As someone who has built new muzzle loading rifles as well as restored old rifles and shotguns back to shooting condition and used them to hunt with, I can agree that you will have an enjoyable time building or restoring one. HOWEVER!!! I can't recommend using the barrel you have. If the bore of your barrel is as rusted and pitted as the outside, my suggestion would be to purchase a brand new barrel and build a completely brand new match lock rifle in the Japanese style. When or if you finish making a matchlock rifle using the barrel you have, you will still only have a modern copy of a Japanese matchlock that may or may not be safe to shoot. New muzzle loading barrels are available for about $200 and you can spend your money and time more productively using a new barrel rather than trying to restore the rusty chunk of iron you have. If the inside is as rusty as the outside, you will have to re-bore and perhaps re-rifle your barrel, unless you want a smooth bore. By the time you do all that, you may find that the barrel is still unsafe to fire, particularly if you discover some internal (or external) flaw. As Brian pointed out, removing the breach plug will be the first and perhaps impossible hurdle to overcome. What you would like to do could be quite enjoyable, but you would be better off in the long run starting with a brand new barrel. A friend of mine build a matchlock rifle (using all new materials) and successfully shot a deer with it. There are people here in the US that do use original Japanese matchlock rifles for competition, hunting and just the fun of shooting them. Starting your project using a new barrel would also result in a finished gun in less time than trying to restore the barrel you have which may or not even be restorable. Good luck what ever you do but also keep safety in mind. Alternatively, buy a genuine Japanese matchlock rifle in decent condition and restore it to shooting condition.