bdmtm29, take heed to GGaskills comment: "And 5" and 6" steel round is very heavy." As an example of how heavy this size range can be, our first cannon we made for sale, the 1/6th scale 100 Pdr. Parrott Rifle, started life as a 29" long X 5.75" Dia. 4150 steel round. These weighed 219 pounds before machining! Finished weight is only 45 Lbs yielding bags and bags of lathe turnings and milling machine chips.
Still though, we would not try and dissuade you from using the heavy piece as it will, after all, produce a larger cannon with more curb appeal and capable of snappy golf ball shooting and a very respectable report. Please keep us informed.
Regards,
Tracy and Mike