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In many sci fi universes that don't have handheld energy weapons, they instead use solid projectiles that either go big (Warhammer 40k) or really small (Mass Effect). The former I have a pretty good understanding off. It is the latter I want to know more about, namely;

How small and fast can someone make a bullet in atmosphere before detrimental effects like friction become an issue?

We can for now ignore how the projectile is launched, though feel free to give it a shot (tee hee).

Lets assume that the atmosphere that we are operating in is Earth like.

A question on how damaging such a projectile would be, and how defend against it, will be asked on another post.

Edit: To clarify on "issue" in the question; a good example would be the critiziem of Mass Effect weaponry, namely that if the sand grain sized projectiles are fast enough to ionize the air, then why is the projectile itself not vaporized? And other issues like why does it not get knocked off course from the air molecules etc.

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  • $\begingroup$ I think your question is poorly phrased. What do you mean with "become an issue"? At 5/s you already experience drag, ask any cyclist. $\endgroup$ – L.Dutch - Reinstate Monica Apr 26 at 4:37
  • $\begingroup$ Does the bullet need to be big enough to be handled individually? $\endgroup$ – adamdewolf Apr 26 at 4:42
  • $\begingroup$ No. It does not. $\endgroup$ – Seraphim Apr 26 at 4:44
  • $\begingroup$ I think you need to specify some specific criteria- "before effects like friction become an issue" doesn't tell us what you have in mind. Does the projectile need to break the skin? Break the skull? Break a concrete block? How thick of concrete? Are you fighting humans or dragons? At what range does it have to do these things? $\endgroup$ – David Apr 26 at 17:21
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Assuming a handwavium weapon that can accelerate the projectile as needed, you can have single atomic nuclei moving at close to the speed of light. In fact, such projectiles are fairly common, since extra-solar sources are continually bombarding the Earth with them: https://en.wikipedia.org/wiki/Cosmic_ray

The problem with using them as a weapon is the extreme case of using any small, high-velocity projectile: they're likely to pass through the target without doing much damage. And in the case of really small projectiles, the damage is most likely dying of cancer a few decades later :-)

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In addition to what jamesqf said in his answer, another drawback of small projectiles is that they are more effected by drag. Small objects have a higher surface area to volume ratio than large objects of similar shape. Objects with higher surface area to volume ratios lose energy to drag faster than if they had a lower ratio. So your tiny bullets will have relatively short range. This is why real sniper rifles tend to be relatively high-caliber. When a bullet falls below the speed of sound, it loses a great deal of its stability and thus, accuracy. Higher caliber bullets stay supersonic longer than smaller bullets.

If your bullet is molecular or smaller in scale, this distance will be similar to the Mean Free Path. On Earth, the mean free path is about 68nm.

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