I participated in a CW reenactment years ago at Ft Steilacoom, WA (near Seattle) where a member had made a 3/4-scale Whitworth, using 1 1/2" high-pressure gas pipe. Its O.D. was 6" with 2 1/4" thick walls. He also had heat-shrunk a reinforcing ring around the breech area, that also had the breechblock hinge forged into it. (The owner/builder was also the owner of a small metal-fabricating shop, which included a small casting facility, and machine shop) The barrel was about 6 ft long, and tapered to about 4" at the muzzle. The hardest part was making the breach, having a triple-leade threaded breech cap that sealed the breach in one rotation. The owner would ONLY shoot blanks, no projectiles at all, not even golf balls. The cartridge looked like a salami wrapped in foil. (with corresponding humor, the Gun Captain would occasionally substitute the command "Load Cartridge" with "Slip me your salami...")
I big regret is never taking any pictures of it...