Originally Posted by tracker12ga
Is a huge objective high powered scope really need to watch vapor trail consitently? I wa wondering if it is possible to do this with a quality pair of binos. if a spotting scope is needed, can it be a compact one?
The main requirement for a vapor trail is to have an atmosphere where the dew point is near the ambient temperature. Then the sharply increased then decreased air pressure from the passing bullet will cause droplet formation. It's the same process as contrails off an aircraft. In Arizona most of the time vapor trails don't form at all. It doesn't take much of a scope to see the trails if they form. You can see them naked eye in good conditions.
You may also detect the bullet path from the optical refraction of the air compressed by passing bullet. It's a smaller effect than a vapor trail and has no persistence. I've only noticed it shooting my 50 BMG. I see it though the rifle scope (14x50) usually at ranges from 300 to 600 yards and it's visible in dry air. The background makes a big difference in how visible it is. You'll see nothing with a uniform background (like sky).
I don't find either very useful for determining where a bullet has impacted. Dust puffs from the ground are better in dry locations but don't happen on damp ground. Tracers work but each type has a limited burn duration. They can also start fires.
All of the above are useful for estimating wind deflection.