Mirage - what happened

[rebel]

Guys, I am really trying to wrap my head around something that happened to me about a month ago. I am hoping the more experienced will weigh in on this.

I went to practice for the Hickory groundhog shoot one day. Temps around 44 degrees, light (5 to 7 mph) to no wind blowing, straight at me. Long story short, realized my dope at 300 was a minute high, adjusted, every thing fine. Used my dope card at 500, hit on target. 300 yards in the shade, 500 had boiling mirage. Fast forward to the shoot, conditions were perfect - overcast, 52 degrees, no wind. My shots at 500 were a minute high. Discovered my mistake later at home - my scope height was set wrong in the ballistics calculator. I had adjusted for that at 300, but figuring the 500 dope worked the day I practiced.
Everything I have read on mirage says the type I was shooting (during practice) in would make the bullet impact high. However because of my dumb mistake I had the scope set 1 MOA high at 500 already. So my shots went a minute low on practice day - I just didn't realize it

I hope I have given you all enough info to explain why, on practice day, my shots weren't clear over the target.
Thanks in advance.

 

[dellet]

Did the gopher on practice day look like this?

Not sure I understood the question. Probably can't help, but would like to understand.

Zeroed at 300 on practice day, calculated up for 500 was on. With mirage.

Game day, same zero worked at 300, at 500 same adjustment shot 5" high in no mirage.

You think it should have been higher in practice, or low instead of high in competition?

 

[ClassCat]

On both days do you know what the Barometric pressure and dew point were ?

 

[4xforfun]

You would need to be off by a brazillion % with your scope height to get a 1 moa error at 500 yards with a 100 yard zero!!

 

[Tim s]

Little or no wind from you to 500 yards ???
That would be called an at atmospheric impossibility.
Without even the very likely mirage considerations, you you realize how little wind would be required for an inch or so of elevation change at 300yds-500yds.

 

[Depart123]

Strong sun is worth 1 moa and a medium boil is worth 1 moa. Sometimes they cancel, sometimes they combine if going in the same direction. There isn't enough specific info in your post.

The 5-7mph wind at 500 would have a significant effect on the sun canceling mirage. The target would have had a 1moa apparent movement depending on the wind.

 

[ClassCat]

You would need to be off by a brazillion % with your scope height to get a 1 moa error at 500 yards with a 100 yard zero!!

Leave Her out of this.She's Brazilianan.

 

[DR_2297]

One thing I learned a long time ago... don’t get married to your zeroes.

 

[rebel]

Dellet, I was afraid that sounded confusing, let me reword that. My original dope was screwed up because of an input error to my ballistic calculator. Scope height was set at 1.5 and should have been 2.5.
Just concentrating on the 500 yards, my impact should have been high during practice. It wasn't. Looking at the corrected dope after the match that night, it should have been 8.4 MOA of up instead of 9.5 MOA which I had dialed.
What confused me and my original question was, Why did it hit dead on in practice when the mirage was boiling and the scope was set 1 MOA high?
Meangreen, yes, 500 yard target was bathed in sun. Target was boiling.
Classcat -I don't have the barometric pressure for either day, but the Jarret rifle guys and I were discussing how dense the air was the day of the match. 97% Humidity, much less the day I practiced.
And thanks for the bikini model!
For the rest, I am fairly inexperienced at match shooting. I do a lot of Varmint hunting and Target, just a match every now and again. Sorry I can't word this like it happened. My juju is off today.

 

[rebel]

You would need to be off by a brazillion % with your scope height to get a 1 moa error at 500 yards with a 100 yard zero!!

Run the information on your ballistic calculator. I was surprised to.

 

[rebel]

Little or no wind from you to 500 yards ???
That would be called an at atmospheric impossibility.
Without even the very likely mirage considerations, you you realize how little wind would be required for an inch or so of elevation change at 300yds-500yds.

I meant what I felt at the firing line. Through the scope, 500 was calm. Sorry for the confusion.

 

[ClassCat]

I meant what I felt at the firing line. Through the scope, 500 was calm. Sorry for the confusion.

Search explanation of Barometric Pressure and Dew Point. Your POI will change.

 

[4xforfun]

Run the information on your ballistic calculator. I was surprised to.

I did...140 vld @ 2900FPS 100 yard zero 1000 ft elevation at 59 degrees, I used 1.5 and 2.5 inche scope height and ended up with a .7MOA change. My 8 year old could tell the difference between 1 1/2 and 2 1/2 inch scope height difference.

If you can't guess within 1/2 inch, you REALLY shouldn't be worrying about a 500 yard Zero!!

People have been blaming scope height errors for years trying to explain why they miss at XYZ yards. It isn't that big a deal ...even if you are off by anything less than 50% error.

Of course, there is always "stacking", but then you have to spread the blame around somewhere else.


Tod

 

[CharlieNC]

In my experience it typically takes more than shooting at one distance on one day to assure the ballistic calculator is "trued". Shoot multiple distances across a range of atmospherics (BP and temp); then adjust your velocity such that the calculated results match the range data. It's not unusual for limited dope as you gathered to fail due to numerous causes, and an organized dope book is valuable to sort this out in short order.

 

[rebel]

Search explanation of Barometric Pressure and Dew Point. Your POI will change.

http://riflebarrels.com/ballistic-effects-of-altitude-temperature-and-humidity/
Among the other great articles I read , this one resonated with me.
Appreciate the help.