Here's a picture of the 2nd gun I built - It's a 2" calibre smoothbore Parrott style. A high-pressure hydraulic tube, with a big aluminium jacket cast around it. This was proofed for 8oz load, blanks only.
I built the carriage using ready-made wheels that were once fitted to a Royal horse carriage. I bought them from the company that makes the carriages the Royal family ride in. This gun is now used by the South Western Artillery, based in Bath, Avon. The carriage can be disassembled in a minute or two and transported in an estate car, along with the barrel. The wheels need a roof-rack !