How would you design a firearm for a fairy?

Oh, that's easy!

The size and speed of the bullet only matters when you're relying purely upon kinetic energy to take down the target.

However, if you have relatively tiny fairies who literally can't handle the recoil of a weapon with a decent kinetic energy, change the paradigm.

Instead of trying to make a gun capable of punching a lethal hole in what is essentially a kaiju to these fairies (a fairy weighs around 0.5kg, while an average human is 75kg, that's a 150:1 mass ratio in the humans' favour, way more than humans vs elephants), make a gun capable of delivering a lethal toxin to their enemies.

There are many neuromuscular-blocking drugs such as curare, or other toxic agents such as strychnine that are relatively rapidly lethal that could be placed in lethal quantities within a projectile that would be able to penetrate human skin, yet would otherwise not carry enough energy to be lethal except with a very well-placed shot.

So, as long as a fairy can get a poisoned shot from a gun into a human's flesh, biochemistry will do the rest fairly quickly. For a small being who is far better placed to make the best of available cover than a human, taking a shot and retreating until the toxin has done its job makes far more sense than standing their ground and trying to take down a human with what amounts to the fairy-sized equivalent of elephant guns.

So, with toxins, a fairy can be lethal with what amounts to a fairy-sized revolver or a light rifle. They wouldn't need to use any particularly heavy weapons against regular humans.

That presupposes that humans don't go after fairies while wearing body armour. However, when an armoured-up human comes along, the fairies can break out their equivalent of crew-served artillery and deploy airborne toxins... but that's an answer to another question.

Additionally, once it becomes known that fairies use poisons, they'll become feared just for that. A fairy wouldn't even need to carry a gun to be able to poison an opponent, since these poisons could be delivered by a fairy-sized dagger/needle, though guns (or bows) would allow a fairy to deliver a toxin at range.

Frame Challenge - They can't

I sat and thought about this, looking at some options like 2mm Kolibri - but what I kept coming back to is that the limiting factor is their lack of Mass to absorb the recoil.

Bearing in mind, 2mm Kolibri is considered both very innacurate (due to the light weight of the bullet and low muzzle velocity of being subsonic) and also barely lethal - I can't find any hard data on lethality, but estimates say it would be only lethal with a well placed shot to the face.

But either way - that's not the point.

I did some very rough calculations on how much recoil force firing the Kolibri Car handgun would impart - it's a tiny force....

But it would still throw the fairy back 5 cm

That would be the equivalent of throwing me back 1 Foot from the recoil.

The problem is thus:

• Increase the bullet weight = increase recoil force
• Increase Muzzle velocity = increase recoil force
• Increase Firearm weight = Unable to shoulder and weild it
• Increase Barrel length = More weight and unable to shoulder

Daniel B's answer of Recoilless Rifles is a good idea - except within the tech period, they didn't really exist.

Realistically, to have enough energy to reliably kill a Human sized Target, your Fairies will need to use a Crew-served Weapon, like a .22lr in a little tripod/mount like artillery

OK, to be fair, this is the second Frame Challenge... but let's see what we can do.

Average human adult male mass: 91 kg.

Average bull elephant mass: 5,000 kg. (I'm being reasonable and using an Asian bull elephant, not the larger African bull elephant.)

Ratio: about 1:55

Why, do you ask? The first thing that came to mind was "elephant gun." After all, humans hunt whomping big critters successfully, so why not fairies? If the ratio is smaller, so much the better, right?

On the surface this sounds plausible. Elephant guns have a kick! Well, "human guns" have a kick, too, just scaled down to fairies. Maybe it would make more sense if it was a fairy-sized howitzer, but still, you do have a real problem.

Chemistry doesn't scale down very well

At a molecular level, chemistry doesn't scale enough. Drink a glass of chlorine and you're dead as a doornail. Pour a cup into a pool and you're (more or less) protected from the natural consequences of too many people sharing too little water.

It's easy for a human to double, triple, whatever is needed to throw a big enough slug far enough and hard enough to bring down elephants, rhinocerous, etc.

But you're scaling it down. An elephant can be killed with a large, fast bullet — but a human can't be significantly hurt with a small, slow BB. There isn't a long gun that can be made utilizing gunpowder (or any other similar explosive propellant) at a fairy's scale that can hurt a human.

I think...

A foot tall isn't inconsiderable. So when it comes to suspension of disbelief, it's not impossible to believe the fairy equivalent of an elephant gun, well aimed, could bring down a rampaging adult human.

But in reality you'd likely need something closer to the bang of a scaled down nuclear detonation. The bullet is too small, the kick too hard, the propellant simply not strong enough.

Build a suitable harness and fly

The question doesn't specify this, but everyone knows that fairies have wings and can fly. That's a defining feature of a fairy. If they didn't have wings, they'd be some other small mythological race; perhaps a brownie, for example. So with that in mind, let's solve the problem.

Recoil creates three issues.

• Applied force on the contact point on your body. As anyone who's fired a shotgun knows, it hurts your shoulder.
• Problems with aiming when the force creates off-axis motion of the gun. All human guns have the barrel on top of a mounting structure, creating a force above where you're holding it which rotates the barrel upwards.
• Pushback. Getting blown backwards off your feet is its own cause of injury. (Or think of cannons rolling back on their mountings.)

The first of these is always going to be a problem for fairies, even if they're stronger per pound than a human. Most likely they're going to need some significant damping structure between them and the gun. That naturally leads us towards the concept of a fairy needing a harness and substantial mounting structure to hold the gun. Let's assume they're strong enough to support all this, and move on.

Then we have off-axis force. This is going to be pretty brutal for fairies too. If we suppose a significant mounting structure though, you could essentially mount this to a breastplate in front of you, ensuring that the fairy's centre-mass is behind the gun barrel. The fairy will still have all the problems of recoil, but at least the bullet will go where they intended, and they'll only be travelling straight backwards when the recoil hits. Attach a damper between the gun and the breastplate to soak up the immediate shock, of course.

And finally we get to the most interesting problem, which is equal and opposite reaction. There's no way to get around it - a fairy is going backwards, fast. But is that a problem? It certainly is on land, sure. But ships famously can mount much heavier armaments, because the fact that they get pushed back across the water isn't an issue for them.

So, the fairy takes to the air. When they shoot, they get pushed back exactly as you'd expect. But at that point their wings become airbrakes to stop themselves, and all they need to do is allow enough stopping distance behind them. No problems.

For bonus points from this, also consider the biggest problem in a firefight, which is announcing your location to the enemy who wants to fire back at you. If you're a fairy, the location of the gunshot is very much not going to be the location of the fairy half a second later!

That which is possible does not happen when good doctrine demands something else

Why portable man-guns will be too awful to be the best option

Can fairies invent recoilless small arms for killing humans? Probably. Will they use them, probably not.

The smallest guns capable of harming a human in the midwest were the tiny derangers women often carried which weighed about 1 lb and held a single shot. But these things rarely killed anyone since their muzzle velocities were so low that they basically hit like the .22cal pellet guns we used to shoot each other with as kids. They sting like hell to get hit by, but even basic clothing will usually stop them, and hitting exposed skin from more than a couple of feet away is likely to just leave a welt. More importantly, it takes a minimum of 2 pounds of force to set a spring strong enough to detonate a blast cap; so, a single fairy would need a winch or something to rearm the firing mechanism, even if they had extra ammo. Also, true recoilless weapons tend to only be half as powerful as their with recoil counterparts; so, you'd have to content with that as well.

So yes, they could make something, but it would not be good or effective as an infantry level weapon system.

Why doctrine will focus on bigger cannons instead

Instead let's consider this from a doctrine perspective: Do you want 4 troops with ineffective weapons and one bullet each, or a team of 4 fairies manning a rapid fire cannon than can reliably take a human down with each shot. The logical answer is that they will CHOOSE the cannon every time, even if the deranger is a possibility. Once you are thinking in terms of 4lb weapon systems, you can make something comparable to Colt P1840. These 45cal revolvers weighed about 2.5lb which you could break into 3 components that all weigh under 1 lb: the barrel, the cylinder, and you would replace the trigger and handle with a spring traversing carriage. Then your 4th team member could carry up to 28 rounds of ammo.

Based on the P1840, your carriage would need about 4 inches of spring recoil distance to absorb the shock using its own weight without flipping or sliding your cannon out of position.

What this means for your gun crew is that your 4 fairy team could fire at and kill humans from dozens of yards away, and they could fire 6 shots in rapid succession with 2 working together to pull the hammer mechanism, 1 to aim, and 1 to fire. And then to reload, they could all work together to quickly refill the cylinder. So, not only is this going to be more powerful, but it will be able to fire more shots over time.

Lastly, you need to consider how humans will fight fairies. Humans already had multi-shot shotguns at this point in time and many were seasoned bird hunters able to shoot down fairy sized targets at pretty long ranges. Or worse yet, humans had punt guns which could drop a whole squad of fairies with a single shot. This means that fairies will be much better off fighting humans on the ground where they can entrench, and their small size makes them harder to spot. This means they need to focus on cannons with longer ranges and well hidden entrenchments if they want to get close enough to fight humans.

I present the 0.15 inch Mankiller Heavy Rifle:

This is a 0.16lb (73g) rifle with a bullpup layout to accommodate the 7.2in (183mm) heavy barrel in a manageable overall length of 8.8in (223mm).

To keep the length as short as possible given the extremely long barrel it is a single shot rifle loaded through a removeable bolt that includes a padded recoil pad. (The design is similar to the LAR Grizzly or Maadi Griffin).

There is also a substantial muzzle break to help mitigate recoil, and the rifle is intended to be fired from the prone position using the integrated bipod.

This fires the 3.81x20 cartridge using a 3.81mm projectile weighing 22 grains (1.43g) at 785 feet per second (239ms^-1) for a muzzle energy of 30.12 foot pounds energy (40.84J). Although ideally suited to targets in the 45lb body weight category, with correct shot placement, it is capable of stopping an unarmoured human sized target out to 50 yards. Felt recoil is on the order of 1.2 joules.

The cartridge itself is a straight case rimless design with a slight taper to aid extraction.

Realistically 30fpe is pretty marginal for a human target, but the projectile should be capable of penetrating a human skull and reaching vital areas of the brain. It would have to be a central nervous system shot though, relying on bleeding would be ineffective given the small, low velocity projectile.

The recoil would be quite a substantial thump for a fairy but I think it would be manageable from the prone position.

Ofcourse, fairies are much stronger, proportionally, than humans, so you could just make the weapon heavier to reduce recoil further, not much can be done about the unwieldy size however - you need barrel length to get decent velocities - especially with early 20thC tech. The ballistics otherwise seem fairly plausible.

Two other things to consider:

Firstly in warfare armour was extremely rare in WWI because the infantry rounds of the period were, at that time, practically unstoppable. That is definitely NOT true of this thing. Human soldiers who knew they would be fighting fairies equipped with anti-human rifles would almost certainly wear some armour. Low quality late medieval plate would have no trouble stopping the 3.81x20.

Secondly the very short engagement ranges (from a human perspective) would make fairy snipers very vulnerable to counter fire from almost any human weapon. They would only barely be out of grenade range!

Banzai!

Let's face it: no smallarm that a fairy can handle without injury even packs the punch and accuracy of a snubnose .22 short. That is already light artillery for fairies.

But there is an option, provided the fairies are desperate enough to order literal suicide missions: strap one of their own with explosives and charge the human with the equivalent of an anti tank mine. It puts the fairies into the doctrinal camp of the WW2 imperial Japanese. Kamikaze. Banzai.

Drop a firearm that fires once in free fall

Assuming they do not have to hold the firearm the entire time, and the firearm does not have to be rapidly reusable, they could they just fly up to a human-sized target (your fairies can fly, right?) with a gun that fires a split second after it is dropped. This does not negate recoil, but it changes the result of the recoil from "injure the fairy and send it tumbling back" to "lose the gun and have to retrieve it".

Caveats:

• The firearm would not be easy to re-use quickly. After all, it will be across the floor.
• The fairy has to fly relatively close to their target.

Assuming the target is just as vulnerable to bullets as we are, they would be dead regardless.

Extremely small caliber bullet with poison

Imagine being shot by a 2mm Kolibri. A fairy could easily carry the weapon and the recoil would be negligible. It's stopping power is terrible, though. It'll sting if you get hit, but it's not going to kill you.

Now imagine if the tip contains a potent toxin such as ricin. You're going to die.

PICK-GUNS.

Imagine a tiny shotgun with a war-pick tip: where a rifle would have a bayonet, the pick-gun has a downwards pointing spike, and the crosshairs above it are long and sharp too.

When a Pairy wants to shoot the pike-gun, they stab the pick into the ground (or an enemy) and brace the gun against recoil. Now the gun can be high enough caliber to down one of the Biggies (humans) with a single shot!

When out of ammo, the pick-gun (gun-pick?) is still an effective war-pick.

Really simple answer - scale it. Assume fairies have better aim than humans due to their small size, and just scale everything accordingly. Scale the weight, recoil, caliber and everything else to the size of the fairy, and just give them better aim than a human. Given their small size they might not have as good depth perception as humans, but this is fantasy anyways, make your own rules. Don't make things difficult for yourself, just scale it to human firearms and give the fairies an accuracy boost to counteract their small size. Hit a specific blood vessel and BAM, dude goes down.