Ever since this game came out, I have been amazed of the poor attention to detail of this part in this game.
I have been wanting to do a mod but don't have a game and there are none local to inspect - as it is calling for a mod.
edit: 8-2-13
What I was thinking was a 3D print ABS black plastic authentic looking civil cannon would be perfect, but after getting dimensions and doing a few sketches it was deemed that people wanted it to look realistic in which a 3D print could not achieve due to what had to be housed and still be kept to scale. Therefore I had the thread moved out of my member location to the general Stern mods section and cleaned up to represent the new direction of a great solution from a member here.