Guys,
This is from an earlier topic about lining rifles barrels:
T.J.'s 3652 Neltner Road, Alexandria, KY 41001, (859) 635-5560, Mike Sayers, President. These guys make a large selection of hammer forged liners for modern calibers from 0.17 to 0.50, about 40 different ones. For muzzle loaders they make a 0.460 bore/0.476 groove, 1-48, 5/8 OD and a 0.510 bore/0.532 groove, 1-48, 11/16 OD. I used the 0.510 liner and was very pleased with the result as seen in the photos above. T.J.'s sells these for $3.75 per inch. I believe that Track of the Wolf sells these also, but at a higher price. Obviously the price of the liner will exceed the price of a new barrel, so it only makes sense to line only the most valuable barrels, line the really cool twist iron barrel, above.
For smoothbore barrels I just use regular thin wall tubing, either carbon steel or stainless. 5/8 thin wall tubing may work, although you may need to polish/sand down the OD a bit. I get tubing from McMaster-Carr and install them with slow setting epoxy. Not too difficult a job.
Jim