Shooting low at 500 yards

[PHammer]

My first question here on the forum. After consulting two of my friends much more knowledgeable than me, they have advised I put my question here. I am relatively new to precision shooting and reloading. I have been preparing for about the past year to compete at the Hickory Ground Hog match. Practice, practice, and more practice with a lot of learning too. I prepared a good working 6mmBR round with Varget, good OAL, CCI450 primer, and Berger 105VLD target bullet. Practice on Thursday before the shoot was great at 100, 300 & 500 yards. I could not have asked for any better. The practice conditions in the late afternoon was sunny and clear about 60 degrees very little humidity and about a 5-10mph cross wind.
At the Ground Hog Shoot early on Saturday morning it was 50 degrees to start, with a steady cross wind about 10mph. The American and Ground Hog flags were standing out! 100 and 300 yards grouped and performed very well. Then the 500 yard group was shot about 11:15 at about 65 degrees, a little more humidity and a little mirage. The wind had dropped down to about 5mph or less and it was from behind heading straight toward the target. This shot is on a slightly downward angle too. These three shots did not group as in practice and they were 3 inches low from the aim point. The shooting is done prone and quick, I would say competition jitters but my daughter shot too and her shots were as mine, 3 inches low. She did outshoot me in the match however, but please don’t tell anyone about this!
My question to you is “Do you have any ideas why these shots did not do as in practice and fell on the target low?”
Thanks for your reply, I am really trying to figure out this fascinating and enjoyable sport.

 

[bheadboy]

if I understand your description of the wind/mirage, I think the problem is he following,

When the wind is straight from behind, it will wipe the mirage out, thus any elevation adjustment having been for cross wind rising mirage effect will be wrong as the target will have been lower than you see it.

go to the straight wind and the elevation effect of mirage is wiped out. and you shoot low.

Bob

 

[CatShooter]

I think your problem rests with bheadboy's comments and that fact that it appears you didn't adjust for " This shot is on a slightly downward angle too."
You didn't define the slope but slope angle is a critical factor in accuracy.

If the slope was downhill enough to effect the shot, the shot would have gone high, not low.

 

[bheadboy]

Catshooter is correct in that a down hill shot, will go higher, the sharper the slope the greater effect, however most ranges have only a slight slope to them so the effect would most likely been a modest change in the effect of the tail wind wiping the mirage.

Bob

 

[6BRinNZ]

Do you adjust your scope for a 500 yard zero? If so what ballistic data did you use to get the come ups?

 

[JamesnTN]

Ok guys what he has is the GH shoot allows for practice the day before so he had all his scope adjustments for each distance already down. What happened was his 500 yRd adjustments were 3" lower from day before. Welcome to that nasty world of conditions and being able to read them. That's what happened different day different conditions.
The guys who win usually team up in 2-3 shooting the same rifle and allowing a relay in between so they can see the targets before the next one had to shoot, by the time it gets to the last guy they have that rifle dialed in to win in most cases.

 

[Outrider27]

I never developed a reliable ability to shoot mirage conditions well, but I learned a lot about the rudiments of the effects of mirage by aiming where the target looked to be through the scope and seeing where the groups actually printed with a proven rifle and loads and reliable scope settings from days without mirage. What OP described fits exactly what I've observed when shooting with a wind coming from behind me on a day with conditions that would encourage mirage. I'm with bheadboy's analysis all the way; it sounds like classic mirage tricks.

 

[CaptainMal]

Your daughter outshot you?

Shhhh. That means you are a good teacher.

As for the low, my take is like everyone else, mirage conditions changed, air pressure changed, there were unknown wind changes downrange, your parallex changed in the scope, eye position, bla bla bla etc.
It's not as easy as it should be.

 

[rain164845]

Have you run a box test on your scope?

 

[Ben]

The hard to explain conditions are one of the things that make that match so much fun. Just when you think you are starting to figure it out something unexpected will happen.

 

[LCazador]

Just wondering did you make notes of the previous day's conditions. That is scope adjustments, wind speed, and wind direction on prevailing conditions. And did you flag ( use wind flags) all your practice shots?

 

[MarkS]

PHammer,

That is the Hickory Ground Hog match. The range looks like a peice of cake, but it's not. This range has no angle, maybe 10' lower than the firing line. If you are gettng wind coming in or going out, you will see vertical in your groups, or impact. The range has a high hill on the right side, when the wind blows, it does strange things down range. If you shot good in practice, something changed. I have done the same thing myself there. I shoot great groups but not in the center.

Mark Schronce

 

[TroyMN]

At the Ground Hog The wind had dropped down to about 5mph or less and it was from behind heading straight toward the target.

I remember from a wind clinic that in a head wind you hit high, tail wind you hit low. Doesn't that explain it?

 

[Toby Bradshaw]

I remember from a wind clinic that in a head wind you hit high, tail wind you hit low. Doesn't that explain it?

A pure headwind will cause the bullet's impact to be low, and a tailwind will have the opposite effect. Think about the time that the bullet spends in air masses moving in different directions and it will become obvious why a headwind has this effect.

Surface features of a range (berms, trees, etc.) can make a lot more difference than the nominal wind direction/speed, though.

 

[MarkS]

Surface features of a range (berms, trees, etc.) can make a lot more difference than the nominal wind direction/speed, though.
[/quote]

Toby you could not be more right. The Hickory Ground Hog range is only open one week a year, it is crop field most of the year. You could not set up a range with more stuff, to deal with, if you tried. It is a great match, everyone needs to try it.

My match is coming up in 2 weeks come give it a try. http://forum.accurateshooter.com/index.php?topic=3838144.msg36362222#msg36362222 We have our on set range stuff, it is on Windy Gap Mt.

Mark

 

[TroyMN]

Think about the time that the bullet spends in air masses moving in different directions and it will become obvious why a headwind has this effect.

That is exactly what I thought. The reason I remember it the way I described is it did not make sense to me, not logical. The explanation of why; a bullets shape creates lift and the head wind causes more lift. I have no statistical significant first hand verification of this. The wind clinic was at MN Elk River club (2013). Taught by members of the USA F-class team, I took their word as fact. I could have remembered it incorrect but...

 

[PHammer]

Thanks to all of you who replied to my question. The information you provided is really comprehensive and excellent, so again thank you. These answers also tell me just how much learning I have in front of me !