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Imagine a small metal canister, with a narrow exit on the top. There is a sort of door covering this, with a metal plate that is removed from the side. The pressure inside of the canister acts as a propellant, launching the weapon forward and propelling it through flight.

My question is: would this work? Some reasons I can think of that may cause this to not work:

  • Horrible accuracy
  • The opening would have to be too small to be plausible
  • The canister would run out of air instantly, and fall to the ground shortly after
  • The cost/energy of producing them is ridiculous

Imagine modern or slightly above modern tech, and enemies at distances of +/- 50 meters.

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  • $\begingroup$ here's a Question i asked that is similar hope it can give you some pointers worldbuilding.stackexchange.com/questions/105342/… $\endgroup$ – Creed Arcon Mar 3 at 2:59
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    $\begingroup$ "The canister would run out of air instantly, and fall to the ground shortly after" - The laws of physics say that second part doesn't happen (at least, in a way that makes the weapon ineffective). Consider your typical projectile fired out of a gun. It stops receiving thrust "instantly" (as soon as it exits the barrel), but can remain airborne long enough to accurately strike targets hundreds of meters away or more. $\endgroup$ – aroth Mar 3 at 3:32
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    $\begingroup$ "gun-less" - do you mean barrel less? $\endgroup$ – Mazura Mar 3 at 18:37
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    $\begingroup$ Water rocket with a bladder to allow horizontal operation might work. $\endgroup$ – KalleMP Mar 3 at 19:20
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Here at Weapons R Us, we strive to offer the latest in amazingly inaccurate & astoundingly cumbersome weapons. In fact, we offer two models of precisely the weapons system you're looking for!

enter image description here

First, our Basic Can & Stick; this is our easy to understand, step by step how-to instructional video, shot on site at our manufacturing plant in Siberia.

Second, our Deluxe Rocket Can; this is Myth Busters trying our our latest product!

And finally, at WRS's Backcountry R & D facility; here we are testing our anti-aircraft launch mechanism.

Basic model takes a bit of coaxing to activate, but is quite effective. Our deluxe model is, generally speaking, an instant success! Contrary to mythconception, this weapon system is relatively easy to mass produce and not terribly expensive. As you can imagine, even the cans are recyclable!

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    $\begingroup$ Can't stop laughing at the first one... $\endgroup$ – Redwolf Programs Mar 3 at 1:48
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    $\begingroup$ I admit it. I watched all three of those videos. Ha ha ha! $\endgroup$ – Cyn Mar 3 at 2:32
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    $\begingroup$ Darwin Award Winner wannabees. $\endgroup$ – Justin Thyme the Second Mar 3 at 3:09
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No, this is not practical

You mention in your question the concern that "The cost/energy of producing them is ridiculous", which implies you're concerned about this being a practical or mass produced weapon. From that perspective, no, this doesn't work, for the simple reason that you are firing your pressure vessel at the enemy, instead of using the pressure vessel to fire a projectile at them. The latter approach has historically been used extensively, and there is no justification for not taking this approach with pneumatic weapons.

In all of the video instances noted by @elemtilas, non-combatants are using extant pressure vessels in a very wasteful manner. With any of those devices, it would be more practical to use the large canister to fire larger number of lightweight projectiles, i.e. flechettes, and then still have the canister to use when you're done!

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  • $\begingroup$ Keep in mind: the typical Youtube video largely consists of ill informed boys saying things like "oo, I wonder if this will explode? --- let's bang on it and find out!!" The question doesn't actually require the weapon to be "practical", which is a different consideration entirely. As a rejoinder: one could simply strap a high explosive charge and shrapnel packets onto the CO2 tank...oh, wait. We already do that ... they're called missiles! I'd argue it's far more practical to produce a simple pressurised canister than it is fiddling around with propellents, at the prescribed ranges. $\endgroup$ – elemtilas Mar 3 at 18:00
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    $\begingroup$ Girandoni air rifle, circa 1779, used on the Lewis and Clark Expedition, +1 $\endgroup$ – Mazura Mar 3 at 18:35
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    $\begingroup$ @elimtilas I didn’t mean any disrespect to your answer, but by asking whether “the cost/energy” is prohibitive, production is implied, and that pulls in practicality. Yes, it’s simpler to produce a pressurized canister, but there’s no reason to then launch the canister itself: use it to launch the projectile of your choice. As the Holman projector shows, air pressure based grenade launchers are a viable weapon. $\endgroup$ – Daniel B Mar 3 at 18:36
  • $\begingroup$ Oh, I didn't take it as disrespect! My point was simply simply that the OP asks if it will work. Cost/energy and so forth seem to be ruminations on why it won't. To be honest, compressed gas tanks are cheap and plentiful. Hospitals and clinics have thousands of these things. Every house has at least one, maybe two (fire extinguishers!) and every public place has usually multiples. As for shooting the canister vs shooting pellets, well, I guess that depends on the end goal of the weapon's design. After all, why build a tank when you can just send ten thousand boys to shoot at each other? $\endgroup$ – elemtilas Mar 4 at 16:50
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A resounding 'yes' at +/- 50 meters.

The compressed CO2 cylinders used in air guns have enough force to launch the cylinders at least 50 m.

They are used to propel rocket car models, rockets, and such. We had a stray (one that came out of the end of a rocket) ricochet back and forth in a classroom 20 m long. Left dents in several places in the walls. The trick would be to launch them in a tube just slightly larger than the gas cylinder. The firing pin would be exactly that - a pin that punctures a small hole in the nozzle end. We used a hammer and a nail. Put a hardened bullet-type nose on them, and maybe a trailing rigid tail or stability, and you would certainly have a weapon that would hurt.

These CO2 cylinders can be made quite inexpensively, although probably not cost competitive with bullets.

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What you are talking about is really just rockets that use compressed air instead of a chemical reaction to develop pressure. Accuracy can be controlled using the same mechanisms such as tail fins. The duration of the propulsion with compressed air can be extended using reaction mass such as water, as mentioned in another response. Production costs will depend on many other factors that you control in your setting.

Compressed air is obviously inferior to chemical based propulsion since we (humans in general) know how to do both and we don't generally choose compressed air. I would venture a guess that the main problem is weight. Canisters that can handle high pressure gas are heavy. Adding reaction mass to extend the propulsion adds weight. If you add a payload such as explosives then you are adding more weight.

You should also consider whether your tanks contain compressed air (like a SCUBA tank) or contain gaseous mixtures in liquid form (like basically every other high pressure tank). The liquid will add even more weight but it also contains more potential energy than basic compressed air.

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