Think of firearm technology similar to that of today with better materials, machining, etc. A colony of humans has discovered a way to make propellant that expands at near light speed. What advantages and disadvantages would this weapon have? What technological refinements would be required to make it usable?

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    $\begingroup$ i haven’t got time for a full answer right now, but the short answer That immediately springs to mind is: this is a Bad Idea. $\endgroup$
    – Joe Bloggs
    Commented Mar 26, 2018 at 14:18
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    $\begingroup$ @JoeBloggs Yes, a handgun with propellant that expands at near light speed isn't a personal weapon, it's a weapon of mass destruction (including the vaporisation of the wielder) with a large blast radius. $\endgroup$
    – Mike Scott
    Commented Mar 26, 2018 at 14:19
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    $\begingroup$ I agree with Joe, and raise an XKCD What If: what-if.xkcd.com/1 $\endgroup$ Commented Mar 26, 2018 at 14:20
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    $\begingroup$ Like a nuclear hand grenade, it is a perfect demonstration of the difference between technical brilliance and wisdom. $\endgroup$ Commented Mar 26, 2018 at 14:20
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    $\begingroup$ The question makes no sense. Propellants are not characterized by their expansion speed, but by the amount of energy they release. I believe that you don't understand how a fire weapon works. The propellant burns very rapidly (you may assume instantaneously), and is converted into an equal mass of hot gas (how hot depends on how much energy is released by burning the propellant). This gas is initially confined to the volume which was formerly occupied by the propellant; it will exert a force on the projectile, accelerating it forward. The propellant expands exactly as fast as the projectile. $\endgroup$
    – AlexP
    Commented Mar 26, 2018 at 15:58

9 Answers 9


When smokeless powder was adopted in 1884, it was quickly found that it had two to three times the energy density of conventional black powder, and if loaded to black powder specifications would cause immediate destruction of both the weapon and often the shooter.

The solution here is the same as it was in 1884: Use less propellant, and moderate its release of energy. This makes it less like an antimatter bomb as other answers claim, and more like a fission fragment rocket, a current proposal for harnessing relativistic particles as a propellant. The difference between a useful propellant and a bomb is, primarily, the rate at which energy is released. If you can produce relativistic particles at a controlled rate, you may have a useful propellant.

So if we scale back the emitted energy to not be wildly beyond what conventional chemical propellants provide, what's this propellant good for? Well, the faster a gas can expand, the higher the maximum velocity of any projectile propelled by it. So there is a niche application for this technology there:

High-velocity weapons systems

Conventional firearms currently struggle to reach 2km/s as a direct consequence of limits on the rate of gas expansion. One workaround currently used for scientific research is the light-gas gun, but this has never been practical for military use due to the need for an external supply of the light gas in question.

Railguns and coilguns have been an attempt to get around this problem entirely, but have obstacles of their own. Electrical energy storage density comes nowhere close to that of chemical propellants, which makes them implicitly bulky and complex.

With high-energy-density, high-expansion-velocity propellant, you can get railgun-like performance from a long-barreled variant of a more conventionally-propelled firearm. Colonists on another planet may find this useful either for long-ranged artillery, or surface-to-space gunnery.

There's also another, more mundane application thanks to the compactness of this substance as a propellant.

Compact, caseless ammo

With your propellant now so incredibly compact, one of the biggest hurdles in caseless ammunition is solved. Rather than enclosing the bullet in a brick of propellant, a microscopically tiny amount of propellant can be affixed to the rear of each bullet.

There are associated technical hurdles to overcome (the heat dissipation afforded by casings, and the process involved in clearing an unfired round being the primary two), but if this can be made viable then ammunition becomes lighter, easier to transport, easier to carry, and allows for magazines of significantly greater capacity.

  • $\begingroup$ This was exactly my first thought, with one addition to your caseless ammo explanation. The "ammo" could be a tiny "fuel pack" semi-permanently affixed to the gun (to be replaced after some large number of rounds were fired). This would allow the magazine to carry only the bullets, and no propellant whatsoever - the magazine and ammunition is now inert and harmless, which would simplify transport and storage. $\endgroup$
    – dwizum
    Commented Mar 28, 2018 at 18:00
  • $\begingroup$ @dwizum I had that thought too- but then I figured that if the propellant is so enormously powerful that only a tiny amount would be necessary, then molding it into the bullet would simplify the mechanical operation of the gun. You do have a good point about simplifying transport/storage with inert projectiles, though. $\endgroup$
    – Catgut
    Commented Mar 29, 2018 at 12:50
  • $\begingroup$ I had a consulting gig that involved logistics for munitions stores. It's incredibly complicated and expensive. Making the consumable portion of the ammunition inert would be an enormous benefit. Locking the propellant to the firearm would mean you only have to protect the (non-consumable) component that you're going to protect anyways, and your consumable is much cheaper and safer to manufacture, transport, store, and protect. $\endgroup$
    – dwizum
    Commented Mar 29, 2018 at 13:46

(...) propellant that expands at near light speeds (...)

It doesn't matter whether the projectiles themselves would go near light speed... Just the propellants doing so would already be quite hazardous.

The very first XKCD - what if article deals exactly with that. The scenario is a baseball being thrown at 90% of the speed of light. It is a very fun read, and, like many other XKCD what if's and questions that have the tag, anyone around the phenomenon proposed in the question gets disassembled into particles in a very spectacular way.

TL;DR: at near light speeds, particles with mass have enough energy to cause nuclear reactions. Here is Randall Munroe's artistic conception of what happens when the mass in case is that of a baseball:

My eyes! It burns!

We could assume that your fictional ammunition propellants, for a single shot, will have considerably much less mass than a baseball. Regardless, the effect is the same... The shooter will have been vaporized before their target is hit.

Perhaps you would like to develop a laser pistol or rifle instead? The shot will be at proper light speed, and the gun would be relatively safer to use.

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    $\begingroup$ Hmm thanks for the input, so its a suicide weapon, lets hope ISIS dosen't get ahold of it. $\endgroup$
    – Efialtes
    Commented Mar 26, 2018 at 14:27
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    $\begingroup$ @Mark The energy involved at those speeds is astronomical (literally). Depending on the mass involved you're talking weapons that can take out anything from small buildings to large planets. $\endgroup$
    – Tim B
    Commented Mar 26, 2018 at 15:18
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    $\begingroup$ This is exactly what came to mind when I read the question title. @Mark the article is a great read, I highly suggest it (and all his other "What-if" articles for that matter). $\endgroup$
    – thanby
    Commented Mar 27, 2018 at 11:57
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    $\begingroup$ @user2357112: You missed an opportunity to make a Public PSA, but there's always the next time someone says "ATM machine" in your general hearing. ;-) $\endgroup$
    – tonysdg
    Commented Mar 27, 2018 at 14:35
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    $\begingroup$ Note that nuclear reactions may be a bit low energy for what actually happens -- at near-c speeds, common atomic matter gains kinetic energy that passes the quark-guon binding energy of protons, let alone nuclear binding energy of atoms. The difference is probably pretty minor (you get a huge fireball). $\endgroup$
    – Yakk
    Commented Mar 28, 2018 at 20:01

In point of fact, you do not want fast propellants in firearms. Here's what happens when you load a fast-burning pistol powder into rifle cartridges: http://bulletin.accurateshooter.com/2015/01/what-happens-when-you-load-pistol-powder-in-a-rifle-cartridge/

This is only due to the difference in detonation rates between two types of gunpowder - all of which are classified as low explosives. High explosives like TNT can have detonation velocities that are 10X or more higher.

For a propellant, you want something that detonates fairly slowly, in order to give a "push" to your bullet. High explosives detonate so fast that the produce a shock, shattering the surrounding material rather than shoving it.

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    $\begingroup$ Nice link, that's a little terrifying $\endgroup$
    – thanby
    Commented Mar 27, 2018 at 12:05
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    $\begingroup$ Technical language here is important: You do not want something that 'detonates' at all. The term for the slow, controlled release of energy through combustion is deflagration, whereas detonation is precipitated by a pressure wave going through the substance. High explosives detonate, propellants deflagrate. Propellants can be induced to detonate under certain conditions, and then you have a Bad Time. $\endgroup$
    – Catgut
    Commented Mar 27, 2018 at 17:59
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    $\begingroup$ @Catgut: You have a point, but I think the important difference is between a "shove" that pushes a bullet out the barrel, and a sudden shock that shatters things. After all, you can get a quite effective "shove" without either detonation or deflagration, as in an air rifle. $\endgroup$
    – jamesqf
    Commented Mar 27, 2018 at 18:42
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    $\begingroup$ I appreciate the insight this answer provides into the fundamental misconception in the OP. $\endgroup$ Commented Mar 28, 2018 at 1:18

A colony of humans has discovered a way to make propellant that expands at near light speeds.

Let's take a look at what we're actually dealing with here. I assume your intent is that you want to get projectiles going at relativistic speeds. If you look at the formula for relativistic kinetic energy it's pretty easy to derive that going at $\sqrt{0.75}c$ means having as much kinetic energy as you do rest mass energy - in other words, for a 10 gram projectile to get moving at about 0.866c you need to convert 10 grams of propellant into kinetic energy for the projectile with 100% efficiency (not possible due to the second law of thermodynamics), or convert something like 100 grams of propellant with 10% efficiency.

Let's assume you have 5-gram bullets, and you can get about 50% efficiency from your propellant (this is stretching believability, though). So you need a way to convert about 10 grams of mass into energy very quickly. Antimatter. The colonists are using antimatter.

Here's a Wikipedia link I really like - Order of magnitude (energy). If you look at this chart at the $10^{14}$ row you'll see "Energy released by annihilation of 1 gram of antimatter and matter". Now look up at the $10^{13}$ row and notice this entry - "Yield of the Little Boy atomic bomb dropped on Hiroshima in World War II". 1 gram of antimatter and matter is about 3x the energy yield of Little Boy, so the 10 grams you need for a relativistic projectile gives you about 30x that energy yield.

What technological refinements would be required to make it usable?

In short, you need a gun capable of withstanding a point-blank nuclear blast and directing it entirely at whatever you're aiming at. (The propellant isn't the big news - the unobtainium, applied phlebotinum, handwavium, or whatever they're using to make these guns is.) Even then, you're still going to be obliterated by all the nuclear reactions set off by the gun with all the air between you and your target.

What advantages and disadvantages would this weapon have?

This gun shoots nukes. That's the only real way to think of it. I'm going to say it has strict long-range-only usability, and only in zero atmosphere conditions.

Note: As Allan says in his answer, you also have to do something about recoil. Otherwise your gun with suddenly be behind you (with the parts of you that are behind the gun no longer being attached to the rest of you), and you don't want to be standing in front of your nuke gun at point-blank range.

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    $\begingroup$ To not make it too subtle: this gun doesn't so much "shoot" nukes as detonates them while they're still in the chamber (or the literal moment they leave the barrel at best). $\endgroup$
    – mtraceur
    Commented Mar 26, 2018 at 22:21
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    $\begingroup$ @mtraceur you definitely want it happening in the chamber - if it happens after it's left the barrel, you have no hope of directing all of the energy away from you. $\endgroup$
    – Rob Watts
    Commented Mar 26, 2018 at 22:23
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    $\begingroup$ Imagine that gun recoiling. $\endgroup$
    – user9981
    Commented Mar 27, 2018 at 17:07
  • $\begingroup$ @Magicsowon yeah, if you don't manage recoil (like Allan's answer), you're going to have a bad time. $\endgroup$
    – Rob Watts
    Commented Mar 27, 2018 at 17:16

If you assume that the bullet is fired at extremely high speeds then regardless of what the propellant is or does then the immediate problem you have is gun recoil. If your bullet flies off at 100X the velocity of a regular gun then you have 100X the recoil in your gun. Forget the damage it will do to your target, consider instead the damage it will do to you. To counter this you'd either need something supporting the gun (eg. powered armor or a gun mount) or some kind of Star Trek style inertial dampening (really that is misnamed https://en.wikipedia.org/wiki/Inertia_negation) to reduce or remove the recoil of each shot.

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    $\begingroup$ Firing this gun will create a city-block shattering fireball + mushroom cloud and you are worried about recoil? $\endgroup$ Commented Mar 27, 2018 at 14:43
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    $\begingroup$ @Renan i.imgur.com/9cjD2oE.gifv - given the energies involved, if you don't manage the recoil you'll have a city-block shattering fireball in both directions. One of those is created by the gun itself flying backwards (and whatever pieces of you happened to be behind the gun). $\endgroup$
    – Rob Watts
    Commented Mar 27, 2018 at 16:00
  • $\begingroup$ @RobWatts fair enough. +1. $\endgroup$ Commented Mar 27, 2018 at 16:57
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    $\begingroup$ Not if you use the propellant in a "recoilless rifle" design: I.e., enough propellent is directed out the "back" of the rifle to offset the momentum generated by mass leaving the front. (Doesn't mean you don't have to worry about what's "behind" the firearm; just that recoil mitigation was solved a long time ago.) $\endgroup$
    – feetwet
    Commented Mar 27, 2018 at 20:15
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    $\begingroup$ @feetwet something gives me the impression that the "recoilless rifle" design is going to need a few tweaks to work with the nuke levels of energy we're dealing with. $\endgroup$
    – Rob Watts
    Commented Mar 27, 2018 at 21:05

I must slightly disagree with a bunch of other posters. Assuming sane costs this propellant actually does have a use. You have a reasonably stable substance with antimatter-level energy release. As Mauser suggested it would make a good warhead for your shell, but you asked about using it as a propellant.

The "gun" I am picturing is too big to be mobile, it could only serve as a fixed defense for some high value installation. Take a convenient mountain, drill a hole in it. This is fitted with a large but otherwise ordinary (but smooth bore) artillery barrel and a substantial chamber is hollowed out behind this. In that chamber you put a bunch of water and a tiny amount of propellant.

Since you have no traverse capability with the gun you will have to use guided shells. You'll also need a bunch of these (but you'll need them anyway as this gun will have a very low rate of fire.)

This gun can outrange conventional artillery an ordinary artillery piece has no way of accelerating it's shell to a speed higher than the expansion velocity of whatever propelled it. (There are some very complex workarounds for this that involve providing additional propellant while it's going down the barrel, but even those have limits.) Your velocity will only be limited by what your shell can take during launch (longer barrels will increase this) and what it can survive without being destroyed by it's passage through the atmosphere.

Note that something of this sort has actually fired one shot: The "propellant" was an atomic bomb.

  • $\begingroup$ Of all the answers that propose that the weapon can actually be fired without utterly destroying its user, this is the only sane one. $\endgroup$ Commented Mar 27, 2018 at 14:46
  • $\begingroup$ Great idea. You may very well be able to provide some amount of ability to aim the round. Just as a revolver is able to fire a bullet with a gap between the revolving chambers and the barrel, you could provide adjustments to the barrel with an imperfect coupling to the water chamber. $\endgroup$
    – Mike Vonn
    Commented Mar 28, 2018 at 13:46

Based on the other answers, a gun, cannon or similar device is simply impractical. However Rob Watts may have come up with a way to fulfill the conditions: instead of a gun you need an antimatter rocket.

While making a bullet sized antimatter rocket may be problematic, if we envision a very advanced technology capable of making an antimatter trap that can fit in a bottle or soda can sized projectile. The ten gram propelling charge waits suspended in a vacuum until the rocket is fired, whereupon normal matter is allowed into the reactor chamber. Powerful magnetic fields guide the charged reaction particles out the nozzle to generate thrust, however a fireball of gamma radiation is also being produced, so this is not a man portable weapon.

enter image description here

Basic antimatter rocket

enter image description here

Controlling and directing the reaction. Charged mesons are guided by the magnetic coils. Uncharged mesons and gamma rays will exit in all directions

The fireball of hard radiation will kill any living thing nearby, while the intense thermal radiation will set anything inflammable on fire. This will be a tiny and very intense version of the "SLAM" (Supersonic Low Altitude Missile) developed in the late 1950's.

enter image description here

Now imagine this is the size of a wine bottle...


You have a very energy dense propellant, it just needs to be used carefully. Do not think of it as a propellant, but the highest of explosives. Below is a prescription for a very effective weapon based upon this "propellant"

  1. Put a very small amount of propellant in a fused shell
  2. Fire at target using conventional propellants

A firearm firing this ammunition would likely be able to destroy any vehicle or structure.


I think the limits will be on the strength of the "gun" and chamber design. I had this idea today and it isnt explained anywhere. Using high energy propellants in small arms or higher calibers. I would think there is limits, but i am sure bullet size could be lowered, barrel and chamber heavily reinforced. Or why not a two stage bullet design? A small charge to get it going then a high energy rocketball type ammo? I keep thinking guns using this tech could be super tiny. Bullets that look like nails in a super high capacity magazines. Using tiny amounts of propellant the weight is all projectile, if you can contain the blast using existing materials. Maybe using special vents to ensure gun doesnt explode. Lots of questions.

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    $\begingroup$ Please be aware that in an answer you are supposed to answer the question, not ask additional ones. For that you can use comments or write another question on your own. $\endgroup$
    – L.Dutch
    Commented Aug 13, 2018 at 7:08
  • $\begingroup$ Thats pretty useful idea. $\endgroup$
    – Efialtes
    Commented Aug 20, 2018 at 13:29

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