Hi Ken,
As Fernando said, it could be a toy due to its tiny size. That being said,I think it could still be a miniature signal gun. Some were indeed diminutive and were used to signal the hour on ships, serve as alarm clocks of the day. Here's a wiki page with an example cannon like yours. If it were only a toy, I don't think it would have the English proofs like yours. Congratulations on the neat find!
https://en.wikipedia.org/wiki/Sundial_cannon