Use a router to hog out or cut out as much of the waste wood you can, then use either a section of the barrel, or the barrel itself as a scraper to remove the rest of the wood. All that is needed is a sharp squared edge on the end of the barrel, and a lot of elbow grease. There are round scrapers sold by gunsmithing supply houses, like Brownells to do this kind of work. Even most modern, round, gun barrels have a slight taper to them, so this is a common problem for all stock makers.
One of the smarter approaches( defined as any better idea than I have thought up on my own! :blah: ) to doing this kind of work is to mark the sides of the stock along the barrel mortise with ledger lines, spaced every 1 inch or so. Along the side of the stock, or if the forestock is wide enough, mark the diameter of the barrel at each of these "Stations". This gives you an instant guide to use in checking the progress of your work as you scrape the taper to the barrel channel. :thumbsup: