Mirage - how much point of impact shift?

[Len Backus]

I was shooting my newly remodeled rifles yesterday at 500 and 700 yard targets. The sun was bright and from behind the targets in the morning period. Later, the sun was just over the targets, sometimes behind a cloud by then. Wind was shifting direction and intensity.

How much does mirage shift the point of impact in these changing conditions?

 

[Dave King]

Len

Here's a link to an interesting article about mirage that I hadn't read until today.
http://www.m-s-co.com/sso/articles/article7.html

It makes short work of the question of quantifying the effect of mirage on projectile impact. (I rather enjoyed the method he uses to quantify the change due to mirage.)

Good to have you back on the site.

 

[Len Backus]

You must use the same search engine as I do. I saw that this morning, too.

So --- David, how much is the greatest POI shift you think occurs at 500 and 700 yards?

 

[Darryl Cassel]

Hello Len

As the temperature rises, so does the bullet.
For example, if you shoot in an early relay lets say 9:00 AM at Williamsport and win your relay, leave the clicks on the scope, and at 2:30PM during the shoot-off and after the sun has a chance to raise the temp 20 or 30 degrees, your bullet will print up to 12" high. This is with a straight up mirage picture. Have witnessed this many times.

The mirage is VERY hard to read at Williamsport and other 1000 yd ranges also. It is a true indicator however, of what is happening between point A (Shooting bench) and point B (Target)at longrange. You have to shoot several shots in the sighter round to know how much the mirage is effecting the bullet downrange. When longrange hunting, this is where your hunting partner with bigeyes is very important.

I don't believe anyone or any program can tell you what mirage is worth at all ranges or in every shooting senerio.
There are just to many variables in longrange shooting.

Probably the more one reads into this shooting game, the more confusing it gets.
A hands on approch as to drop charts (actual fire) and mirage is more benificial to the shooter. We all know that NO ballistics program is 100% correct every day. Even your actual fire will vary from day to day. The same thing seems to work with mirage. It is hard to determine day after day.

Darryl Cassel

[ 07-01-2001: Message edited by: Darryl Cassel ]

 

[Warren Jensen]

Darryl,

It would be interesting to try to quantify the mirage component of the observation, "Sun is up, shots are up". You could do this by keeping detailed records of the air temperature, the ammo temperature, the air pressure, humidity, and shot placement. You could later calculate each component of a rising point of impact and the remainder would be caused by mirage. The components can be calculated very precisely if they are measured precisely. You would have to also place some sort of value on the mirage at the time of each shot.

I do not think that this would be valid for anything but the conditions on a certain range. But as we all know, different ranges have conditions that will pattern and thus perhaps the mirage could be patterned, also, for that range. I suspect you guys do this intuitively already, but being an engineer I have a compulsion to try to quantify everything.

 

[Len Backus]

I am trying an experiment. A couple days ago I set up a rifle with its attached 22 power scope aiming at a 700 yard target. I recorded the sight picture at the start of the experiment --- 8PM. I left the rifle out on its setup overnight and through the next day at 4:20 PM.

As you know, the reticle shifted apparent location on the target over time as the sun and wind directions as well as temperature changed.

The target is to the east. At 8PM the sun was from the west behind clouds and a slight wind blew from the west. Temp was 80 degrees.

At 5:30 AM the sun was from about the norhteast and wind was from the north. Reticle was 2 inches right and 2.5 inches up. Temp had dropped to 60 degrees.

At 8:00 Am wind was 5 - 10 from the north. Temp was 65 and reticle was up from original by 5 inches and moved to the right 2 inches. Sun was from the east and now higher in the sky.

At 2 PM wind was 5 - 12 mph from the northeast. Temp was 82, sun was out from the the southwest. Reticle was close to original spot, hard to see in the mirage.

At 3:30 PM wind was closer to north at 5 - 10. Sun was still bright and reticle was at original spot.

At 4:20 the bright sun was from the west, temp was 85. Wind from the north at 5 - 10. Reticle was back up 5 inches from original and but shifting right and left of original by 3 or 4 inches.

I will try this again a little more carefully --- but for now what does it mean?

[ 07-03-2001: Message edited by: Len Backus ]

 

[Warren Jensen]

Len,

To begin with, you have to be sure that your rifle holding setup is not the culprit, due to expansion or contraction of the various holding parts. My first inclination is to look there.

Find a fixed, fine point that is not too distant, i.e. less than 50 yds. and see if the POA is moving during the day.

 

[Len Backus]

I'll try that next time. It will be easier to see the target during the distorted times of day.

My rest setup was classic benchrest in nature. Concrete bench top on wooden 6 x 6's into the ground. Heavy competition style front rest with sandbags front and rear.

 

[Darryl Cassel]

Hello Warren

As you know, all the math and records you keep can be thrown out the window during a 1000 yard match with the ever changing wind directions, the sun going in and out of cloud cover and with a time limit on your 10 shots fired at the target.

Under controlled conditions where time is not a factor, a complete rundown and printout of conditions may work to a degree but, not 100%. You still have too many variables.

We at the Williamsport 1000 yd range welcome ANYONE to come and try to dope the wind and mirage match after match. We have not found ANYONE to do it since the club opened 34 yrs ago.

As far as complete records go, you would have to have a very thick book (or carry your computer) for each rifle and bullet that is fired in a match or hunting field.
We don't have that much time in match or hunting conditions.

The record holders at 1000 yd matches fire the rounds as fast as they can get them down range before a condition change takes effect. That works sometime but, not always. They do get caught.

Even in the hunting senario, the partner with the bigeyes is the important person on the team. There is not enough time to try to go over figures that a person hopes will place the bullet EXACTLY where it is supposed to go. The weather conditions, changing wind direction and the animal walking and changing distance on you is a real problem.

The best way we have found to overcome some of the problems is to have a good drop chart that is very close, fire over the animal (or 100 yds ahead) to get the windage correct, count on your partner for accurate information and then click down to the animal and kill it.

I understand your position on the controlled shooting and records but, as you know, all the math in the world is not EXACT in this shooting game. The records and math will tell you what the bullet is SUPPOSED to do and not what it really does once it leaves the barrel. If it were that easy, there would be new records broken every match and there would be no need for a sighter round over an animal. One shot kills from 1000 yds to 2000 yds would be made everytime you lined up on an animal. This just does not happen and I would like to see ANYONE do it shot after shot every time out.

There are just too many CHANGING variables from point A to point B

At Williamsport we watch the wind flags and mirage and TRY to determine what the bullet is doing in the 6 min sight in time. If the condition changes a complete 180 degrees right after your 6 mins are up, your up the well know creek without a paddle.
You must be able to see where your bullet is going and where the impact is to make your final scope adjustments.

Just another thought on the placement of bullets at extreme longrange.


Darryl Cassel

 

[Len Backus]

Darryl and Warren

A realistic hunting range for my current abilities is from 500 to 700 yards on a deer-sized animal. At these ranges and under somewhat typical changing conditions, what would you venture to be the amount that POI will shift due to mirage? Perhaps give some quantified theoretical examples?

An answer would help me put the issue into some perspective.

 

[Dave King]

Len

I don't have a measure of the amount of apparent target movement due to mirage but I'd think one could build a grid and measure it.

Perhaps a grid of alternating black and white squares (checker board) with a center aim point. Pick a day with appreciable mirage and setup at 100 yards. Record the outer bounds of the distortion. It'd be nice if the wind was from only one direction rather than switching but a series of wind flags would be a good indicator.

I saw some pictures that Ian M. took through his riflescopes and tried it with my video camera. It works well enough and I'd imagine that if you built a contraption to mount a video behind a scope you could record onto video the entire mirage determination affair against the grid.

I'd do this in one sitting and not a time-lapse series.

[ 07-04-2001: Message edited by: Dave King ]

 

[Warren Jensen]

Len,

I think Dave is on to something. It would be my opinion that the mirage affects the image in two ways and any experiment should address each way.

First, as you have mentioned, there is a POA to POI offset. This is where the image is being offset up,down, left, or right.

Second, there is the image distortion itself, where the clarity of the image outline is being reduced and is much harder to distinguish from the background. This makes exact POA placement less precise.

Any experiment should try to quantify the image offset and the image distortion, separately, in my opinion.

Darryl,

Believe me I understand the complexities of longrange shooting. I also believe that these can be measured and corrections applied. For example, we just spent a time at Yuma Proving Grounds getting exact trajectory data, meter by meter, on quite a number of cartridges and bullets. This is exact flight data not mathematical models. We can apply correcting values for the flight variables, such as wind and it's various vectors, temperature, pressure, etc. The result is precise trajectory prediction. The difficulty is in the measurement and calculation. Many times the conditions will change, even if you have the measurements correct, before you can calculate the correction. What you need is immediate correction values. Also, you need real time imputs to make those corrections. We are working on this and will have our first generation of our Advanced Ballistic Computer available in about 60 days. Our goal is to work towards a product that does real time measurement and correction display.

I am intimately familiar with the long range spotter round technique, having used it extensively 30 years ago. The equipment you use today is much more sophisticated than we had then. What my particular challenge is now is to put the first, and perhaps the only, round on target at long range. We are closer to solving this than you might think. There are many times when spotting is not useful or is counterproductive, night being a prime example.

I am not trying to be argumentative or combative but we are working on similar projects coming from two entirely different perspectives. As an example, range estimation in my arena has to be totally passive. No laser rangefinders here. Anybody with a radar/laser detector can find you. We have a passive range determination system in the works, and it is as accurate as lasers. Also, when shooting past 2000 yds. you have to have continuous wind vectors from muzzle to target, and again, we have a system in development. This is not Star Wars stuff, but available technology. If you solve the range, the winds, the optics, the temperature changes with ammo and air, the pressure, the humidity, spin drift and coriolis effect, with a cartridge that has a stable and accurate projectile you can put that first shot on target. You reduce and then eliminate the errors a piece at a time and eventually you will get there.

Again, this is in no way an attempt to minimize your skill and expertise, which I have the greatest respect for. I am learning from you and your fellow competitors and hunters constantly.

 

[Darryl Cassel]

Excellent information Warren and I thank you for the work you are doing. That ballistics program sounds "top of the line" not to mention all the testing.

Bob McCoy, whom I'm sure you knew, was a friend of mine and did similar testing for the Government.

After coming to Williamsport (many years ago) and checking out the results of all the 338 cartridges that were being fired, writing down everyones loads for such, he decided to have two longrange rifles built for the military. Wouldn't you know, they were chambered for my cartridge, the 338/416 IMP?

Thanks again for the fine information.
Please keep us informed as to results you find. It will help us in the hunting fields and at the range.

Darryl Cassel

 

[Warren Jensen]

Darryl,

I did know Bob. A fine gentleman. We are trying to expand upon his work.