[Hectorvictorious]

I'm a newbie to long range shooting, with the longest actual gun range around being 300 yds. So I've never dealt with mirage or ever really had any other factors to deal with. I've read an awful lot of stuff that mentions watching bullet path. ????????

 

[Guest]

Don't feel bad about your three-hundred yard range! I live outside of metro Atlanta, where all i have is TWENTY FIVE YARDS at an INDOOR range! Unless i wish to drive for an hour and a half in metro Atlanta traffic to gain access to a 75 yard outdoor range! Now don't you all feel sorry for me and my long-range guns that will never reach their potential, or even get close to it?

Well, after that little tirade, lets address the question. The only mirage that i can think of as affecting a long-range marksman is the appearance of heat waves in the high magnifcation of the scope. This will invariably destroy your ability to shoot or even see your target clearly. Thankfully such guns as the Sig SSG 3000 are now issued with mirage shields. But I have never heard of waching the bullet's path. Anybody else have?

 

[Nekekami]

English isn't my primary language(Not even when it comes to shooting), but what I guess that it means is to calculate your shot such that the bullet will not end up in a tree or rock halfway out to the target just because you only concentrated on compensating for wind drift, but not checking what's in between(Which could be a bit embarrasing if it happens during an excercise. And under combat conditions, embarassment will most likely be the least of your worries.....). Check for a clear shot, including the trajectory.

 

[DarkShadows]

bullets path = vapor trail. basicly the bullet creates a vapor trail and a spotter with a spotting scope is able to pick that up as the bullet travles down range it looks like a V. if the sniper misses with his first shot the spotter seeing the vapor trail is able to recomend windage and elevation settings to correct the miss so the second shot is hopefully on target

 

[Jeffvn]

I thought it was the visible evidence of the vortex or hole created by the supersonic bullet traveling down range. Either way, it is something that can bee seen through the spotting scope.

Cool picture of this event with a .308 bullet at supersonice speed. from a good site on how bullets fly.

http://www.nennstiel-ruprecht.de/bullfly/

I can't get the image file to load, but go here and look at the preasure wave in front of the supersonic .308 bullet.

http://www.nennstiel-ruprecht.de/bullfly/fig2.htm

 

[Nekekami]

I can't say that I've seen that vapor trail very often, not even in cold air(-15 degrees Celsius and so). Maybe 1 shot out of 40 or so... Basically, vapor trails(condensation) occurs when the airflow becomes unevenly distributed and disrupted as it passes over a surface. That's why a fighter jet that is flying in a straight line might not show any condensation/vapor trails, but as soon as it starts to pull tight tight turns, it will. Another major factor is the atmospheric conditions such as humidity, air pressure etc.

 

[mele]

Okay, I guess I better step in on this one.

We call it trace, which is actually different from vapor trail which is visible to the naked eye, but looks kind of similar. Trace is "always" there, but easier to pick up in certain lighting conditions. As was mentioned above, there is a pressure that builds in front of a bullet as it travels down range, this pressure actually will cause distorion of the light. This distortion causes a visible "wake" to be seen if you are lined up just right and with GOOD optics. If you have bad optics that isn't transimitting much light, etc, you will have a hard time picking up the trace. If you are the shooter, you will not see it, not after recoil. So, here are few tips to learn how to pick up the trace, and once you see it, it becomes very easy to pick up.

As the spotter:
1 - Line up to the side of, and slightly behind the shooter. The idea is to try and get as close as you can to directly behind of and in line with the rifle. Do NOT touch the shooter, just get as close as you can. (never disturb the guy pulling the trigger.

2 - If you have a variable power spotting scope, put it on 20x, this seems to be the ideal mag for picking up trace and field of view.

3 - Adjust your site picture so the target is in the bottom portion of your field of view. This helps pick up trace as the bullet "lobs" in to the target.

Make sure the spotting scope is focused, and then pay attention. If you are engaging targets under 200 meters, you generally will not be able to pick up the trace.

Mirage is a completely different topic, and is bad for target picture, but good for estimating wind.

MEL

 

[Nekekami]

Mel:

Ah, thanks, now I understand what he meant(Since english isn't my primary language).

But, I have to ask, what would the lighting conditions have to be, and I think that being further to the north, the conditions for it to be visible are fairly different.

 

[mele]

You can see the trace in all lighting conditions (well, middle of the night doesn't count). It just so happens that it is easier to see in certain lighting conditions. Bright and clear light seems to be about the easiest, overcast makes it a bit more difficult, as does haze and other things that can block visibility the further down range you go.

MEL

 

[Nekekami]

What you're referring to, however, is refraction and diffraction, which is highly dependant upon the angle of the incoming light. The light source in question is the sun, and since the angle of incoming light(as well as the wavelength, due to atmospheric conditions) will be different in Huddinge, roughly 59 13 N 17 56 E compared to, let's say in Washington,DC, roughly 38 55 N 77 0 W, what would the effect be? If I remember refraction and diffraction correctly, the effects would be different at the two locations.

 

[david]

The position of the sun doesn't matter so long as it's a bright day. The distortion caused by the bullet is not of the sun, but of the image of reflected light off the background from the sun.

Diffraction (the difference in spread between different wavelengths of light) is irrelevant here.

The bullet compresses the air around it to a differnt density (and so different optical density) so that light is refracted differently through this air. This is why you can see the bullet path.

The easiest way is to use a tracer, but they can be bad news for barrels.

 

[Nekekami]

David:

Ah, but the refraction of the light becomes different if the sun is in a different angle. If the sunlight goes in at a 7 degree angle, the effect will be different from it going in at a 12 degree angle.

Also, diffraction is part of the equation. The atmosphere is not uniform in density, so wavelengths will differ a little between different places on earth and how far the light has travelled through the atmosphere and thus has been filtered., and thus alter optical properties.

 

[david]

Nekekami, i don't want this to become a physics debate, but i'm doing a PhD in physics right now.

I understand the concept you're picturing, but it isn't realistic for 2 reasons:

1. For sunlight to be coming in at 7 degrees, you're going to have to be shooting 7 degrees off the sun.

2. As i said before but maybe not very clearly, the distortion you see is not of the sun but of light reflected off everything else. If you fire a bullet towards the ground from an altitude, you still see a distortion. This has nothing to do with light directly from the sun. You see a distortion of light _from the ground_ that reflects sunlight, and diffuses it to such an extent that it doesn't matter where the sun is (which is why you can see outside in every direction).

When you talk about diffraction (i really think you're talking about refraction), remember that all light is then affected by the atmosphere in this way anyway.

Diffraction doesn't matter, it happens all the time but is irrelevant for this particular thread.

 

[mele]

Heh this is great stuff! My degree ended up in mathematics, but I've always loved physics!!

To put it in perspective, I've not noticed any real difference in being able to pick up bullet trace based on where I am in the world. The amount of brightness and particulates (haze, fog, dust) in the air HAS made a difference.

MEL

 

[Nekekami]

David:

This already _is_ a physics debate. :wink:
And studying for PhD doesn't say much. Which field are you going for?

The angles I mentioned were just examples. However, you forget that reality is not just isolated calculations. Different wavelenghts scatter differently, and since you have both direct light and diffuse light(scattered from the surface), the effect might not become visible(Understanding this might be easier if you start doing some photography in addition to physics. Doing photography, I've noticed that reality doesn't always jive with physics).

My spotter and I tried it this weekend, and he saw it about once per twenty-five shots, and he spotted it more easily at around 13-14 than at noon or around 16.

 

[david]

good job for going out and testing! i wish ranges over here were as accessible.

Erm, i'd think most people would agree that someone doing a PhD in physics (high-yield nuclear research), meaning i've spent the last 4 years of my life in physics with refraction and diffraction being way way below my level, does say something.

to say that reality is not just isolated calculations, and that reality and physics do not always jive only means that your physics isn't up to scratch (no offence at all).

i don't understand why you insist on diffraction mattering so much, i very well know that different wavelengths are scattered differently but again that is not the cause, you're over complicating it. It's a refractive phenomenon.

Diffraction is either (depending on the definition):

1. the bending of light around an apeture of same order of magnitude of size to its wavelength ... this obviously doesn't happen
or
2. the spreading out of wavelengths due to a refractor having different refractive indices for different wavelengths. *This effect is negligible* because the change in optical density of the air around the bullet is very low, so the *differences* in spread between wavelengths are not observable.

i drew out a diagram to show what happens, but don't know how to get it onto the message. it's really just light bending differently around the bullet because the air around it is highly compressed. The reason its hard to see is because the bullet is small and moves very quickly.

you seem to know much more about shooting than i do, but i'm the authority on physics :wink:

 

[mele]

Only saw it once per 25? Oh wait, I forget you were probably using your Sabot standard issue rounds. A smaller projectile doing 4000+ FPS would be much more difficult then our 175gr doing 2580fps purely because of mass and velocity! We watch trace on ~90% of our shots, if the conditions are right, we see it EVERY time.

MEL

 

[Nekekami]

David:

The problem is, we're not able to replicate what you say should be happening, as often as you say it should be happening.

We tried again this morning, this time shooting across a 400m lake towards our targets. Weather conditions are bright, sunny, no clouds, no mist, no haze, 12 degrees celsius, 1024.1 hPa air pressure, 68% humidity. Same thing this time. If it's just an ordinary refraction effect, those should have been fairly good conditions.

Mel:

Actually, no. When I'm shooting that many rounds in a row, I use ordinary FMJ ammo(We've got a large store of ammo nearing it's best before-date, so I can shoot close to how much I want.... I just need to save a couple of thousand rounds for the two MG crews... =) )

 

[mele]

Nek,

You should be seeing it much more then that, especially with that high of humidity (generally helps with bullet trace). Like I've said before, I've been all over, and shot in all kinds of conditions, and generally we will trace. Some times its harder, but still seem to catch much more then you are now

MEL

 

[Nekekami]

Mel:

That's why we're puzzled too. We've been out this morning to make sure that it's not our optics acting up, and the optics are not at fault. We're starting to think that maybe the background is so mottled and uneven in pattern that the disturbance is hard to notice because of that.