So I THINK the way it works is that after a certain distance, some weapons muzzle can't be heard, but at some point the bullet breaks the sound barrier and you hear that. So there's a *pop* from the bullet and a *whizz* or *shoop* sound from the bullet breaking the sonic barrier. I think that's what they said. So SMG's basically don't have enough velocity to break the sound barrier, but for some reason people can account for drop, so at a certain range, especially with suppressors, you can't hear them at all.