Caused by condensation of moisture in the atmosphere, due to the drop of pressure immediately behind the bullet. Similiar to the streaks which seem to be attached to the wing tips and some control surfaces of jets during some maneuvers. Not to be confused with the engine exhaust contrails, which is a completely different situation.
It is also possible to actually see some bullets in flight, when the light is exactly right.
Personally, I have seen bullets from the 45 ACP (big and slow) when sunlight was from over my shoulder as I was shooting.
I can't remember ever seeing the vapor trail from bullets----just haven't done enough rifle shooting under the right conditions, I guess.