Dog, you mentioned a Napoleon 1857, it is a great looking cannon. My avatar is an 1857. You are on the right track of design, but lack the profile will always look like an O2 bottle that makes noise. One of the difficult things with a cannon is the deep boring needed. You can purchase cored 4140 steel 4" d with a 1.5 cored hole, then plug the end at the cascabel, and have the barrel profiled.
If you intend to leave the cannon out in the weather be sure to fit a plug to keep out rain and other things.