Question about setting zero in sunlight and shadow

[Curmudgeon]

I was at a midrange NBRSA match Sunday and before the start we were talking about wind when it was mentioned that ones zero would change from sunlight on range to shadow from clouds overhead and darkening the range. A storm was upon us and we were going to complete the match not waiting for the best conditions. Clouds were passing overhead and changing the light on a regular basis. One of the shooters mentioned they noticed as much as 10 inches at a match in Sacramento at 1000 yards. Does anyone have some input on this phenomenon? Much appreciated.

 

[JohnKielly]

Hereabouts, the iron sight shooters have the mantra, "light up, sight up; light down sight down." It's usually attributed to the effect of mirage being diminished once the sun is lost.

One MOA seems about right.

 

[Park ranger]

People see the target differently in different light conditions. To me, the target looks much bigger when it's cloudy, so my 6 O'CLOCK hold I would have to add clicks. When it was bright, the bull looked smaller, so I was into the black more, and would have to turn my sights down.

Yes, now I put a scope on my service rifle, it doesn't matter. I imagine F class is more about mirage distorting where the center of the target actually is. Just a guess..

 

[snert]

I do not shoot competition. What I do know is that at 10Am my zero is zero. At 2PM when the sun is high and clear, i come down as much as four clicks on a 500 yard groundhog, or I shoot over it. Go get some food, come back at 5Pm and it is back to zero/zero.

 

[watercam]

Since you are a BR shooter, and thus shooting with a scope, the iron sight phenoms above may not be applicable. Storms passing through bring three potential and variable conditions to the table - wind, temperature change and variable atmospheric pressure. All of which can 'adjust' one's point of impact.

 

[CaptainMal]

Cooler air is more dense. Offers more resistance than the less-dense warmer air.

Shooting under varying cloudy conditions can be like dealing with a yo yo target.

 

[Scott Harris]

I have not noticed that effect with a scope, even at 1,000, but maybe i'm not paying close enough attention. It does make sense that mirage appearing and then disappearing could cause the target image to shift....something to test next time!

 

[Curmudgeon]

This is an image that bends my mind. A yo-yo in the wind on a scudding cloudy day. Shoot and enjoy and infrequently excellent conditions will present themselves. Thanks all and have a pleasant Thanksgiving.

 

[Fast14riot]

I noticed it on range 6 in Sacramento this year after the controlled burn, the black infield in direct sun would create massive mirage, passing clouds it calmed down considerably.

With aperture sights, it's about how much light is being let in and allowing sharp focus on your front sight. I can go from a .045 to a .038 aperture in the same match depending on conditions throughout the day.

 

[XTR]

I have not noticed that effect with a scope, even at 1,000, but maybe i'm not paying close enough attention. It does make sense that mirage appearing and then disappearing could cause the target image to shift....something to test next time!

I'm with Scott on this one. I've never noticed it with a scope. To me the few times I've played with iron sights on a service rifle using a 6 o'clock hold it makes sense that the bottom of the black is sharper when the sun is out so my hold is higher by a click.

As for the change from morning to afternoon. I've seen 20degres temperature shift from the first relay in the AM to the last in the afternoon. All other things being equal plug 72F and 92F into JBM and see how much that changes your zero at 1000 yards.

 

[500Stroker]

Cooler air is more dense. Offers more resistance than the less-dense warmer air.

Shooting under varying cloudy conditions can be like dealing with a yo yo target.

I have experienced this first hand.

 

[dkhunt14]

When the target lights up the shots go high. Normally about 3 to 4 inches with a big bright change. I have seen it many times with a heavy gun that absolutely tracks dead nuts. Matt

 

[Immike]

Cooler air is more dense. Offers more resistance than the less-dense warmer air.

Shooting under varying cloudy conditions can be like dealing with a yo yo target.

The density and temp changes are measurable and move slowly but mirage is not a constant. This is the shooting in shooting competitive when it becomes a factor. It gets bigger when there is high ground in the field and the lower you are to the ground. It can't be measured accurately guys, you must be Flexable to it and aware of it.

 

[SF-Guy]

I was at a midrange NBRSA match Sunday and before the start we were talking about wind when it was mentioned that ones zero would change from sunlight on range to shadow from clouds overhead and darkening the range. A storm was upon us and we were going to complete the match not waiting for the best conditions. Clouds were passing overhead and changing the light on a regular basis. One of the shooters mentioned they noticed as much as 10 inches at a match in Sacramento at 1000 yards. Does anyone have some input on this phenomenon? Much appreciated.

There is definitely a shift in the perception of where we think the bullseye is when affected by light or mirage. You are right on the money "Lights up; Sights up (so come down) and Lights down; Sights down so come up"
Like mirage, light will affect where we see the target and the distortion is more in humid conditions and much less in arid conditions. I was happy in the sand box where mirage was less and humidity was just an unfounded rumor.
Scopes are also affected but the magnification plays tricks on us and "we are closer to the target" with the magnification. If you have not seen light and mirage through your scope it is not something to fear, but respect. I recommend laying down behind a spotting scope and spending time watching conditions - then shoot in them all you can. I like a data book and keep track of conditions as each one is effectively Zero info.

 

[SF-Guy]

I'm with Scott on this one. I've never noticed it with a scope. To me the few times I've played with iron sights on a service rifle using a 6 o'clock hold it makes sense that the bottom of the black is sharper when the sun is out so my hold is higher by a click.

As for the change from morning to afternoon. I've seen 20degres temperature shift from the first relay in the AM to the last in the afternoon. All other things being equal plug 72F and 92F into JBM and see how much that changes your zero at 1000 yards.

Cool beans. yep, cold air has more density. An old guy named Torricelli walked up a mountain to measure the barometric pressure and watched it fall as he went up. A barometer setting is how pilots know where the 'bottom of the deck is" and set the barometer for the place they intend to land. they get that via radio and reset their altimeter to that barometric pressure and if you watch as the calibrated altimeter comes down to 0 ft, the wheels hit the deck. Temperature and density vary inversely. raise the temp and lower the density. he molecules in a cubic foot of air are the same in number and expand with rising temperature and contract with lower temps. why is this important to know? Shoot across a roadway in the summer and watch your impacts rise. The same over water. This is why i frown on police snipers training in football stadiums (empty with no people in the stands) and think they have a solid solution and zero at a given distance and angle...not so fast. no people = lower temp and no thermals. but, hey, shooting anywhere is fun right?