I'm wondering if a world, given the capabilities of at least early 20th century industrialization, but without any substance equivalent to black powder/gunpowder would go about producing ranged weapons. I understand air rifles exist, but would they be the primary instrument of armies?
closed as primarily opinion-based by Aify, ShadoCat, L.Dutch - Reinstate Monica♦, sphennings, Marshall Tigerus Jul 17 '18 at 19:27
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Possibly using motorised flywheels, like a pitching machine but instead of pitching single baseballs it fires shells or bundles of steel darts, for larger installations. The crossbow is still a worryingly effective weapon given modern construction techniques and materials crossbows, and even bows but they're harder to use, can be quite efficient even in the face of modern body armour.
I would expect to only see air or spring powered rifles for extended long range applications, sniping the enemy officer corps for example.
Not air rifles, but steam powered weapons. See for example the Holman Projector, actually used in WWII.
For personal armament, airguns are the way to go.
Air-powered weapons are a surprisingly simple technology- the Girandoni Air Rifle was developed in the late-1700s and used militarily by Austria in the early-1800s. It had a muzzle energy comparable to modern hunting crossbows, but could fire at a very high rate with accuracy and significant killing power.
This is an example of an airgun being militarily viable and competitive with firearms, over two hundred years after crossbows were rendered obsolete. With firearms out of the equation, crossbows are not likely to make a comeback- technological developments into the 20th century made airguns more powerful and, more importantly, logistically easier to manage.
They're simple, accurate, easy to use, more powerful than a crossbow (some modern airguns are capable of 1kJ or more of muzzle energy), and capable of a much higher sustained rate of fire. The primary disadvantage is the reliance on finite air pressure, but this can be mitigated with swappable air tanks, a single large reservoir, or an air compressor.
Fundamentally, the difference between firearms, airguns, crossbows, pitching machines, and any other sort of projectile device comes down to the source of propulsion energy. Firearms are extremely efficient and powerful through the use of chemical propellants, but if chemical propellant is unavailable, air pressure is the next best thing.
Numbers have a quality of its own. Muskeets replaced bows not because they were better, but because they were easier to teach to use to untrained troops. Iron (steel) swords replaced bronze not because they were better, but because they were cheaper and iron ores were commonplace while tin ores were rare.
While there are ways of making gunpowder-less guns, they are all rather impractical. Just like laser guns, we don't use them because they're not practical, not because we don't know how to build one. Chemical explosives are a very convenient way of propelling projectiles. If you don't have these, the mechanical force of a bow is the next best thing. Other methods, like compressed air, steam or magnetic fields are far more expensive, clumsy, unreliable and/or failure-prone, so I guess your civilization would aim for state-of-the-art repetition crossbows.