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mirage question

 
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  #1  
Old 02-23-2011, 03:27 AM
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mirage question

How bad can mirage efect your 100 yard groups? Im looking for something to explain why my decent shooting rifle shot extremely poorly yesterday. Any experience with this would be helpfull as I have none.
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Old 02-23-2011, 04:53 AM
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Re: mirage question

On a day when it's really boiling (relatively calm and the mirage is thick), it can play evil tricks on us.

One time I had the gun pretty much locked in dead steady. It was one of those days when the mirage makes you wonder; but anyway, I saw the target move nearly an inch when the wind picked up. Of course, the target didn't really move but it may as well have, because it looked like it did. If not for having the gun on a dead steady rest, I would've never seen it.

Weird stuff!
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Old 02-23-2011, 05:42 AM
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Re: mirage question

Thats what it was doing to me. I couldnt get a better group than 1 1/2". Is that level of bad normal? Used to shoot between 3/4 and 1", then I modified the trigger to drop the pull and get rid of the horizontal string I was getting with the 6lb pull.
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Old 02-23-2011, 06:39 AM
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Re: mirage question

Mirage could do that to you, but other issues could also. Does your scope have parallax adjustment? What kind of rest are you using? Barrel clean? Change any ammo components?
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Old 02-23-2011, 08:09 AM
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Re: mirage question

Scope was adjusted correctly, clean bore, same box of amo. Black hills .223 match. The rifle has fired exactly 50 rounds, one box. Nothing changed from last time out other than trigger pull and mirage. Oh, rest was sand bags front and back.
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Old 02-23-2011, 10:20 AM
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Re: mirage question

During bench rest comp prep my testing revealed as much as 1.5" POI increase from sunrise until 10:00 on a typical Idaho day.

Mirage is you enemy until you learn to dance with it. It then quickly becomes your friend.
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Old 02-23-2011, 03:40 PM
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Re: mirage question

...not an expert here, but here's my take which I'd love to get feedback from the experts on... in terms of "learning to dance with it" as royinidaho puts it...

Unless I'm mistaken, mirage doesn't move bullets, wind moves bullets. And, mirage is a reflection (pun intended) of what the wind is doing.

Boiling and/or changing is a big problem

However, if you observe mirage closely along with reading the wind, and if it remains constant/consistent in one direction, then your POI may have shifted. But, it shouldn't have a huge impact on your group size relative to the distance you're shooting.

So, depending on what the mirage/wind was doing, that could be a possibility.

But like others stated, there are many other variables that affect group size.

I would think that an estimate of the min-max wind on that day run through a ballistics program with a correct BC for your bullet and MV should give you an order of magnitude as to how much the wind might've moved your bullet around.

Unless there were up/down drafts, which is less likely at 100yds than 1k, then it's not likely going to account for huge vertical dispersion. ...except possibly as pertains the the right hand twist and some slight upward/left movement with a 3 o'clock wind and a slight downward/right shift with a 9 o'clock wind.

I digress, but... My 15yo son was shooting at 500 yds about 2 weeks ago and one of the other shooters on the line pointed out that focusing your scope at 500yds, you could see the mirage moving from left to right and focusing at 300 yds, it was running the exact opposite direction. It would do that for 10-15 minutes and then swap directions.

As a result, his group size opened up about the right amount by my SWAG (142gr SMK, .595 BC, 2825mv, 500yds, 5mph, ~7") and most of the dispersion was horizontal. I had him aim dead on every time and not try to compensate so that we could see the effects. But, if he can learn to "read" it well in real time, then he should be a much better shooter for it.

thanks!
richard
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