Again, thanks for the input. I was trying to keep the thread focused on the question of replacing the barrel. The best I have been able to get out of this gun was 4 inches at 25 yards off the bench for three shots, but frankly I think that was a fluke. A more typical performance, again off the bench was needing 10 shots to hit an 8.5 x11 sheet of paper 5 times. I know that the flintlocks are there own kettle of fish, but that is a lot worse than what I can live with.
I have been shooting BP guns since I was a child in the 60s, though not many flintlock rifles (I have shot primarily flintlock smooth-bore muskets and pistols, match locks and wheel locks, cap-lock rifles and rifled muskets, cap-lock breachloaders and revolvers). I travel in a crowd of BP shooters and was shooting with the local BP club when I was working on loads for this gun so I got good input from some good flintlock shooters when I could not find a load that would work. I traded this gun from a friend, another experienced BP shooter. When I had trouble building a load, I called him and he said he had the same problem with the gun. Despite the problem, I am in the gun very right (about $500 in trade goods and the kit for it is currently about $880) so there is room to fix it if it can be done.
If it is not possible to replace the barrel, counterboring the muzzle would be a solution of sorts. At least it would keep the original length. But we are not there yet. I am doing some measurements on the barrel to see if any of the available barrels folks have mentioned will work.