I recently started to wonder how an animal could evolve a gun. So I decided that here was the best place to find people that are smarter and/or have different ideas than me.

  • This creature has an endoskeleton, about 1ft (30cm)tall, 1ft (30cm)wide, and 3ft (1m)long without the neck.

  • Its neck is 1ft(30cm) long and very flexible to watch for predators and aim the gun.

  • The tail takes up 1ft(30cm) of the body and is used to keep balance when rearing on its back legs to intimidate predators and/or a mating competitor, and reach food from higher places.

  • The creature can swim well enough to not drown in the water, and cannot run faster than about 15mph(24.14kmph), and it can't climb or fly.

The gun cannot be a limb that picks up an object and just throws it like primates do, but have a mechanism that shoots it at least 6.5ft(2m). How would the animal actually shoot something at its predator?

  • $\begingroup$ Hi, Fafo. It looks like you're trying to answer your own question in the body of the question. Biological gun questions usually focus on propulsion methods, how to grow a projectile, energy efficiency, etc. Can you focus on the mechanism to avoid having this question getting voted down as too broad? $\endgroup$ May 23, 2023 at 16:45
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    $\begingroup$ check out cone snails, they have single use darts and specialized "arm", that functions as a blowgun, but with water and venom. $\endgroup$
    – trotzt
    May 23, 2023 at 17:12
  • $\begingroup$ @trotzt they're basically contact weapons though. They're no more guns than cnidocysts are. $\endgroup$ May 23, 2023 at 18:34
  • $\begingroup$ could the speacilized "arm" be attached to the head? $\endgroup$
    – fafo
    May 23, 2023 at 19:24
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    $\begingroup$ May I suggest translating/adding metric measurements ? $\endgroup$
    – breversa
    May 25, 2023 at 15:01

13 Answers 13


So, what you've described (whether intentional or not) sounds eerily similar to the Tyranids from WH40K and their Bio-Weapons:

Details here

In the Lore - there's different types of Ranged bio-weapons, from Acid firing rifles (think like a spitting cobra, but on steroids) to the Deathspitters (which fire a living creature that has several nasty properties).

However, as much as I love 40K, the biggest problem with this concept (and yours) is Range. A Muscle contraction can only produce so much force and spraying a liquid can only go so far - 10 metres is realistically your limit, even a modern flamethrower doesn't have a range much beyond 100 metres.

Whereas a modern rifle round (say 5.56) is effective out to about 400 metres from a standard AR platform, with high quality ammo and a high quality rifle, you can get good groupings out to almost 1,000 metres.

The key here is the pressure required to get a projectile to go that far - which is 55,000 PSI - simply too high for any biological system to contain (for reference the greatest pressure that any part of a Human can withstand - the Femur is less than 20,000 PSI).

To conclude - So long as your 'gun' is only shooting at ranges of around 10 metres - some combination of Acid/Venom and a muscle contraction would suffice. But if you want something that approaches a 'real' gun, then biological limits mean you can't get and contain enough pressure to give a range equivalent to a firearm.

  • 8
    $\begingroup$ Please report to the Martyr for evaluation if you are in possession of heretical knowledge as defined by the great inquisitor. $\endgroup$
    – Trish
    May 23, 2023 at 21:00
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    $\begingroup$ @Trish I serve the Golden throne and know no authority higher. $\endgroup$ May 23, 2023 at 22:56
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    $\begingroup$ Once you are using a pressure-launch, that pressure-chamber getting reinforced by evolution isn't all that implausible. I mean, rodents have iron-reinforced teeth. Human enamel has a 5.6 GPa elastic modulus; bone caps out at 3 GPa, and mice teeth break 14 GPa elastic modulus. Steel has a 210 GPa, much higher; but an evolutionary process that literally builds something Steel-like atom by atom and layer by layer is perfectly plausible (in a SF or Fantasy setting). $\endgroup$
    – Yakk
    May 24, 2023 at 15:06
  • $\begingroup$ @Yakk I'm reminded of the various snails and similar that incorporate iron into their shells as well. $\endgroup$
    – Ruadhan
    May 25, 2023 at 9:27
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    $\begingroup$ @TheDemonLord: This answer correctly eliminates muscle contracting propulsions, but is ambiguous about chemical reaction pressure a-la bombardier beetles. An animal could theoretically evolve that had a hard launch chamber that it could fill with explosive chemicals and a shot. You end with "only shooting at ranges of around 10 metres" but the rest of the answer seems to imply that even that is impossible. $\endgroup$ May 25, 2023 at 16:10

So how did it evolve?

Other answers seem to be focusing on what the end point might be, but you asked how a gun could evolve, that is, what the stages of evolution could have been to result in a gun.

First, take bombardier beetles as a starting point.

enter image description here

They can blast noxious chemicals from their rear ends as a result of a chemical reaction between hydroquinone and hydrogen peroxide. These chemicals spontaneously combust when they come into contact, providing the force to shoot out the payload. We may suppose that your organism's ancestors had a mechanism like this, shooting a liquid mixture at a relatively low force, to burn and poison predators at close range.

From there, we can imagine that there could have been mutations that altered which particular explosive chemicals were used to propel the poison. More powerful explosive chemicals would produce greater range and higher temperature, which would be more effective at deterring predators.

A more powerful explosion would also require a stronger blast chamber, so the ancestors of your gun-creatures would have evolved stronger materials to contain the blast (thicker bone, chitin) at the same time as they evolved more explosive chemicals.

We could even imagine that if they lived in an iron-rich environment, like the scaly-foot gastropod, they could start to incorporate iron into their blast chambers, making them even stronger.

From there it's not a stretch to imagine that one of the ancestors of the gun-creatures happened to grow a bony protrusion at the front of the blast chamber. This protrusion would break off from the force of the explosion and could have helped to injure the predator at greater ranges, in addition to the poison. It would be the precursor to the bullet.

From there evolution would be a straightforward process of improving what's already there:

  • More and more powerful reactive chemicals until the animal naturally produces something like gunpowder
  • Stronger and stronger blast chambers to contain the explosion
  • Improved shape of the blast chamber and bullet, towards something like a rifle barrel
  • The ability to regenerate the bullet (as well as any damaged parts of the blast chamber) after it is fired
  • Gradual loss of the vestigial poison component of the payload, in favor of an increased explosion for a faster bullet
  • Eyes and brain structures to help aim the bullet more accurately at range

By the end of this process, the creatures could have something similar in range and power to a musket, though it would be single-shot and they would have to take time to regenerate the damaged blast chamber and lost bullet afterwards. What initially was a last-ditch defensive tool against predators has become something much more powerful and efficient, so that the creatures may evolve to become apex predators themselves.



This is an example of Nature did all the hard work for you. A gun is, essentially a tube of some length from which a projectile is expelled at some velocity. Every male on the planet knows about this and has almost certainly given the thing a try.

So, what is a penis and how does it work? Well, it is, essentially, a tube of some length through which a couple different kinds of projectiles are expelled with force. The evolution of your creature on its path towards weaponising this arrangement would involve two components: musculoskeletal and saccular.

Skeletal: I would suggest that a bacculum be already in place and that the penis thus be permanently rigid. This would allow the creature to engage its weapon at any time without the need for reproductive signalling to occur.

Musculature: There are already muscles in this region, but your creatures will be evolving muscles that more forcefully expel the projectiles and more energetically rotate the penis around its axis along with improved nervous systems that allow for penis-eye coordination for aiming.

Saccular: Your creatures will evolve one or more bladder analogue structures that produce some sort of material for use as a projectile. A strong acid, for example, perhaps in conjunction with a sticky mucus.

These three components could arise from some reproductive or excretory precursor system. A new and better use has been found for these early precursor systems and are now widespread in the population. The advantage here is that all the structures are already in place in the likely mammalian creature you propose: it doesn't require any massive physiological overhauls or involve invertebrate solutions.

  • $\begingroup$ Now, if I remember correctly, the urethra even has spiralled walls to make the urine go straight, just like a real rifle would have spiral grooves to make the bullet rotate and go straighter. I'll see if I can find anything on this So that's another fun little fact for you:) $\endgroup$
    – Katy
    May 25, 2023 at 9:20
  • $\begingroup$ A correction: it is not the same, this is an old misconception, but the design of the very end of the urethra does cause it to spin. I'd assume a similar mechanism. $\endgroup$
    – Katy
    May 25, 2023 at 9:24

I imagine any answer will require a decent amount of handwaving. The two methods I could think of would be a biological spring, or two chemicals that when mixed together produce a large amount of gas.

The idea for a spring-powered projectile launcher is inspired by this insect that has evolved gears, which to me puts the idea of a powerful biological spring into not-totally-implausible territory.

The idea for the creature having stores of two separate chemicals that it mixes to produce thrust comes from the bombardier beetle, which does the exact same thing to expel boiling hot fluid at other insects. With some handwaving, its possible your creature could expel these two fluids into the 'barrel' of the gun, which will accelerate the projectile and cause it to shoot out.

Another important consideration is the projectiles. Your creature will either have to grow the projectiles itself, which would likely allow for better aim and range (as you could argue evolution caused the projectiles to take on an aerodynamic shape) but would make ammunition far more valuable, or it could suck rocks up into the barrel, which would result in poorer aim and range, but much cheaper ammunition and the potential for scattershot.


Use a scaled up nematocyst

A nematocyst is the tiny harpoon like structure used by jellyfish to sting thier prey. A nematocyst is basically a biological spring that is compressed inside of a fluid sac. To fire the nematocyst, the jellyfish inflates the sac with extra water pressure causing it to rupture when this happens, the spring is both launched out by the pressure build up and it decompresses causing it to spring out ejecting at an acceleration faster than a modern riffle... now, part of what makes nematocysts so darn fast is that they can take full advantage of the the square cube. If you were to scale a nematocyst up to the size of a gun sized weapon, material science becomes very different. Since you are working with an endoskeleton animal, a similar structure could similarly evolve to use a horn like material.

Based on what we know about actual spring weapons made by humans from horn, we can predict that a spring loaded horn weapon could fire at speeds ranging from 200-300 feet per second giving you a weapon similar in nature to a recurve composite bow and arrow.

How it might have evolved

Imagine an animal similar to an impala. It's horns already have a generally spring like shape. Now imagine if a mutation caused these horns to be overgrown with a sinuw layer that keeps it from breaking skin, and instead compresses it. Then when a predator tries to eat it, the stress causes a build up in blood pressure causing the horn to rupture the layer stabbing the would be predator as it tries to bite the back of the mutant impala's neck. Evolution does its things and this becomes a dominant gene.

Over a few million years, these horns become more specialized springs. They reorient forward so they can be used more offensively, and as they get better and better at springing out, they start breaking off sometimes when they spring out... which gives them more range so again they continue to evolve until they are designed specifically to fire off at will.

enter image description here


You've got loads of options, most of them unfortunately painful for the creature in terms of recoil and side effects unless buffered by an insensitive and durable shield of hard tissue, for example a bone mass used as a launching pad. For mobility's sake, the back with a hard shell makes the most logical location for shielding the creature from the recoil of its own projectiles if they are powerful enough to pack a punch. It could also jettison parts of a limb at high speed and and regrow them. Whether digits, tail, back or any other location, a bowl-shaped bulwark of durable tissue is required to prevent significant self-damage.

Propulsion offers many methods, each with significant constraints.

Air power

The creature could use pneumatic chambers within the skeletal system, like some dinosaurs have. The creature can channel high pressure into a launch site by flexing its body in a way that pinches or seals pneumatic valves. Once released, the membrane covering a given exit must seal and regrow together with the projectile, after which it can be launched again. Likely no soft tissue can withstand high enough pressure to reuseably launch a projectile at lethal velocities.


The creature could have an internal mechanical "arm" that loads and launches grown spines from a glandular magazine using the end of a long lever bone flexed with a strong but short muscle around a short rotator to achieve high velocities through a curved bony guide similar to a partial gun barrel. https://en.wikipedia.org/wiki/Spear-thrower


Yeah, chemistry. A pair of glands mix reagents or a spark is provided to a fuel reservoir which ignites or expands a gaseous substance rapidly to launch pebble-like growths at an adversary.


Considering the barrel pressure, your creature probably won't be able to fire regular bullets   But you could try the Gyrojet, where instead of firing bullets, your creature fires very small rockets   However, the accuracy of this bullet type is very poor and your creature may need to be a pack hunter to use it effectively

  • $\begingroup$ Bear in mind that the inaccuracy of Gyrojets in real life was due to a manufacturing defect in the ammunition, not something inherent to the concept. $\endgroup$
    – Ruadhan
    May 30, 2023 at 13:08

Lots of discussion here around bullets and chamber-pressures, but what if we looked at the real-world for inspiration?


I love gyrojets.

They're the answer to a question we didn't ask.

"What if we made the bullets rocket-propelled?"

There are a few major advantages to rocket-propelled bullets.

  1. Low muzzle-velocity
  2. Next to no firing-chamber pressures, you can launch one from anything that can withstand the heat for a split-second.
  3. They keep accelerating as long as their fuel holds out, so they can exceed conventional bullets in their calibre very easily with the right fuel.

Your creature grows biological gyrojet bullets and launches them from a specialised horn-like organ.

The mechanism would be somewhat like growing an egg, enter image description here Each projectile would be complete in and of itself. A spike of bone with a hollow section full of a powerful rocket-fuel-like substance.

The "firing chamber" would consist of a pair of glands which secrete chemicals that burst into flame on contact.

This is essentially the same as the Bombardier Beetle (mentioned elsewhere) and various other insects with similar capabilities.

If they do this when there's a projectile in the chamber, the projectile ignites and immediately launches as a rocket.

Its muzzle-velocity would be pretty piddly, but at a dozen meters away it'll have reached bullet-like velocities and be lethally dangerous to anyone not wearing a bulletproof vest.

The creature with a biological gyrojet launcher is probably not going to achieve the same velocities, range and accuracy as a machined weapon, but imagine a nest of them..

A predator sneaks up on the nest, knowing that their guns aren't very dangerous at close range.

Then it gets taken out by a sentry lurking in a tree 20 meters away.

The minimum-range limitation I think would add to your worldbuilding quite well.


The animal could have a similar mechanism to the sandbox tree, which has fibers that contract and create a lot of force until it explodes / shoots out.

That mechanism can of course be more complicated and involve several organs to create a bone projectile, a barrel bone, fibers and maybe even explosive gas, which all aid in creating and shooting a projectile.

EDIT: As requested I'm adding a possible option here

One possibility could be that the animal has a barrel shaped hollow bone. This bone has a mechanism (muscles and fibers) to "load" the bone with a bone / shell projectile. After the projectile is loaded the body starts to produce an organic explosive which is the "gun powder" in that case. The primer could be some small organ or part of an organ, which gives off a shockwave or electrical signal to detonate the explosive. Such a mechanism would obviously not make the animal able to fire like a machine gun but it is realistic that it could fire a couple bullets per day (It would need to consume a lot of resources including the materials needed to create projectile and explosives tho. Most likely it'd need to actually devour the bones of its victims for the minerals)

  • $\begingroup$ @BlueSkinandGlowingRedEyes It entirely depends on how you plan to create your fantasy world. $\endgroup$
    – GoldNova
    May 24, 2023 at 14:05
  • $\begingroup$ @BlueSkinandGlowingRedEyes One possibility could be that the animal has a barrel shaped hollow bone. This bone has a mechanism (muscles and fibers) to "load" the bone with a bone / shell projectile. After the projectile is loaded the body starts to produce an organic explosive which is the "gun powder" in that case. The primer could be some small organ or part of an organ, which gives off a shockwave or electrical signal to detonate the explosive. Such a mechanism would obviously not make the animal able to fire like a machine gun but it is realistic that it could fire a couple bullets per day $\endgroup$
    – GoldNova
    May 24, 2023 at 14:19
  • $\begingroup$ @BlueSkinandGlowingRedEyes I added it :) $\endgroup$
    – GoldNova
    May 24, 2023 at 14:24

This is probably a comment more than an answer, but given that no one else has bothered I feel compelled.

Allow me to introduce you to the bombadier beetle.

This little bug has evolved an interesting defense mechanism. It shoots hot steam out of its but, produced by some interesting chemistry involving hydrogen peroxide, a few organic catalysts, etc. You can read the link for details if interested.

It's basically your gun, as-is. We need to scale it up, and we need to put a bullet and barrel into the mix. As we extend the chamber for the gas to be expelled from it will give us a bit more velocity and accuracy. This could be made of any of a number of biological substances. Bone, horn, even something closer to what we find in teeth.

And the bullet could be made of nearly any of the same substances, as long as they're relatively fast-growing. It may have to go days between "reloading", given the biological nature of the ammunition, but even venomous snakes are reluctant to bite for concern that they might not have enough later when they might need it, right?

Evolution has already delivered a solution to your problem (or at least a prototype). It only took 4 billion years or so!

  • $\begingroup$ yes i am familiar with the bombardier beetle, but i think of it as more closer to a flamethrower and to scale it up would make it much harder to gather sufficient amounts of those chemicals $\endgroup$
    – fafo
    May 24, 2023 at 16:29
  • $\begingroup$ No. It's not closer to those. There is no "burning fuel" being squirted out its butt. It's the mechanism you want. Hot gas is expelled with significant energy. Just put a bullet in front of it. $\endgroup$
    – John O
    May 24, 2023 at 19:32
  • $\begingroup$ @John_0 I wasn't saying it was a flamethrower. I was thinking that that mechanism would more likely evolve into a flamethrower before a gun Givin that the resulting liquid that fires out is boiling. I wasn't trying to say that you were wrong or that it wouldn't $\endgroup$
    – fafo
    May 24, 2023 at 21:26

Just here to add something (to be honest, I didn't read carefully all the other answers so if someone else already talked about this, I'm terribly sorry).

It's amazing how Nature always finds a way (Jeff Goldblum anyone?). There exists a kind of shrimp that evolved a small, but very powerful, spring-action powered hammer (with a very complex assortment of bones, tendons and muscles). The scientific name is Gonodactylus smithii if I'm not mistaken, commonly called purple spot mantis shrimp. I've seen it in documentaries, the bigger ones can generate a force up to 1500 Newtons. Regrettably, the english wiki is terribly lacking in information about it, but you should be able to research more about it with the information I've given (It's all I know about it).

From there, I would think it's only a matter of figuring a way to create a projectile (could be a piece of regenerative bone) and some barrel-like structure to transform this friendly shrimp into a long-ranged shooting beast. Give it eagle eyes for accuracy, and you're basically done.

The data is extracted from my memory from the documentary I mentioned, so It would be wise to double check the numerical information, but nevertheless it is the strongest force generated by any natural animal mechanism. Hope this helps!


Following on to many of the others, I would go with the bombardier beetle analogy as far as propulsion. For the chamber, lots of creatures generate crystal structures, and the snails that incorporate iron into their shells have already been mentioned. Carbon is one of the most readily available elements; if we're hand-waving anyway, why not a creature that can secret a carbon fiber lattice of some sort?

As to the actual projectile, chicken gizzards may make a good answer. Chickens (and other birds, I'm sure) eat rocks and gravel to assist in digestion. Your creature may do the same thing to provide ready bullets. And maybe if it's out of internal bullets, it just picks another one up and stuffs it in the hole. Maybe the carbon fiber lattice is flexible enough to allow its musculature to adjust the fit for any random rock within a given size range.



Some things like it exist in nature already, years of evolution could lead to strengthened muscle to deliver at greater ranges.



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