The Armstrong Disappearing Gun
For anyone on the other side of the world there is another of Armstrong's products near Dunedin in New Zealand. In response to an 1886 scare about an invasion by the Russians they installed a "disappearing gun" at Taiaroa Head, at the end of the Otago Peninsula (there are a few others that are since lost). It is held just underground with a set of gas springs that will push it up in order to fire. The recoil will then push it back into its hidden position.
One for the Geek's Guide to New Zealand, maybe....
This is the one I've visited. I note from the wikipedia article that there are others in various locations, some in working order - the one at Taiaroa Head isn't.