I am looking for plausible mechanisms to legitimize (or replace) the physics-based weaponry used in the fictional world of my latest story.

My story utilizes a range of weapons (like Starship Troopers, Aliens, etc), including handheld and vehicle-mounted. Ultimately, I wish to avoid common tropes found in SciFi today -- like plasma bolt cannons, phasers, etc -- and seek to provide my readers something new, possibly using other states of matter in the process.

Assault rifle

One idea leverages a coil-gun like weapon (the size of today's typical assault rifle) fed by a backpack filled with some kind of fluids or gases. When the mixed in the gun's chamber, a chemical reaction occurs and the round is launched by magnetic (or some other) means down the barrel. By the time the round exits the weapon, its density has increased -- e.g., by a factor of 1000 -- to the point where even small caliber bullets have a punch like a shot from a tank cannon.

Ship-mounted cannon

Space-faring warships utilize similar-shaped cannons to today's naval battleships, though the ordinance, again, utilizes some unique state found in physics that is devastating (though not planet killing) and could start off as one state (ideally lightweight) and then create a state that can produce the desired effect.

Both of these have "hand wavy" power sources that are compact and sufficient to provide whatever power the weapon needs--growing larger as the weapon grows in size, of course. My story is not driving the hard-science angle (such as The Martian), but I wish to tap into something science-based to avoid creating generic "death-ray guns" in my story.

Sadly, my research has stalled after a couple of months trying to derive something remotely plausible. I've gone down the path of researching ferrafluids, neutron degeneracy, super-solids, and many other exotic forms of matter. To date, I haven't been able to come up with even "hand wavy" SciFi weaponry that passes even my admittedly low bar for believability.

Hoping someone with deep physics knowledge would be willing to provide either a quick suggestion or, ideally, work with me to create two "passable" weapons detailed above for my story -- the assault rifle and the ship-mounted cannon.

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    $\begingroup$ Welcome to worldbuilding Tim. The question you posted on Physics.SE was migrated here earlier today, and I rejected it with the motivation that " "I am fishing for ideas" is not the kind of questions we take on Worldbuilding. We want a defined scope and a precise answer evaluation metric". If you want an answer from worldbuilding, try to comply with our standards. You can find more in the help center. $\endgroup$ – L.Dutch - Reinstate Monica Apr 17 '20 at 19:21
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    $\begingroup$ Before doing anything else please remove the irrelevant bits from yoru text - it's way too messy as it stands. You need to provide a brief statement of what weapon you're trying to build and what you want from us. Note that the hard-science tag requires strict scientific accuracy, something that's completely inappropriate for what you want. science-based is all you need. $\endgroup$ – StephenG Apr 17 '20 at 19:41
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    $\begingroup$ By the way - what you want has been described in some works of sci-fi, and also some videogames such as X-Com: Enemy Within. Your weapons looks like a particle cannon to me, which is a miniaturized particle accelerator. The point is ionizing a gas and then shooting the ions at extreme speeds - you're shooting a bunch of small particles that carry more kinectic energy than conventional bullets. $\endgroup$ – The Square-Cube Law Apr 17 '20 at 20:07
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    $\begingroup$ "Ultimately, I wish to avoid common tropes found in SciFi today" - Why? If you don't have a good reason other than you want to, I'd advise against it. Tried and true tropes are tried and true tropes for a reason - because they're good and readers are familiar with it. Don't try to reinvent the wheel, unless you genuinely need a new wheel for new circumstances or your story is about what happens when a new wheel gets introduced. $\endgroup$ – Halfthawed Apr 17 '20 at 20:19
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    $\begingroup$ I'm not sure if you understand this, but WB SE is not a discussion forum. It's a straight Q&A site. You ask reasonably focused questions and people try and answer them. People vote on questions and answers if they feel they're useful. What we don't do is work out ideas in discussion posts - the site is just not designed for that. Comments are sometimes used to ask for clarification but are not intended for extended discussions. $\endgroup$ – StephenG Apr 17 '20 at 22:19

You've got two different weapons in your question, but I think they can both be tackled with the same solution: hand waving nuclear fusion.

Assault rifle

The First Law of Thermodynamics says you can't get something from nothing.

In a closed system, the total mass-energy can neither increase nor decrease.

But whoever said this was a closed system? You've got a perfectly good atmosphere just sitting there. Your bullets are modified Bussard ramjets, using sophisticated nano-scale hand waving to fuse atmospheric nitrogen, oxygen and hydrogen into ⁵⁶Fe. The main mass of the bullet is teardrop shaped; there is an outer shell with a similar shape, except the leading (pointy) bit is replaced by a funnel. Air is directed in between the main mass of the bullet and its shell, moving 'round the teardrop to the back, where the airflow creates a higher pressure. This is where the fusion takes place; the molten iron that results gets dumped into the hollow inner part of the main mass of the bullet, but the bullet's reached its target by the time this starts to melt the parts controlling airflow.

The fusion reactor emits vast amounts of light (and somehow nothing else, other than the iron), which is directed in a tight beam out the back of the bullet; a small elliptical reflector dish attached to the gun (around the barrel) directs the light back towards the place where the bullet will be by the time the light reaches it (flexing as the bullet travels to ensure that this occurs) to impart more momentum to the bullet as well as maintain the intense temperatures required for fusion. This means that the gun gets all of the recoil of the heavy, iron-filled bullet that hits its target – but you can carry a lot more ammo, and the recoil lasts the duration of the bullet's flight.

None of this intense light enters the barrel, because diffraction, and it doesn't get scattered by the air, because it's been scooped up by the bullet and there isn't enough time for surrounding air molecules to get that close unless it's a very hot day. In such conditions, you would be able to see a really bright beam of light – perhaps white, though it'd need to be a pure, single-colour frequency if you wanted to use the "because diffraction" excuse (i.e., if it's visible light, it needs to be a colour of the rainbow).

The momentum gained by the energy sent out as light is just enough to accelerate the mass gathered from the air to the same speed as the bullet. The bullet neither speeds up nor slows down – it just gets heavier as it flies.

Due to the light-speed limit, there would be left-over really intense light that wouldn't be able to bounce back and hit the bullet. If you're willing to suspend some engineering realism, this energy could be captured and stored for launching the next bullet.

(I'd need to run the numbers to see how plausible that 1000× figure is, but most of the issues of plausibility there can be fixed by making the initial bullet mass lower. Or perhaps 1000× is an exaggeration for marketing purposes.)

In practice, this would look like a gun that fires glowing globs of liquid / gaseous / plasma metal through people with a sound like thunder, and perhaps a flash of really straight (perhaps colourful) lightning.

Ship-mounted cannon

It's easy to make a planet-destroying ship-mounted cannon; just make a Mount Everest-mass black hole (something more than 1 Pg should be safe enough to handle without glowing too much) and drop it at the planet in question.
okay, maybe producing a black hole isn't all that easy…

Destroying spaceships, on the other hand, is relatively easy: fire really tiny, really fast rocks. Lots of them. Punch holes through everything important. In space, the only thing in the way of your rocks is the spaceship, and people find it hard to survive when their engines are broken and their atmosphere is rushing out of very rapidly widening cracks in the hull. (You need a lot of these holes, because there are a lot of airlocks inside a spaceship.)

If you want your cannons to fire more traditional cannonballs, instead of really tiny shrapnel… just use hydrogen bombs, then follow up with copious quantities of ClF₃. You will have burning craters, in space, which you've got to admit is pretty cool. (If nuclear weapons are prohibited, any powerful explosive will do, and the ClF₃ will become all the more useful.)

A combination of these techniques is probably best; put holes through everything, but only bomb and burn important stuff like the fight and flight systems.

Spaceships are fragile.

  • $\begingroup$ Wow! Thank you for your super detailed explanation! If you're interested in getting a free copy of my book--presuming it ever makes it to print--please shoot me a comment one my author's facebook page. You've earned it! facebook.com/authorTAFenner/ $\endgroup$ – TIM FENNER Apr 18 '20 at 23:35
  • $\begingroup$ wizzwizz4 -- The more I read your answer, the more intrigued I become. If there is any way I could further pick your brain -- I would be most appreciative (and I promise not stalk you afterwards!). If you're game to continue fleshing out this idea, please give me a shout on my author facebook page OR let me know how I can best reach you. $\endgroup$ – TIM FENNER Apr 18 '20 at 23:46
  • $\begingroup$ We could take further discussion to chat. I'll just make a room now. $\endgroup$ – wizzwizz4 Apr 19 '20 at 9:38
  • $\begingroup$ @TIMFENNER Let's continue this discussion in chat. $\endgroup$ – wizzwizz4 Apr 19 '20 at 9:40

Tesseract bullets.

Imagine a 3 dimensional box made of cardboard. Using a hydraulic press you smash it into 2 dimensions. The flat sheet you have now is considerably more dense than the box that you started with.

Your bullets work similarly. They are 4 dimensional objects but tenuously so - they are balanced in their extradimensional extent like a paper clip folded into a spring. On triggering the mechanism, the extradimensional mass of the bullet balanced outside our 3 dimensions collapses over a period of microseconds back into 3 dimensions.

The resulting bullet is much more massive and possibly unstably so as the atoms comprising the bullet are packed much more closely than they like to be. It is also extremely cold - pushing the mass out through the 4th dimension to make the bullet takes a lot of energy input, and is functionally equivalent to compressing a gas. On relaxing back to the native 3d dimension it is the equivalent of a compressed gas expanding, and endothermic.

4th dimension aside; there is no free lunch and increasing kinetic energy for nothing violates the laws of physics. 1/2mv^2 needs to stay constant. Let us look at some numbers.

enter image description here

Turning a 1 gram rifle bullet into a 7 kg shotput slows it down to the speed of a fast fastball. It is going to leave a mark, for sure but not blow a hole thru a tank.

But if you start with a railgun projectile... enter image description here

7 kg at 340 m/s is like a cannonball as it exits the cannon. That will be the desired wallop.

One could make a case that a wallop is a wallop, so why not just shoot things with the railgun projectile? Possibly because the 1 gram bullet might ricochet, or possibly continue through the target and out the other side, much of its kinetic energy retained. Those issues are less problematic with a cannonball.

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    $\begingroup$ Thank you for your super detailed explanation! If you're interested in getting a free copy of my book--presuming it ever makes it to print--please shoot me a comment one my author's facebook page Cheers! facebook.com/authorTAFenner/notifications/… $\endgroup$ – TIM FENNER Apr 18 '20 at 3:51
  • $\begingroup$ What about if the gun makes the bullet less stable in the fourth dimension so that when it hits all the mass crashes back to the normal three dimensions? Then it could retain its speed and have more mass for when it counts? $\endgroup$ – NomadMaker Apr 18 '20 at 12:15
  • $\begingroup$ A heavier same size projectile also has advantages in terms of conservation of momentum - a railgun projectile has issues with slowing down due to drag, a 7kg projectile the size of a 1g projectile not so much. $\endgroup$ – Daniel Vestøl Apr 18 '20 at 17:02
  • $\begingroup$ One problem is with the small-arms version: a 7kg projectile shot out of the rifle at killing speeds would also throw the shooter (or just the rifle) back due to conservation of momentum. $\endgroup$ – NomadMaker Apr 18 '20 at 19:44
  • $\begingroup$ @NomadMaker - you could sidestep that by accelerating more smoothly over a longer period; for example a railgun rather than a chemical explosion. Or use a rocket and let the exhaust gases deal with recoil. $\endgroup$ – Willk Apr 18 '20 at 20:06

It's not plausible, under science.

The first law of thermodynamics states such-

The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.

You can't increase the density of a material without adding more material or changing the volume.

If you want a vaguely scientific gun like this, you need a new source of mass.

So, have the bullets not be bullets, but a tracking beacon. You fire at a target, and the gun or the ship opens a portal up to a heavier projectile which smashes them. Wormholes are theoretically possible in physics, and so this is appropriately sciencey. In the gun or the ship you could have a coiled ring, which uses powerful electromagnets to accelerate projectiles to immense speeds. When the tracking beacon hits the target, a portal teleports in instantly sending the projectile to the target.

  • $\begingroup$ Very creative! My story has the concept of wormholes for travel, though they require tremendous amounts of energy to use (to keep this from being Star Trek/everybody flying around easily. Still, very much appreciate the response! $\endgroup$ – TIM FENNER Apr 18 '20 at 3:54
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    $\begingroup$ Why not just make it require tremendous power to use it safely? You can make a super unstable portal which will kill anyone who goes through for a lower power amount, or a stable portal for massive power. It doesn't matter if you kill your bullets. $\endgroup$ – Nepene Nep Apr 18 '20 at 10:40
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    $\begingroup$ If your weapon allows to create wormholes at a remote place via a projectile, why send another bullet through that wormhole? Instead let the instable wormhole kill the target by ripping a football sized hole in it. $\endgroup$ – Skillmon likes topanswers.xyz Apr 18 '20 at 18:32
  • $\begingroup$ Because making portals into solid objects exponentially increase the energy requirements and isn't as easy as teleporting objects, because they wanted an object that increases in mass. $\endgroup$ – Nepene Nep Apr 18 '20 at 23:47

AI weapons

The problem with most weaponry today is in order to shoot someone you must give them a chance to shoot at you also.

To solve this problem you can create "guns" that fire miniature drones instead. the drone can then seek out and shoot the enemy. If the enemy shoots them down, that is OK, it is just a drone. Normally in that situation your soldiers would be shot, but not anymore.

Your large ship weapon can behave in the same way. instead of firing heavy ordinance, the weapon instead is an armored drone carrier that deploys the drones into the enemy ship or over the enemy position.

Depending on how small you can get the drones there are two additional benefits, First, they become harder to hit, up to the point where only other AI weapons can shoot them down. Second, if your drone fly with wings and in hordes they can simulate the sound of angry bees. Many armies will stand against your soldiers and tanks. It is hard to hold the line against a mass of swarming insects that are preceded by the sound of angry bees.

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    $\begingroup$ Not quite what I am going for, but still appreciate the effort! Thank you! $\endgroup$ – TIM FENNER Apr 18 '20 at 3:52
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    $\begingroup$ Wouldn't snake-like instead of bee-like be more effecitive, at least against Indiana Jones? $\endgroup$ – Hagen von Eitzen Apr 18 '20 at 9:29


I've narrowed what your looking for down to a solution which meets a few criteria:

1) You are looking for a weapon which is based upon a physical mechanism (non-magical) that can is roughly analogous two modern day equivalents: the Main Battle Rifle (MBR) and Heavy Artillery.

1a) For the MBR I'll use the following restrictions: a small arms weapon capable of being wielded unaided by the average human, is capable of continuous fire, is re-loadable in a modular fashion (if applicable) and is capable of inflicting lethal damage to the average human up to ranges of at least 300 meters.

1b) For Heavy Artillery you mention the addition analogy of navel artillery, capable of being mounted on spacecraft and inflicting severe damage to the constructs on which the heavy artillery are mounted (presumably other spaceships, but not planets). I'll assume that the artillery are not hard guided (no missiles), while some sort of "soft" guidance is allowed in analogy with modern day artillery. For real world analogies, I'm presuming your thinking of 12 - 16 in. guns.

2) The physical mechanism by which these weapons operate is "mostly realistic". Since "mostly realistic" is very open to interpretation, I'll restrict my solutions to only those which do not violate known symmetry laws in physics (most of the Standard Model). Fictional physics I'm assuming can be used, but only as long as it does not violate already established laws.

3) The Physical mechanism by which the weaponry operates is not a common trophe of science fiction. Thus, I will rule out the following: Laser beams, Plasma projectiles or bolts, Gauss guns, "Ray" guns, as well as real life projectiles: metal and polymer bullets, depleted uranium and tungsten bullets and kinetic projectiles. The caveat here is some elements of the above may be used, but only if they are coupled with a mechanism\material which is not typically associated with the weapon system.

4) The weaponry should incorporate unique and unusual physics and/or materials in its' design. Thus mostly ruled out are chemical firing mechanisms and electric firing mechanisms.

Possible Avenues:

With these restrictions in mind I'll present some possible solutions. Unfortunately all of these criteria eliminate a lot of possibilities. I'll start with high levels of handwavium and proceed downwards.

A) A fifth force: You can do all sorts of fun stuff with a fifth force without explicitly breaking known physics. As long as the fifth force involves improbable scenarios and as long as that scenario doesn't explicitly violate known laws it can appear plausible:

A-1) For example, does this fifth force interact with matter? Perhaps as an interaction between masses but unlike gravity the interaction is strictly repulsive. How is this force triggered, perhaps by running a current through an exotic material. Now you can have a gun whose power is limited by the structural integrity of your material and electric current. Combine this with high-temperature super-conductors and you have one hell of a weapon. Of course the constants for such a field to work have to be just right and you still need to respect conservation of energy, but hey its a fifth force and can be tailored to how you like it.

A-2) Of course invoking a fifth force will come across as very contrived to anyone knowledgeable about physics, but it can provide the playground to explore really cool phenomena which can be used for all sorts of other things. Working through the physics of new fields and particles interacting can given rise to constructs well beyond weaponry. This approach is also very conducive to effects and weaponry which is not already (over?) explored in science fiction.

B) Less contrived than a fifth force would involve materials and phenomena, but interesting enough be be unique. Superconductivity is an obvious choice which will allow all sorts of electromagnetic weaponry, but this is quite common in science fiction. Other areas along this avenue are explosive propellants which are not commonly used in Sci-Fi and which have no real-life counterpart. For example, inducing nuclear emissions in some exotic material to produce a powerful explosive reaction which could result in more energetic projectiles. While no real cases of induced nuclear emission have been found suitable for this purpose, there have been some proposed cases which might serve for insight into what this phenomena might look like (see Hafnium controversy).

C) At the bottom of the rung are possibilities found IRL, unfortunately most of these are already found somewhere in fiction and science fiction. You can have caseless ammunition, Self-guiding (smart) munitions and miniature atomic weaponry. Each of these can be tweaked to be unique but will not appear very exotic.

Where to go:

I would imagine that it would be easier to work backwards than forwards. By this I mean that it would be easier to flesh out a weapon system and some characteristics that this weapon system has. For instance, what does the rifle look like? Does it have any characteristics which stand out? Perhaps it needs a very cylindrical metal chamber, a long barrel and fires what appears to be bright bolts of light. Perhaps it can fire rapidly, but has a serious over-heating problem and is loaded with cartridges containing bullet shaped rods, but no propellant or casing. The more descriptive the more you can contrive some physics by which this sort of weapon could be made.

Working forwards on the other hand is not easy if you want to be consistent with the physics. For example, if you want to incorporate an IRL exotic states of matter, degenerate electron matter for instance, this really narrows down what you can do since there already exist very good theories of electron degeneracy pressure and when it is applicable which is essentially only in the conduction band of super cooled metals, or in super dense, high gravity star systems such as white dwarfs. Unfortunately real life physics is very restrictive which I believe explains the tendency of all science fiction, especially the sort which tries very hard to not break existing physics laws, to sort of converge on the same ideas and concepts.

  • $\begingroup$ Wow! Thank you for your super detailed explanation! If you're interested in getting a free copy of my book--presuming it ever makes it to print--please shoot me a comment one my author's facebook page. You've earned it! facebook.com/authorTAFenner/notifications/… $\endgroup$ – TIM FENNER Apr 18 '20 at 3:56
  • $\begingroup$ I like hand-waving explanations where you allude to advanced physics, but don't explain the details. Characters having a discussion about how it works and saying "I thought it had to do with the quantum states of electrons." and a physicist saying "you clearly didn't get it." show the gun does what you want, but is based on physics that have yet to be worked out. $\endgroup$ – DWKraus Apr 18 '20 at 19:32

use actual guns

With current technology, we can create actual rail guns, actual lasers, and self-aiming guns. The reason we don't is that all those things have large maintenance costs and are hard to justify in combat. Lets say you need special multi-dimensional ammo (a cool idea by the way). That probably needs highly specialized equipment and trained individuals. If the factory that makes those breaks down or is unable to supply you, you are screwed. But if your guns take 9mm or 7.62x51 NATO rounds then you can resupply from allies, local suppliers, or even from the enemy. While the idea of huge over powering weapons is cool, most people who get shot don't say they were glad they were not shot with something bigger, if they can say anything at all. getting shot with a good bullet from a good gun is sufficient in most cases. Also, while these big guns have large power sources, a backpack, and a barrel that requires it to be mostly vacant, normal guns will shoot in extraordinary conditions. There is a reason one of the most popular weapons in the world is the AK-47.

In "Lord of War" (2005) they say this

"Of all the weapons in the vast Soviet arsenal nothing was more profitable than Avtomat Kalashnikova model of 1947, more commonly known as the AK-47, or Kalashnikov. It's the world's most popular assault rifle, a weapon all fighters love. An elegantly simple nine pound amalgamation of forged steel and plywood, it doesn't break, jam, or overheat. It will shoot whether it's covered in mud or filled with sand. It's so easy even a child can use it, and they do."

The point is a good weapon will work under any circumstance with minimal maintenance, so your weapons need to do that also.

Weapons that require a back pack and management of three resources will always be beat by weapons that can be disassembled by high school drop outs and shoot mud-caked bullets in the rain.

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    $\begingroup$ I assume that, for the purposes of this question, the theoretical weapons being discussed are reliable enough to be a feasible choice for militaries. Sure, modern firearms — after centuries of trial and error — are more reliable than today's directed-energy weapons and the like, but back when firearms were new, melee weapons were much more reliable than those, and we don't still use clubs and swords all the time. $\endgroup$ – parasoup Apr 17 '20 at 23:20
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    $\begingroup$ That's right. The weapons in question would be considered the most durable and most used weapons on the field. Regardless, thank you for the response! $\endgroup$ – TIM FENNER Apr 18 '20 at 3:54

Your assault rifle is inherently impossible to use. Guns are subject to Newton's third law. If you have a gun that fires a round that hits like tank cannon then the gun recoils like a tank cannon. Your user dies.

Your weapon also seems impractical for most combat. Rifles are for shooting at infantry, your round hits with far more force than is needed to kill infantry. While there are repeated stories about the military deliberately designing their rifles to injure instead of kill (to make the enemy take time dealing with the injured) what's actually going on is much simpler: the smaller bullets are lighter, a soldier can carry more stuff. For general combat you always want the lightest rifle that can do the job.

Now, sometimes you have to deal with fortifications. You don't do that with your normal antipersonnel rifle, though. (Although some rifles also mount a grenade launcher for this purpose.)

The only reason to build infantry rifles that hit like a tank is if you are facing an enemy that requires being hit that hard to stop them. The Apocalypse Troll by David Weber comes to mind--the heroine carries a hand weapon that hits with the power up to that of a small tactical nuke because her enemies are tank-like cyborgs.

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    $\begingroup$ Hi Loren -- thanks for the reply! Just as you highlighted at the end, my infantry must face a variety of situations and the weapon can be "dailed" into whatever is needed--antipersonnel, armored vehicles, structures, etc. $\endgroup$ – TIM FENNER Apr 19 '20 at 2:00

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