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.