he'll be done. Good luck and use 60 grit.
I'm not sure why I'm even wasting my time replying to this but, why would someone just show a photo of a $50,000 plus machining center for opening up a barrel channel. We have all read Nesika Chads posts and realize he likes to brag about his cnc equipment. A very very small percentage of people doing gun work can afford to use cnc. I run cnc and manual machines in my shop but I don't brag to the world about it. How about some down to earth answers and leave the high all mighty " I run CNC " crap out.