Unless you're going for absolute period correctness, You might be able to afford a pearlitic cast iron barrel with a cast in shelby tubing liner with a plug welded by a certified pressure vessel welder.
A friend of mine, since passed, had one done that way, a full scale replica of a U.S. Army M. 1841 Mountain Howitzer. It was proofed to 30,000 cup. The originals were brass, far too expensive now, but the Iron tube with liner was less than $500 in 1987. Original caliber was 4 5/8". Terry had his made to fit a beer can filled with cement or plaster of paris. (2 5/8").
Three Hawks