So you have 3 loose pipes fitted together and a vent drilled thru all 3? Basic "rules" are the truniun diameter is equal to the bore and extend out the diameter of the bore from the barrel. They are located where the center of gravity is for the gun. the only "barrel" you have is the long barrel. Having additional steel around will not prevent a failure, like a chain it is as strong as the weakest length. The wall thickness in bronze or iron is typically equal to the bore. With modern seamless mechanical tubing you can get away with 1/3 the thickness depending on the material properties.
In other words to be "correct" for a 1 inch bore you should be 3 inches in diameter at the breech whether for looks or strength depending on the material used. Your truniuns should be 1 inch in diameter and extend out about 1 inch, the barrel should near balance on them with slightly more weight on the breech. I hope this helps.