Thanks El Caz, good post. I agree with Dom, as far back as I can remember I've been fascinated with the (usually one-offs) incredibly detailed miniature artillery models that craftsmen like this gentleman produce.
You're right Zulu, but this guy isn't putting on airs, he comes straight out and says that he doesn't really know much of the history involved with the cannon he made. I also think you're on target in thinking that he read "tar bucket" somewhere, then further conjecture led him to assume that the tar was actually intended to 'slow' the wheels down. It became known as the "tar bucket" because 'pine tar' actually was used in a mixture with 'tallow' to make the lubricant for carriage wheels before the later 19th century.