Cannons were made by several methods over their history (forged, wrapped, cast with a bore, cast solid and drilled, etc.) but the best of them, before the mid 1800s, were cast solid and then bored to the appropriate caliber. The weakest part of any casting is in the center of the mass of metal. As the metal solidifies from the liquid state, it shrinks. The outside of any casting chills first so the weakest material will be in the middle. If the bore is cast in place by placing a core in the mold, that weakened material ends up between the bore and the outer mold line of the barrel. Not good. If the gun is cast solid, the weakest material is in the center where it is then removed by drilling the bore. In these modern reproduction barrels, the liner is usually DOM (Drawn Over Mandrel) steel tubing with a welded breech. The best is seamless but some DOM tubing is also seam welded and then drawn. The liner is then usually installed in the drilled gun barrel with epoxy. In the large 19th and 20th century Navy guns used on cruisers and battleships (8 inch bore and larger) the rifled liner was installed by shrink fitting into the outer barrel...an immensely complicated and delicate operation.