Bullet vapor trails visible in spotter scope

[HotRails]

Has anyone ever observed this phenomenon? Any of you engineers know what causes it? Is it caused by the heat that results from the friction of the bullet moving through the air?

If not it is (sort of) visible here.

http://www.youtube.com/watch?v=emqzwJ5nSdI

[B.D.Dubloon]

Pretty good AK shooting at 300 yards.

[Guns R Tools]

Long time ago I was shooting scoped 22 LR lever action and I was able to see the trail. It was wild. It moved so much side to side in helical motion.

I heard that if you stand behind shooter with spotting scope focused slightly before/after the target you can see trail better.

[elsolo]

rapid depressurization of the air, and the water vapor drops out.

Ever see a whiff of 'smoke' when unplugging an airtool from the hose, similar thing I think.

[Scratch705]

isn't that what military train their spotters to do? place the spotting scope just slightly out of focus so that the contrail is visible.

has something to do with the distorted air caused by the bullet as it moves through the air?

[socaldsal]

Yes, under the right conditions you can observe this.

[dwtt]

isn't that what military train their spotters to do? place the spotting scope just slightly out of focus so that the contrail is visible.

has something to do with the distorted air caused by the bullet as it moves through the air?

That's what my Gunny told me. He was a sniper in his younger years and told me they would look for the vapor trail to help spot the shot. Of course, he sometimes would send a Pfc down to motor T to retrieve a can of beep, so you had to keep on your toes with him.

[aplinker]

compressed air has a different refractive index, so you see a variation in light that passes there.

The physics is similar to mirage, but the effect is localized for the bullet path.

[kicker72]

I was spotting with my Leupy on 12x and have seen this out of a 308 PTR. It looks almost unreal when you see it. We shooting out past 500 yards so you could see the bullet arch and drop .

[randy]

It's called trace and if you position yourself close to the shooter you can see it. If you are in other postions you can see it also just not as often. Spotters use it to guide the second shot in if needed in a sniper team.

[SDJim]

It's called trace and if you position yourself close to the shooter you can see it. If you are in other postions you can see it also just not as often. Spotters use it to guide the second shot in if needed in a sniper team.

Anyone that's scored for someone at a HP Rifle match has observed the trace.

It's not uncommon to ask for assistance from an observer to help you get on paper if your firing a rifle with out good zeros.

The best way would be to have your scope right next to the shooters rifle, but that is impractical at a match. Second best is to get low behind the shooter right in line with the projectiles path.

[tomhenry]

Maybe it was my imagination, but I saw a "trail" behind full charge 357 magnum loads fired out of a Colt Python one summer. It was extremely hot and extremely humid (Kentucky), and I saw the trail with the naked eye.

[Fjold]

We were watching the bullet tracks at the SCPRC shoot last Saturday morning. Almost every shot could be seen.

Go to an IHMSA Handgun silhouette match and even with binoculars from behind the shooting line you can see those big old pumpkins out of the 44 mags arcing through the air at 200 meters.

[Nodda Duma]

compressed air has a different refractive index, so you see a variation in light that passes there.

The physics is similar to mirage, but the effect is localized for the bullet path.

I've seen the effect, too.

As an optical engineer I can say that this is the correct answer. More generally speaking, the index of refraction of air changes with density. It also varies with air temperature and the results are similar (ie heat waves in a spotting scope).

Index of refraction describes the velocity at which light travels through a medium. Lenses take advantage of this property by changing the path light takes through the lens, forming images for cameras or telescopes in their most common application.

-Jason

[trinydex]

which function of the lens makes mirage more visable when "unfocused?"