The Gonne, like so many other recent technological devices in Discworld, was invented by Leonard of Quirm. As usual, he had the best of intentions when he devised it, but it turned out to be one of the most dangerous weapons ever conceived in the history of the Disc.
The weapon is powered by a kind of firework mechanism. It consists of a long tube with a feed mechanism for 6 small cartridges that can be fired quickly and with dangerous accuracy over a long distance. This fact makes the Gonne much more dangerous than the common crossbows.
It was so dangerous, in fact, that Havelock Vetinari, who normally keeps anything useful around, ordered it destroyed. But the Assassins' Guild disobeyed the order, and instead secretly kept it under lock and key in their Guild Museum. Disaster strikes in the book Men at Arms when the Gonne is stolen.
With almost supernatural power, the Gonne can possess the mind of the man who uses it. It shows him the power he has in his hands, and erases all scruples by telling him what could be achieved with this power. Even Samuel Vimes struggled against this temptation and the only man who seemed to be entirely immune to the Gonne's promises was Carrot Ironfoundersson, who finally managed to destroy the weapon once and for all. Carrot is immune because he has a dwarf's pragmatic attitude: a made thing is just a tool crafted for a purpose. why should I listen when it talks to me, as there's nothing there to do the talking? (This is probably why Hammerhock was killed after his purpose, of performing a minor repair on the gonne, was over. He also viewed it as a device, a clever device but nothing more, and was loudly speculating on building more—a prospect which the Gonne did not like.)
The Gonne has never been seen again. It has been suggested that Carrot buried it in the coffin of Lance-Constable Cuddy. His reasoning was that as the Gonne "died" when he smashed it against a stone pillar, its spirit could accompany Cuddy on his journey into the Afterlife, so as to provide a suitable weapon to fend off the evil spirits that Dwarfs don't believe in, but which may inconveniently believe in Dwarfs. The fact that it was well hidden and no man could re-create such a dangerous device ever again (unless they find Leonard's original sketch and get interesting ideas) was of course purely a secondary consideration.
Contribution to industrialization
The Gonne might represent a quantum leap in crime and warfare, but in the process of manufacturing the Gonne Leonard comes up with an invention which while unnoticed is a cornerstone of industrialization. When Vetinari asked Leonard if someone else could build a Gonne, his response includes:
...the grooves in the barrel requires some finesse, I had to build a quite complex tool for that
The tool that Leonard had built for this purpose was a screw-cutting lathe, one of the cornerstones in the progression from craftsmanship to serial production.
In Roundworld terminology, Leonard's Gonne is probably a self-loading wheel-lock rifle. Though it fires six shots before reloading, it is definitely not a revolver; the six welded tubes of the magazine are arranged in a line, and advanced by a rack and pinion, making it either a repeater or a self-loader. The rack and pinion suggest some form of automatic recocking mechanism, making a self-loader more likely. The grooves in the barrel imply rifling, as opposed to a smooth bore like a musket. The firing mechanism is described as a "tinderbox," which means flint and steel; this could be either a wheel-lock, snaphance or flintlock, but the wheel lock is the only one that would be contemporary with Leonardo da Vinci, whom Leonard clearly resembles.
In Roundworld history, some efforts were made to build repeating firearms during the wheel-lock and flintlock era, such as the Kalthoff repeating wheel-lock of the early 1600s. Repeating firearms would not become widely used, however, until the invention of the metal cartridge in the 19th century, and self-loaders later still. Nothing precisely like Leonard's weapon ever existed, though its various components all did at one time or another.
The museum at Château du Clos Lucé, in Amboise, France, where Da Vinci spent the last 3 years of his life, along with the models (made from his drawings) of a tank (of sorts) and a
spinning-up-into-air-machine screw-drive helicopter also contains a model of a kind of multi-shot machine gun, though it is different in scale (each barrel is a couple of meters long).
In WW2, the standard issue machine-guns used by the Japanese and the Italians worked on a similar principle to the loading system for the Gonne, adjusted to allow for automatic fire. Rather than a loose flexible belt in which the individual rounds were loaded into a cloth strip (as per Russian practice), or linked by re-usable metal clips (as per German), these MG's employed a fixed and rigid "tray" in which the rounds were fixed to an inflexible metal strip capable of carrying no more than ten rounds at a time. At least the British Bren Gun loaded its rounds into a fully enclosed magazine. The Italians fought their war in a desert—they soon discovered their system was an invitation to load sand and grit into the mechanism as well as the round. The Japanese discovered similar drawbacks in the jungle. But this indicates how, with a little refinement to the design, the gonne in the Discworld could so easily have become an Auto-Kinetic Machined Gonne That Carries On Firing Without Human Intervention.
One of the earliest proto-machine-guns, the Mittraileuse of the late 1700's, involved six to twelve independently loaded musket mechanisms. The barrels and chambers attached to a rotating wheel, which as it fired swung the next barrel into place to meet the flintlock. While it took forever to load each barrel, once deployed it could lay down a quick and devastating burst of fire along a tactically vital arc. The French used it as a static weapon in fortresses.
And if you scale up a gonne, or scale down a Barking Dog (a legal weapon on the Discworld), where is the point at which a Gonne becomes an efficient artillery piece, or a Barking Dog a crudely effective but strictly illegal hand-held weapon?