This is what some of the old time Pennsylvania Dutch and Appalachian gunsmiths did.
Remove the barrel from the rifle. “Unbrich” the barrel (remove breech plug).
Place the unbreeched barrel in a fast moving stream. The muzzle must face upstream.
The barrel is to remain in the stream fo 3 days and 3 nights.
This washes the devils (demons) out of the barrel.
It was very important that the muzzle faced up stream and not the breech. This washed the demons out. Placed breech upstream, new demons could be washed in..... not good.
After threes days the barrel was fetched from the stream. It was cleaned and a cross or X was stamped on the bottom flat behind the muzzle. The was usually under the front sight location. This mark would be hidden by the nose cap.
This was a “Hex” mark.
Hence......
Hex Barrel.
The hexagonal shaped was never used for a Muzzleloading barrel. Hex refers to the mark on the barrel not the shape of it.
To this day you’ll here hex or hexagonal referring to BP barrels.
It really worked. They probably bent or trued the barrel during this process. Also after 3 days the barrel would fill with silt mud or sand, when cleaned this helped remove any burs on the rifling.
Most importantly, it washed the demon out and the mark prevented it’s return.
You do not want the demon riding the ball.