In Prior Mt. Bill's book on building trade guns, he mentions using high pressure, seamless, gas pipe, for barrel material, with good results.
I have a friend that inherited a number of South American "monkey guns" a relative brought back from the rain forest. They all have repairs, and some are beyond scary. One in particular, had the nipple threads either burned off, or sheared off, and secured the nipple into the snail by glueing it in with a gob of chickle ( the material used in chewing gum), and then wrapping the breech with a piece if monkey hide with a little hole in it, for just the cap retaining portion on the nipple to protrude. I would suspect that light charges were the order of the day with this arrangement.
Hungry Horse