I've always wondered about hydraulic cylinders. They are quality steel, of course, nicely honed internally. But they are not particularly heavy walled, and would require a breech to be fitted. Threading would reduce the wall thickness further. If one were making a drop fired mortar, I could see hydraulic tube being a reasonable choice. Just don't know about using one for a ml cannon. If I were going to use a tubing product, I think I would be inclined to use heavy walled mechanical tubing.
SignGuy - you're in Ottawa - if you get a chance, drop by the Metal Supermarket over on the east side of town. There will likely be pieces of heavy wall tube on the odds and ends rack that will stir your imagination. Heavy enough material to really thread for a breech, and to mount trunnions, as well as to turn to an attractive contour. Many years ago I made a mortar fron what I understand was a track tensionner from a bulldozer. Scrounged it from a dump, already had a small can sized bore in it. Drilled a reduced diameter powder chamber. Made a trunnion and a block mount. Piece was about 4" diameter, a foot long. It would launch a can about 200 yds, 8 seconds in the air. A good scrap yard is worth checking out.
