I get a kick out of seeing vapor trails and bullet trace from my bullets. For those of you who don't know what this is, check out this video and skip to timeline 3:11sec.

www.youtube.com/watch?v=ZxpnWftvCTo

Bullet trace is caused by the bullet disturbing the air and simply causing light to distort in that spot, much like how heat rising from the ground causes a shimmering effect.

Vapor trail is something else entirely and extremely rare as far as I know, as the weather conditions have to be very much just right, not to mention the background has to be dark enough to see the trail. The vapor trail is caused by a drop in pressure behind the bullet which causes water in the air to condense momentarily, similar to those little white streaks at the tip of a jet's wing.

To view bullet trace, you simply need to have the right combination of environmental factors and be generally behind the shooter. It can be seen with the eye, supposedly, but usually it takes some good optics looking downrange to catch a glimpse of it.

To view vapor trail, you also need the right combination of environmental factors (even more so!), but you can be positioned anywhere to see it, as the white mist is easily visible from any angle even by the naked eye.