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Back story (not essential to the answer)

Humans invade a forested planet and encounter the local intelligent lifeform. These creatures are tiny compared to humans and they see people as huge monsters. They want to drive the monsters away.

The tiny aliens have field artillery that look similar to those of WWII on Earth only much, much smaller.

Question

I shall scale the aliens according to the size of their weapons, but how small can I make the weapons and still be deadly to humans?


Thoughts

If the aliens' field guns were about the size of an average human handgun, then that would be deadly (if the aim was good) but how small of a bore could a handgun (or alien artillery) have and still be deadly?

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    $\begingroup$ HOW tiny are they? And are they allowed to use poisonous material like Polonium or Plutonium? Because a sowing needle from polonium would kill a human. $\endgroup$ – Trish Nov 1 '20 at 1:24
  • $\begingroup$ @Trish has a good question. With the right poison, a blow dart from a grade school straw will kill a human. With sufficient powder, a 0.1mm bearing through the heart will kill a human. Now that we mention it, aim is just as important as caliber. A howitzer that only takes an arm off might not kill a human. Maybe all you need to know is how much energy is required to pierce the sternum and heart? Once we know that, it's all just physics. $\endgroup$ – JBH Nov 1 '20 at 3:25
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    $\begingroup$ 5.56mm is pretty small, and while one hit isn’t likely to be fatal unless your aim is excellent, several likely would be, such as from sustained artillery fire. As a reader, I’d question anything smaller. That also gives you a 30:1 scale vs a 150mm artillery shell, and sapient beings that small are already a (different) problem. $\endgroup$ – StephenS Nov 1 '20 at 4:16
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    $\begingroup$ It seems to me that the aliens might get too small to be inteilligent beings before their field artillery becomes too small to harm humans. $\endgroup$ – M. A. Golding Nov 1 '20 at 17:09
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    $\begingroup$ Polonium kills in a month, arsenic in hours to days, based on dosing, ricin and botox in minutes to hours $\endgroup$ – Trish Nov 2 '20 at 11:27
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That's a complicated question. Several cartridge like the .17 have a .17 bullet stuck on the end of what is basically a .22 cartridge, so while the barrel inner diameter is small, the chamber of the weapon has to be large enough to fit a "normal" sized round.

The smallest practical cartridge is probably the 4.25mm Liliput, designed for the Liliput pistol. There have been smaller ones, such as the Swiss mini gun (2.34mm, assembled more like a watch than a firearm), and the 2.77mm Kolibri, which was supposedly made for women to carry in self defense in the 1920s. They were probably better off throwing the pistol at someone than shooting them: essentially you had to aim for the face in order to cause a wound that might hurt.

The main issue with small calibre rounds is that they're going to have crappy effective range because air resistance is going to slow the bullets down faster, relatively speaking, than a larger, heavier round, and they're going to have less velocity to begin with because of less propellant.

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    $\begingroup$ What if the bullet was..... longer? As if it had more room for propellant. What if the aliens, rather than designing a short, stout barrel, built a very sturdy and long barrel, which could shoot long bullets with a large amount of force? I mean think about it, as humans, we don’t just make “giant handguns” for tanks. We make heavier caliber bullets with long barrels and lots of propellant. $\endgroup$ – Mister SirCode Nov 1 '20 at 3:34
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Size of a pen.

enter image description here

You may be able to go slightly smaller using a weighted needle launched by an tiny Em field, but the overall profile will still be approximately pen shaped.

Recoil, trigger and safety lever size, and ability to hold in hands are critical factors here, any smaller and grips or recoil will be hard to manage.

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Small-caliber handguns, these aliens would be effected hugely by recoil of most guns so I have chosen two that could still be effective yet small enough to be used.

  1. 6mm Flobert snub nose revolver

The 6mm Flobert round may sound weak (and it is) but when used properly, it can deal enough damage to kill a human or similarly sized animal with no problem at all. Although quite ineffective at long range, this gun could be perfectly concealed and shot by multiple shapes and sizes. 6mm Flobert.

  1. 2mm Kolibri

Just in case that last gun wasn't small enough, this 1914 Kolibri could be surprisingly effective against unarmed victims. This tiny 2mm pistol would be absolutely useless at range and would be less effective than a knife but if you insist on using a gun, this could be the perfect option.

2mm Kolibri

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