If this is a plain flint style breech you may be able to trim that liner in the bore from the breech end using a thin file edge after removing the breechplug.
I would cast a lead lap and use a fine abrasive to lap the bore. You’ll need a wooden rod close to bore diameter. Half an inch from one end file a square section 2” long. Just below the square section file a good groove around the rod and wrap cotton string in the groove to make a gasket of sorts.
Insert this in the muzzle end, other end first, till the string gasket starts to enter the muzzle. Now tap it in till the square portion is right at the muzzle. Might have to use less string, or more.
Melt some lead and pre-heat the barrel with a propane torch starting about 4” down from the muzzle. Don’t un-solder anything. When the barrel is hot and the lead is melted good, pour the lap. In a minute push the lap and road out from the barrel and allow all to cool.
Snip and file the excess flash lead off the lap, oil it up, and slide it in the muzzle. Use a short starter to knock it in a ways. Drive it back out. Repeat, with oil, driving the lap further using a ramrod repeatedly till it will pass end to end.
Now load that lap with pumice mixed with oil, or lapping compound, or valve grinding compound and work it end to end 200 times. Reload with grit as needed. Remember the middle gets it both ways so be careful to do extra shorter strokes at breech and muzzle.
Clean the bore thoroughly with patches soaked in whatever is handy. I’ve used turpentine.
Check it out and if this doesn’t do it, a mild re-cutting of the lands and grooves will.