High and Low Impact - Early morning & Late afternoon

[Windpomp]

I have been struggling with High and Low misses in the early morning (during sunrise) and late afternoon (during sunset) and now try to take the shot only after 09h00 when the sun settles a bit higher in the sky.

For the past 4 months I have been doing a bit of range time to pinpoint or predict the high and low hits and this is where I got…...



80% of the time I would get 0.5MOA low groups before 08h00 in the morning.

On several occasions the groups will climb to about 1.0 MOA (0.5MOA above midday zero) for about 30min during sunrise and then drop down to the 0.5MOA below zero where it will stay until after 08h00.

The groups then settle at the theoretical zero and stays there until 17h00.

After 17h00 the groups tends to hit high 0.5 MOA.

On several occasions the exact same high groups (1.0 MOA) would occur in the late afternoon during sunset.

I have worked through a thesis in surveying, explaining the problems in refraction that causes the phenomenon whereby a false image is seen either low or high.
There was however no published solution or prediction on how to determine which one (low or high) one would expect?

How do you deal with this, or do you simply wait until a bit later in the morning?

 

[Canhunter35]

Are you adjusting your density altitude from the cool morning temp to the warm temp at noon then when it gets cool again? The shift in temperature is most likely the culprit

 

[Windpomp]

I am comparing shots fired within an hour on the same day. No visible change in atmospheric conditions.

In the early morning as the sun rises (not always though) the group impacts about 1 MOA high and then settles back down again after about 35min.
The same occurrence in the afternoon.
Quite often the group stays at the same elevation during sunrise and sunset.

My question is if there is a way of predicting when the impact will be high, low of at the same elevation during sunrise and/or sunset.

 

[Canhunter35]

R u parallax free?
Move your head so there's a shadow when looking through your scope. Using the shadow, center the image, thus removing parallax.
How far are you shooting during this occurrence?

 

[Shane Lindsey]

Is the sun facing the target. If it is, it can create the appearance of elongating the target when low on the horizon and shrinking when it is high in the sky. Almost shadow effect.

Front site posts will do the same with the sun. "Sun up sites up".

Not sure I have experienced it with a scoped rifle though. After reading your post again. I don't think this is it.

 

[Frank in the Laurels]

Half a minute of change...question is at what distance and is this only a hunting or competition rifle...the usual culprits, altitude, powder temperature sensitivity, oil in the barrel, Ammo temperature before loading--inside your jacket or laying on bench--can be huge...length of time loaded round sits in a warm vs cold chamber, scope/your viewing of the scope, your head orientation qr cold on your cheek..there's a Ton to pick from.

 

[Windpomp]

Yes, I am paying special attention to parallax during the exercise.

Shooting distance is 500m ( 547 Yards). Groups size varies between 1"-2" groups.

Is there a way that I can upload a video to illustrate a particular miss due to the conditions described?

 

[Greyfox]

Not sure if it is the issue, but you might check the scope. My speculation is that all lense designs/configurations are not created equal. A few years back I swapped out a Huskemaw 5x20x50 that was mounted on my 6.5x284. This set-up performed with very high consistency out to 500+yards under all lighting conditions. When I mounted my Nightforce NXS 5.5x22x50 I began to experience a similar issue of POI changes in low light or high glare conditions. I was never certain if it was me, the scope, or some other effect that caused the problem, but when I replaced the Huskemaw on the rifle the problem disappeared and my original performance restored ever since. I suspected a combination of my aging eyes combined with a parallax effect in lower ligh conditions. The approach and adjustment turning ratios between the two scopes is quite different; the Huskemaw being a fast ratio design that is parrallax free when the scope is in focus. The NF has a longer ratio adjustment and seems to be more difficult for me to discern proper adjustment in low light.

 

[Barrelnut]

Do you change scope magnification for early/late low light situations. Maybe the scope is changing POI at different magnifications?

 

[Rich Coyle]

interesting.

 

[Pdvdh]

I more or less accept the premise that different lighting/sun conditions could affect optical presentation of the location of the bullseye thru the atmosphere and thru scope lens. I accept this without being able to explain the theory. I've experienced, and read of others' experiences, which lead me to my own personal conclusions.

One example, which isn't exactly apples to apples, is the fairly common belief by some very experienced riflemen that their rifles are capable if improved precision at longer distances compared to shorter distances. Which is total nonsense in my opinion. I think their experiences and beliefs are partially caused by optical illusion, optical shifting - perhaps parallax - or some combination of factors that are difficult to place a finger on.

For context, I'm talking relatively small moa shifts in POIs.
If you can figure out all the various cause and effects of what you've experienced, you deserve the equivalent of the Nobel Peace Award.

 

[elkaholic]

I have no doubt that light angle, cloudy days vs bright sun, etc., cause poi shift based on refraction. I learned this shooting over my bench at home at different times of the day when the sun was shinning. I had a rifle that was capable of shooting .o's and was always having to change my horizontal back and forth by at least .5 moa. I finally saw what was going on and was able to duplicate zero if I shot when the sun was at the same angle. Since then, i have tried to zero on cloudy days and live with the fact that it can change with light. Most of our hunting is done in more cloudy conditions anyway......Rich

 

[Pdvdh]

Besides the affect of lighting on POI, last winter I took 4-5 different rifles out and tested their POIs thru differing temperatures over time. Over weeks & months of time. One of my rifles was reasonably consistent over the duration of time on POI.

The disappointing results with the others caused me to accept the fact that if I really want to know where my rifles will impact, I'd best shoot them when I arrive at base hunting camp immediately prior to any long range engagements on game. Most of my rifles experienced slight and variable/unreliable shifts in POI over time. Enough to be problematic at long range. All my testing was at 280yds. One shot per rifle over time in differing outdoor temps and lighting conditions. The sun gets pretty low in the sky up here during the winter. So perhaps the natural sunlight and position of the sun in the sky relative to my target was part of the cause. I don't have enough time in my life to figure out the exact cause of these small shifts in POI over time. There are simply too many possibilities, one of which includes me.

By the way, all of my rifles are carefully free-floated and glass bedded, and they include rock solid one-piece scope rails and quality scope bases. Two are BAT actions with the scope rail manufactured into the action. Most are full custom. All are at least partial custom. I don't attribute the shifts in POI to shoddy equipment or lack of knowledge.

There are simply too many variables to nail it down in any exact manner, IMO. Like the Twilight Zone... the inexplicable.

 

[JDD]

i use to shoot small bore shil and partner with one of the top shooters in the nation!
he mounted a scope to a 4x4 stuck in the ground, and mount a ram target to a board at 100 yards . He checked it on the hour starting at day break. The dot moved through out the day at one point rose above the target then can back down, It made a figure 8! The test in my option was incomplete, it need to be done facing the 4 points of the compass and through the 4 seasons. its got to be the angle of the light and some mirage latter in the day.

I used that data to dial 1 moa up at the end of the day! it was a sudden death shoot off at the southern nationals as the light was diving! Rams at a 100 yards I hit center my opponent missed! I won the class