A fyne gun indeed sir ! Anymore info or pictures on this one? (Length-Bore) We should all have one standing guard outside our homes.
From memory, the length of the barrel was around 5 feet. There was no bore as such, although there was an armature across which the trunnions were welded, to support the rest of the barrel material. This armature was welded up from scaffold pipe. The carriage was softwood, but the wheels were steel-rimmed, the axle holes were welded to the rims with steel rods, forming a random load of "spokes". Then the gap inside the wheels was filled with cement. The metalwork was 3mm steel plate, cut and welded with a MIG welder. I can state with certainly that you shouldn't make a carriage from softwood. After several years outside in all elements, the inside decayed to a light spongy mass, just held together by the ironwork and a dozen coats of paint !
One day, it suddenly all just collapsed into a heap of mouldering dust....... even the barrel fell in half. I daresay softwood is okay for indoor use. The wheels survived though, so I recommend my method of making truck-type wheels.