what size of cannon are you planning to make ?? the easiest way would be to use an lathe .
ok I can understand that it could be problems if its an larger cannon you are going to make ,
but try to find an work shop who have an enough large lathe . the cost to have it done will be just an very small part of the completed gun , and then you are sure to have it done with precision . dont forget that it should be reamed also and not only drilled .
would be very interesting to see an drawing or an master , Im also planning to build a few cast bronze cannons .
it would be for my son and some of his friends who now are training for an peace keeping force in the army .
I attach some drawings that you can see what Im planning , but they aint finished yet . just look at them as rough sketches .
I also got help here at the forum from seacoast artillery with an photo of an extremely beautiful dolphin design that Im planning to use , I attach that photo also .