Few months ago I was once again shooting at 1000 meter targets. This is my "private backyard range" and usually my first shot is within +/- 20 cm in height if I make the ballistic compensation by using external ballistic calculator with 6 DOF corrections. Anyway this time shots of first group were about 70 cm high. It was sunny clear spring day and still some snow on the ground. I think that air was actually a bit colder close to the ground (where my target was) than at my shooting point which is about 3,5 meters above ground. I believe that reason for this unusual POI difference was light refraction. Picture below shows the situation:



I made a calculator (attached) which uses Modified Edlen Equation to estimate refractive index of air as a function of atmospheric conditions and wavelength. And calculates point of impact displacement by applying snell's law. Shooter just has to know atmospheric conditions at shooting level and at target level. I haven't tried it in practice but at least results are reasonable. I have to get accurate fast thermometer for next winter season to test this theory.

the other thing you can do is to fix a spare telescopic sight to your barn, aiming it at a fixed point at 1000m. Then as a function of atmospheric conditions, measure the displacement of the crosshairs.

Quote from: admin on May 30, 2013, 12:50:07 AM
the other thing you can do is to fix a spare telescopic sight to your barn, aiming it at a fixed point at 1000m. Then as a function of atmospheric conditions, measure the displacement of the crosshairs.

Good idea, that blocks shooting error out of the equation.