Hey, umike! We talked at Clifftop - may have been two years ago. We were both playing instruments we built and talked bass construction quite a while. Your single-stringer double-barrel design was about 10x louder than my little teardrop. Loudest acoustic bass device I have ever heard.Fortunately you have the sense and good taste to use it well.
Welcome to TB, if I can help, send me a pm.
When I saw that picture, I KNEW that instrument had to be a cannon! Thanks for confirming this.
George