Hi BoomJ, the bore size is .75cal which is the maximum size allowable under our state laws for "live fire" cannons in New South Wales Australia. A company called Pioneer Gun Works originally made these barrels and sold them for many years and now Newcastle Gun Works manufactures and sells them. They are generally supplied as cast to the customer and are the most common pattern on the firing line here, Regards, Peter B.
P.S. Thanks for the compliment Victor3 and I agree those barrels are very well detailed and are intricate to finish from the casting stage especially around the cypher