During the medieval era of our world, weapons were getting pretty complex, but then gunpowder become commonplace and warfare was reduced to pulling a trigger. During the later end of this period, semi-automatic crossbows were being designed and used by the Chinese.
Could a ballistic weapon be designed in the medieval era that is capable of fully automatic fire?
For starters: The Chinese semi-automatic crossbow is not the only design of crossbow that holds and fires multiple bolts. That design (the repeating crossbow) operates with a combined reload/firing mechanism operated by a lever.
The second, the greek Polybolos operates by the continuous (turns out it isn't continuous, but with a suitable gearing mechanism could be made to be continuous as it's just a change in direction) turning of a windlass, and as long as it's supplied with bolts from the top could theoretically be fired indefinitely. I think this already qualifies as an automatic weapon because there is no discrete action required to launch the next bolt, you just provide mechanical power and it continues to fire.
If this isn't automatic enough for you, however, you could also add a twisted rope store for rotational energy (similar to those used in catapults) and some gearing to make sure that the release of power isn't instantaneous but instead happens at a reasonable rate. Wind it up ahead of time, wait for your attackers, then pull the trigger (which removes a pin that would otherwise prevent the bow from firing when fully retracted). Fully automatic crossbow-y doom.
It would probably have to be a mounted weapon due to the weight of the power store and complexity of the polybolos system, but it's certainly possible.
If you want a simpler version of this: Get a Chinese repeating crossbow and attach it to a reciprocating wheel assembly. Power with a wound rope as before.
It is definitely possible. I mean, to be a little over-simplified, we have them now right? If humans have an idea, it is usually possible, at least to an extent. I am sure that some rudimentary automatic weapons could be built easily if a medieval engineer had the know-how. A little research shows a good number of attempts in different time periods that were fairly successful: Chinese repeating crossbows, ribauldequin, puckel gun, polybolos, etc.
Now with that being out of the way, I think materials are your most important constraint. Many of our automatic weapons, from enormous cannons to automatic bb guns, are machined with very precise tools and materials. The quality of our steel, range of measurement tools, computer programmed machines, new materials like plastic and composites, all make these weapons possible.
So considering that, think of how effective/durable these weapons would be. Rope can fray and break, wood is difficult to hone and cut with precision, metal was difficult to forge and shape. If an automatic weapon was possible it certainly would not be plausible. If you want them in your world remember that they would likely take a group of master craftsmen and would be incredibly expensive. Even then they may not be very durable or effective (read about the first cannons). I would suggest making them a rarity if they are to be added.
It's possible, but not practical.
If we think of early gunpowder weapons, there are a couple of practical limitations:
Metallurgy. The metals of the time are not strong enough, and the machining ability of the time is not precise enough, to build a working breech loading weapon. That requires a strong metal if the breech action isn't overly heavy, and precise machining to achieve a proper gas seal. All early gunpowder weapons were muzzle loaders.
Fouling. Black powder is dirty. It not only produces a cloud of sulfur dioxide smoke, it also leaves a lot of gummy residue behind. That's what hampered the first breech loading rifles - the screw breeches clogged up from black powder residue. I have an accurate (and expensive) reproduction of a British Ferguson rifle (used once in battle in 1776), the first military breechloader. Even with lots of grease on the threads, I can get off maybe four shots before the screw action freezes up from the black powder residue, and has to be cleaned off. As well, the Ferguson uses a flintlock ignition - percussion ignition didn't come along until the early 1800's. Getting flint or slow match to fire rapidly would be just about impossible.
Breechloading firearms did not become practical until the invention of the revolving cylinder that is sealed at the breech, and finaly the metallic cartridge, which contained the fouling to within the barrel and away from the breechloading action.
With kinetic weapons like bows and crossbows, one lacks a reliable power source. A water wheel might work... if you only fight where you have a water wheel. Human power struggles to pull a bow or crossbow for a single shot, so multiple shots is too much for one person. You could use five or six people to power it, but you could also give those five or six people their own bows or crossbows and get the same rate of fire. And the materials of the time may not stand up to the rigorous demands of rapid fire.
One final wrinkle: ruggedness. Battles occur wherever two armies meet, often in unexpected locations, on unknown terrain. A military weapon must be rugged enough to be carried across rough ground, and simple enough for a moderately trained soldier to operate. Rube Goldberg style devices just won't hold up under battlefield conditions.
The practical reality: It is far simpler to train more archers than to try to develop a rapid fire arrow projecting weapon. especially with the materials and machining ability of those days. A platoon of archers is very mobile, and it can withstand damage to parts of it without losing effectiveness (casualties). Gunpowder weapons of that day were far too crude to be considered candidates for rapid fire.
Automatic weapons were developed when the components: the action, the ignition method, and the cartridge method, had evolved to make them practical and rugged enough to withstand battlefield conditions: first the Gatling Gun that appeared not long after the metallic cartridge was developed, and then the Maxim recoil powered machine gun.
I wouldn't see why not? It is possible that, with the right application of tension mechanics to create something fully automatic would still be possible. Take a look at the mechanics of a cross bow for example. It would be possible to create a loader that feeds each bolt after one has been projected. The string pulled back again based on if the trigger is pressed there can be gear mechanics created inside to re-draw the string. catapults could be made automatic too using tension and weight blocks.
How fast this whole process can take is another story and it probably wouldn't be faster than a human would be able to physically shoot draw and shoot again. Bowmen in this time and era were very very very good at their craft. It would be like trying to design a 2 legged robot that could outrun usain bolt. While the robot could run, it wouldn't be able to outrun a human with current technology.
So to answer answer the question, the technology is definitely there to make it possible, how efficient and durable it would be is another. Also may not go as far or shoot as strong because of the way the tension works it may or may not be as strong as humans doing the drawing or giving it that extra umph. In an automated system, it may only provide enough tension to get the release (though you can probably design it so that the draw goes back a certain distance before releasing too).
Yes, but it would be an amusing but impractical toy.
The Medievals were smart enough to realise that complex, cumbersome devices were not going to be as effective as more basic but functional ones. Especially when you're handing them out to squaddies, simplicity and ruggedness are essential.
From http://www.halfbakery.com/idea/Automatic_20bat_20weapon#1317210494
"Thinking about sidearm catapults and preindustrial technology got me wondering about throwing rocks. A simple machine like a sling helps throw a rock farther. Flywheel catapults have been made but you have to stop them to reload. A bat also gets a rock farther but is more uncertain and probably harder to aim than a sling. An arrow goes the farthest but part of this is the aerodynamics of the arrow. Plus an archer or a slinger has a limited rate of fire. The automatic bat defense is a bat mounted on a spinning wheel. The (heavy) wheel is spun fast using gears. Rocks are rapidly dropped one by one in front of the bat. The wheel and bat slows for each one but is back up to speed for the next and does not need to stop to reload.
A human batter can hit a ball 300-500 yards. I am not sure how a machine batter would do with rocks but probably comparable. The automatic bat could be put on a wall for defense or carried on a wagon. I think an automatic bat device like this would be better than a cannon against a cavalry charge, especially if the poor bastards being charged had all day to collect rocks (eg Waterloo)."
If you're allowing gunpowder, the automatic weapons themselves are insanely simple. The chief problem with the weapon is manufacturing a barrel that won't overhead almost immediately (overheating was a big obstacle faced by hand-cranked machine guns of the US civil war.) The second big problem would be the recoil spring. Coil springs weren't invented until the tail end of the medieval period. Still, you could probably justify the early invention of a brass coil spring by a jeweler who was trying out the helical design for another purpose and realized the advantages of it.
The real bugaboo is the ammo. An automatic weapon has to have its ammo be self-contained, nearly identical rounds. For explosive-propelled bullets, this pretty much requires an integrated cartridge. Primers were invented in 1820 (as the percussion cap mechanism for firing a rifle) and brass casing integrating a percussion cap was invented in 1836. The earliest recoil-driven machine gun was invented in 1886, 50 years later. So once you have the ammo, the gun falls into place pretty quickly. Unfortunately, both percussion caps and brass cases relied on the industrial era, when people could cheaply manufacture physically identical objects.
An alternative fully-automatic weapon could be an evolution of the Hwacha. In this concept, a long-burning wick would ignite a rocket, which would push back on a carriage with its exhaust. The carriage would strip another rocket arrow from a hopper, and return to expose it to the wick. (The return motion would be powered by a falling counterweight lifted when the carriage moved back.) Hwachas were never effective as weapons, so there was no impetus to actually develop them when more practical uses of gunpowder came around, but Rule Of Cool dictates that someone, somewhere in the universe have come up with a medieval-tech automatic rocket-launcher.
a Ribauldequin was a medieval cannon which was several small bore cannons side by side that could be fired automatically. So somthing with multiple barrels would be possible. Essentially many fire arms tied together sharing the same fireing mechanism.
Modern automatic firearms fall into 3 categories: gas fed (AR-15), recoil (automatic pistols) and mechanical (such as gatling gun or the cannon in most jets). With the state of technology during the era, the real bottle neck is the cartridge and more importantly the percussion cap that allows the firing of a cartirdge without carrying a lit match.
Your best bet is something with multiple barrels that can be fired with a single match lighting a common fuse.
At least it won't be practical. A firearm won't work without breechloading and reliable primers. Breechloading was used by some early cannon, but those were awkward contraptions.
A crossbow or catapult might be possible, but the principle of a crossbow is to store energy that was slowly accumulated and to release it all at once, for instance with a windlass or cranequin. If "full automatic" means several shots per second, this winding mechanism will be impractical.
One could imagine a design where a rapidly spinning disk functions like a rapid-fire sling.
You can build a Gatling style crossbow/scorpion but the per-bolt stopping power suffers to accommodate the greater rate of fire. You could possibly do something with a high speed flywheel but the tolerances about kill you in terms of engineering the beast with either set up, if you have wide enough tolerances for it to be really reliable it's too heavy and under-powered to be useful and if its build to finer tolerances then it gets unreliable. This is especially true in an era where mass production and standardised parts aren't a thing.
The most low-tech way to build an automatic weapon would be a rudimentary tennis ball launcher. Just spin two wheels really fast and feed projectiles into the gap.
