A great many years ago, I helped cast a cannon barrel of iron. We made a rather detailed wooden mock cannon (less bore) and packed it, I mean really packed it half deep in sand and then added the top frame of the "mold" and packed the other half. The sand was made real sticky by adding molasses (really!) we then cut sprue holes and took the halves apart and took out the wooden model. I wasn't there for the pouring. After the casting cooled, the barrel was x-rayed and then bored out on a jury rigged home made maching that spun the cannon barrel against the stationary cutter. Then it was reamed by hand with a hand forged reamer. The the cast trunnions were trued up by hand with a brace and another hand forged cutter. It was finally vented. Finally x-rayed again and test fired. The whole process took longer than I had patience for but it made for a great little cannon. As I recall, the tube was 28 inches long, about 5 inches in diameter at the breech and 4 inches at the muzzle with a 1.5 inch bore. We made a model for a thunder mug too, but I never saw that finished product.