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Mirage - how much point of impact shift?

 
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Old 06-30-2001, 09:19 AM
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Mirage - how much point of impact shift?

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?
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Old 06-30-2001, 06:27 PM
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Re: Mirage - how much point of impact shift?

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.
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Old 06-30-2001, 09:14 PM
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Re: Mirage - how much point of impact shift?

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?
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Old 07-01-2001, 08:07 AM
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Re: Mirage - how much point of impact shift?

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 ]
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Old 07-03-2001, 07:54 AM
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Re: Mirage - how much point of impact shift?

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.
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Old 07-03-2001, 03:13 PM
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Re: Mirage - how much point of impact shift?

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 ]
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Old 07-03-2001, 03:31 PM
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Re: Mirage - how much point of impact shift?

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.
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