The most peculiar I have seen fired was a multiple-barrel matchlock hand cannon. It had 6 sequential barrels of about 7/16in, arranged with a very slight fan between the barrels. What was neat was that it fired them all from one trigger pull. The stock was a simple pole that the barrels screwed onto. It looked very much like a rake, and probably would have been used against charging troops or cavalry.
The most peculiar that I've actually fired was a black powder blunderbuss. The charge was, "About triple what'd you throw in a musket" according to the owner, and the shot was about a billion BBs. It kicked HARD, and pretty much defoliated the backstop. Basically, what we're talking here is a black-powder Claymore mine with a stock. At 50 feet, the shot pattern had to be the best part of 15 feet wide.