Years back I bought a half scale Napoleon tube from Hern Iron Works in 2 1/4" bore. Built a carriage from Home Depot lumber as actual oak was just way beyond my budget. It served the purpose and once painted you couldn't really tell.
Balls were cast from wheelweight and weighed right around 2 1/2 pounds. I eventually went to casting them from zinc as that would reduce the weight to around 1 1/2 pounds... more zip for the same amount of powder and hopefully a little less stress on the carriage.
I'd always been a little nervous about having that large of a bore in this particular tube.... just didn't leave enough wall thickness to please me. So one of the local board members fabricated a sleeve to epoxy in place that would reduce the bore down to golfball size. Will also fire 1 pound round fishing weights.
Being of Douglas Fir the cheeks eventually started to warp. A year or so ago I found a piece of 2 1/4" thick oak that used to be a mantle on a fireplace... just the perfect size to make into replacement cheeks. Finally got started on that project last month. Everything is pretty much back together again, just have to finish the ironwork over the trunnions and then it will be off to the local range for a much anticipated test drive.
Note: The pictures are of the old carriage. New current one has heavier wheels and much heavier cheeks. The cheeks also now have an iron strap over the trunnions rather than that wood clamp thing. The useless hanging rings are also gone.
One thing about having a cannon in the back of the pickup... sure does discourage tailgaters.