I recently spent more than a month cleaning up the bore on a Krag infantry rifle, and it still isn't "clean". I decided to part it out when I was tired of buying brushes, and it was clear that it would never be a cast bullet shooter. With jacketed bullets, it will do 3-4" at 100 yards from a bench. The bore wasn't just "fouled"--as is often found in milsurp rifles, it had layers of powder fouling combined with metal fouling from both cupronickel and gilding metal jackets.
Foaming bore cleaner did help to speed things up (and to make a mess). I didn't try electrochemical cleaning (like a "Foul Out" from Outers, or a homemade version), but it might be worth a try.
One commonly-used way to "loosen" old fouling is to shoot jacketed loads through the gun, then clean it while it is still warm (or at least soak the bore with solvent). I did this several times--and it did help--but didn't get it all.
Once the bore is clean, if it isn't too bad (a very subjective thing) there are some "tricks" you can use with cast bullets to maybe get enough accuracy for GAF skirmishing. Maybe. It all depends on the bore, and bullet fit.