Yeah I looked at the couple of threads I could find here and google but didn't find what I was looking for.  Since black powder and air/pneumatic guns aren't considered firearms in the non-commie states, would it be possible to build one without doing the paperwork?  Yes, I know it'd only be a single shot but it's as much a proof of concept and "can I do it" thing as anything else.  I know both types were made for some time and were popular in England and France and that some of the old airguns were quite capable of being lethal and of course the new ones are as well.  

The cane gun thread with the .410 barrel got me thinking that you could cut the breach short so that it could only accept a 209 primer adapter, not an actual shell and, to my mind anyways, still be within the rules of BP firearms since you would still be required to front load it with components.  I would be concerned about using it as an actual cane and having the wadding and overshot card become loose from tapping on the ground as you walked.    

Just a novelty idea....