Intersting question

T

Travelor

New member
My wife and I find that our Silhouette RF rifles will change POI of about 1 MOA (downward, checked on the bench) from the morning to the afternoon. During this time the temperatures are changing about 25 degrees and the humidity is changing from 80%+ to about 60% and of course the light is changing as the day progresses. We both shoot Anscutz 1712's with Leupold VariX III 6.5-20 scopes and Eley Club Extra ammo at 1051 fps.

I have posed the question of why this change is occurring over on SteelChickens.com. I know that temperature and humidity change the POI on RF rifles but I do not believe that the light causes a change. The guys on SteelChickens believe that light changes POI with a scope equipped gun as well as the temp/humidity.

Can you guys shed some light (sorry) on the reason for these changes? Surely you see some changes during the day in vertical POI change.

George
 
Light and sight changes

I do not know, relative to scopes. But I do know relative to iron sights that light plays a big part in point of aim and where to hold. More so with a front bead as opposed to a target ring front sight. Also, the more light hitting your eye, the more it will "stop down" and the better ability to focus. Really noticable with old eyes or those on the edge of correct focus.
 
if you have a wood stock it is the humidity change that is causing that. a good bedding job with thick glass and pillars will minimize this.
 
Except for possible mirage I don't see how light can affect the impact very much, but I know better than to shoot if there is a significant change in lighting even though I can watch the scope and see no change. Maybe it's heat.

Your findings are consistent with my own observations. I've noticed that as humidity increases, impact of the bullet moves upward. Temperature by itself does not seem to affect the impact as much. I think it's because of the thinner air at the higher humidity. As humidity increases, so does average velocity and therefore a higher impact. The opposite would be true in a humidity decrease.
 
Any change in air conditions that increase drag will cause bullet to strike lower.

Any warming of top of barrel such as from sunlight, will cause poi to lower.

Any change to lower bullet velocity will cause it to strike the target lower.

Any increase in barrel length, or barrel plus tuner length, due to expansion will lower poi.

Any temperature rise causing front scope mount to expand more that rear will lower poi.

Wind conditions changing to a head wind will lower poi.

Lower bullet friction will cause lower poi.

Water in barrel, or no water in barrel, will cause lower poi.
 
Pacecil
For your info I have seen times when a head wind caused the poi to be higher.

Fred K
 
Pacecil,

Sounds like you've got it down pat. All of this stuff is simple physics and subject to the the laws of physics. Except when it's voodoo, which is 80% of the time.
 
Beau said:
Pacecil,

Sounds like you've got it down pat. All of this stuff is simple physics and subject to the the laws of physics. Except when it's voodoo, which is 80% of the time.
Click to expand...

Beau, no good, you gotta translate from engineer to english.
George, rimfire or center fire, some time start shooting a group in sun- wait for a cloud to come over or visa versa, you'll have your answer.
 
When I used to shoot high power rifle, saying was "lights up-sights up". Light did indeed affect POI, at least with iron sights.

Ken
 
Perhaps, part, but not all of the correct answer is MIRAGE or more specifically the refraction of light rays through differing density mediums. This means light rays are bent and displace the image/target you are attempting to view depending upon the amount and number of changes in the air density from shooter to target.

The most extreme example of this optical phenomenon and the one most people are familiar with is that of trying to shoot a fish in water….aim at the fish and you will miss with every shot.

I “think” what we most commonly observe in a shooting situation is known as “inferior image mirage”. When the ground is warmer than the air just above, the index of refraction is lower near the ground where the air density is lowest. This causes a delay in the wavefront propagating through the cooler air relative the ground, and the wavefront tilts upwards. As it arrives at the shooter, he sees an image of the target below where the target is actually located. Various factors such as the amount of radiant energy being produced by the sun, the temp of the air mass near the ground, and the color of the ground with its ability to absorb or reflect radiant energy will affect displacement of the image to varying degrees.

Any chance this could be correct….I’m all ears.
 
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