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  • Context:

A rebel army with limited resources and an army from a somewhat desperate allied country try to create an alternative to firearms, as the explosives logistics have been weakened and become entirely focused on the few bombs, artillery and missiles they can produce. And due to this catastrophic war, the majority of materials and weapons are being made in a similar manner of today's technology (which is almost humiliating for them to admit, as it shows how low the war dragged down their society).

The solution: creating an electrically driven bow/crossbow with draw weight above 200 kg (using reduction boxes and/or pulleys).

  • The problems:

Well, there are some problems with this idea: the fire rate is not as good as conventional firearms and (I suppose) the structure of this weapon would be heavier than conventional firearms. However, it is silent and cheap to build (again, I suppose).

But the main problem is the speed of which the elastic/bending material launches the projectile (in this case, an arrow or fin-stabilised-bullet [like a tiny APFSDS]). There is a type of bullet-shooting crossbow, but they use slow and blunt projectiles.

In the case of a conventional firearm, the projectile is propelled by expanding gases, but rubber (or other bending material such as carbon composites and so on) is not a gas, and thus, can't move/launch as fast as gunpowder.

What is the maximum speed it could launch something, and how I could go around this problem?

I don't expect the projectile to compete with a bullet's speed, but it would be useful to know this kind of information since even today's conventional arrows can pierce through bulletproof vests (even by just a little in a limited range).

Also, the effective range of today's bows/crossbows are inside of the combat distance of which soldiers would normally engage with targets (350 yards = 320 meters), as shown in the graphic below: enter image description here

  • Possible designs:

Well, I don't think that showing a specific design for a 200 kg crossbow would help you guys on answering this question, because if is something original, it would be quite difficult to calculate things. So I would suggest you assume it is a conventional crossbow design (compound or recurve), but "on steroids".

However, I will suggest some designs:

  • Reverse Limbs Crossbows are very similar to compound crossbows, however, I read in some websites that because of its design, it increases the speed per pound of draw weight, but I can't confirm.
  • A Limbless Crossbow could be an interesting option, since it is more compact and have coiled springs on its interior; Which can be made in a variety of ways, or even be entirely replaced by elastics, allowing for a linear pull using elastics, which would be easier to calculate (I suppose).
  • There is also a Crossbow called Ravin R500E, it can surpass the 500 fps (feet-per-second) and I watched a video saying this specific product was delayed because of its projectile speed, which would create problems involving local laws (or something among these lines).
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    $\begingroup$ The maximum speed of the projectile depends on the stored energy of the system. The maximum amount of energy the system can reach depends on design constraints like purpose of the weapon, maximum weight, production costs, tension limits of the chosen materials etc. Mind also that the fastest speed is achieved by reducing the projectile mass. But an extremely light projectile is unlikely to deliver damage. Speed is not everything. A 400 grain arrow from a 500fps capable crossbow is more deadly than a 22lr (40grains, 1300 fps on average). Of course shot placement is king... $\endgroup$ Jan 12, 2022 at 0:03
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    $\begingroup$ If the hypothetical rebels can't even afford black powder, which can be made with off the shelf, dirt cheap chemicals, they definitely can't afford "electrically driven crossbows" and the charging equipment and battery packs necessary $\endgroup$ Jan 12, 2022 at 0:46
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    $\begingroup$ Re terminology - I'm all for the metric system, but the reason we refer to draw weights in pounds is because it is a unit of weight (= force), where kg is a unit of mass. If you really want to be metric (and make all the archers convert from the units they are accustomed to) then use newtons. $\endgroup$ Jan 12, 2022 at 1:27
  • $\begingroup$ Inertia of the mechanism plays a part in limiting the rate that the force is delivered to the projectile - do you have a particular design in-mind? Perhaps you could specify that (or ask a question about that first). $\endgroup$ Jan 12, 2022 at 1:34
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    $\begingroup$ I would suggest that compressed air, perhaps from cylinders, would be a better bet when explosives are not available. en.wikipedia.org/wiki/Pellet_(air_gun) $\endgroup$ Jan 12, 2022 at 2:16

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Depending on your definition of elastic...

There are spring-piston (or in the newer designs, nitrogen-capsule piston) air rifles that are capable of sending a light-for-bore pellet to mildly supersonic velocity. The same is true of precharged air powered guns, but those are probably outside any reasonable definition of "elastic."

With a spring piston, however, it's elastic deformation of a powerful steel spring that drives a piston at firing to compress the air that pushes the pellet -- and if the pellet is light enough, spring strong enough, and ratio of spring piston diameter to bore large enough, the pellet can slightly exceed the speed of sound (roundly 350 m/s at sea level normal conditions).

Given the recent postings I've read about a supersonic trebuchet, I think it's very likely that a similar leverage system driven by, for instance, a torsion bundle, powerful steel spring (like a crossbow prod), or other elastic system could accomplish the same job, in this case likely exceeding Mach 1 by a good margin (the trebuchet is limited by the acceleration of the falling weight under gravity; a crossbow prod can accelerate that drive linkage much more rapidly).

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