Up right now on ebay is the original incarnation of the Britains 4.7in Naval Gun.  this was a model of a Boer War naval gun, and this toy is the early version with a broad flat spring that gave it tremendous shooting power, it was greatly reduced in the later versions with a coil breech spring so as to minimize the physical destruction the gun wrought on metal figures.  In this version it often knocked heads off figures, and of course this is the precise toy that H.G. Wells used in his famous garden wargames, it's very probably also the gun that Robert Lewis Stevenson used in his attic games.