The one thing that puzzles me is the flare of the muzzle; I've only found one other example in a search that revealed many straight muzzles. I guess it would still work even flared.
The Lyle gun does not have to be too large as it shoots a weight carrying a 'messenger' or light line to the vessel in distress to which a heavier line is bent on to and sent out; then the breeches buoy or whatever rescue rig is set up.
A nice cannon to have; wish it could talk.
These were pretty much the standard in the US at the turn of the century for starting yacht races; newer versions still are.
https://www.youtube.com/watch?v=pWFSVc1DIoo