Re: buying a used blackpowder rifle
There's one or two suppliers that sell underhammer actions and you put your own barrels on them. At that point you need to determine what purpose the gun will have. Round balls guns are really only good to a max of 200 yds. after that the wind and trajectory makes fine accuracy a major problem. For long distances, you need a bullet and faster twist. There are dozens of styles of rifling. Some makers offer only one or two styles and those are general purpose. If you are after minute of angle accuracy, with either round ball or bullet, you will almost certainly need a custom barrel.
Years ago I talked to a guy named "Hoppy" who was a barrel maker. He suggested I read on on rifling styles and then call him again. Hoppy made a very special barrel for me for shooting round balls. Slow twist, grooves five times wider than the lands and the grooves were cut with round sides instead of square sides. I won alot of matches with that barrel. He even put a choked muzzle on that rifle barrel. Talk about some precision shooting. If you are going with a bullet barrel for long range, read up about Harry Pope's system of rifling and the muzzleloader centerfire guns he made. If I were going to build a long range barrel gun, I would use Harry's system and a false muzzle. For thousand yard shooting, anything less thhan a 45 caliber 500 grain bullet isn't sufficiently stable. A 40 caliber might be good to about 700 yds. There's another system of rifling that I don't believe anyone copies today, and that is Alexander Henry's sytem of rifling. the lands were round humps and even the grrooves had raised middles. His barrels were choked just in front of the chamber and again just at the muzzle. Having such a barrel made today would be extraordinarily expensive. With pure lead bullets, they were about the most accurate black powder military guns ever mass produced. Shooting hard lead bullets in aLEX HENRY barrels is dangerous.