Riddle me this? Ballistic calc vs reality mismatch...

[Tripodmvr7]

I did not read through all the comments, but there is a phenomenon called refraction that needs to be adjusted for in the survey world. This phenomenon is strongest when temperatures are rising or dropping fast as in early morning or late afternoon and makes the target appear higher than it is. Entoptics was shooting late afternoon and that could explain the higher impact point. What you see is not always the true image.

 

[BrentM]

Your shoot to data will be off by about .8 MOA using corrected distance only. Those pages are not FM or DA forms so they are likely from a private section training exercise and using riflesmans rule.

 

[BrentM]

I did not read through all the comments, but there is a phenomenon called refraction that needs to be adjusted for in the survey world. This phenomenon is strongest when temperatures are rising or dropping fast as in early morning or late afternoon and makes the target appear higher than it is. Entoptics was shooting late afternoon and that could explain the higher impact point. What you see is not always the true image.

At the distance being shot light bending issues are minimal. I have noticed it is really bad in high humidity, hazy air, and extreme light angles but usually with crappy optics and longer distances.

 

[Stammster]

If taking a 1,000 yard shot with a 25 degree angle regardless of shooting uphill or down hill the calculations will be the same. You must first refer to the chart and convert your degrees angle to cosine. 25 degrees is .906 cosine. So multiple your LOS ( 1000 yards ) by your cosine .906. Now this is where training kicks in. The number 6 in the cosine is higher than 5. So round your cosine up to .91 the equation will look like so.
1000 x .91 = 910 yards. This will be your shoot to range.

The horizontal component is 910 yards, but the TOF is still based on 1000 yd distance. Therefore, factors other than gravity (I.e. wind, spindrift, Coriolis, velocity) are based on true distance.

Also technically speaking, the gravity based horizontal drop also varies per angle, but for all practical aspects in small arms it's approximated by the method you describe (cosine angle x LOS distance to target). Not so much true for heavy artillery at 15 miles.

Angle is a red herring, as the OP was at 2deg at 500 yds. Difference is in hair width, not inches or feet.

 

[flatwater bill]

Lot of good ideas on this thread. Were you shooting over uneven thermals? Part grain stubble part lava rock, for example? Constant temp from 200yd zero to 503 yd group? Thanks for posting..................FWB

 

[Tripodmvr7]

At the distance being shot light bending issues are minimal. I have noticed it is really bad in high humidity, hazy air, and extreme light angles but usually with crappy optics and longer distances.

Normally shooting is done close to the middle of the day when refraction is stable - then its effect is about 20mm at 500 metres. Late afternoon is when the effect is quite pronounced.

To give an example. A colleague of mine did a lot of obeservations in Iran. He observed from a known point and the information supplied listed other surrounding fixed points that could be seen. During the middle of the day one specific point was not there when he tried to observe it. Late afternoon the beacon appeared on a low hill and also the whole village that was below it. At 5km refraction is normally 1.71m, but here many more were visible.

If a barren field is involved the heat rising off that can create abnormal circumstances.

 

[floyd kittrell]

Rarely is the ballistic performance the same as advertised. The factory numbers are under optimum
conditions and only account for one BC. When establishing a drop chart it is prudent to have three over the distance desired and velocities at those points. The BC can change because of performance at different velocities and should be taken into account once you have good actual drop data. then you can find the actual Bullet BC and use it in your programs for best results.

Actual Bullet BC's are never the same as advertised where I hunt and shoot so I have to adjust them to match the drop chart to get good results. I also like to have a 5 to 10 mile wind at 90o to help the calculation better match the actual Bullet BC's.

J E CUSTOM

AGREED , SIR !

 

[BrentM]

Normally shooting is done close to the middle of the day when refraction is stable - then its effect is about 20mm at 500 metres. Late afternoon is when the effect is quite pronounced.

To give an example. A colleague of mine did a lot of obeservations in Iran. He observed from a known point and the information supplied listed other surrounding fixed points that could be seen. During the middle of the day one specific point was not there when he tried to observe it. Late afternoon the beacon appeared on a low hill and also the whole village that was below it. At 5km refraction is normally 1.71m, but here many more were visible.

If a barren field is involved the heat rising off that can create abnormal circumstances.

Heat off a field is going to be thermal/heat wave/mirage, not refraction in the sense being discussed. I've never seen refraction issues be a significant factor in shooting scenarios, especially medium range. There isn't a formula for refraction. It is dependent up on the atmospheric conditions, optics being used, light angle, light intensity, and target reflection. In the most extreme cases and combined with mirage a person could literally not be able to engage a non reflective target. In this case, I don't believe he experienced any issues with mirage or refraction to push him a MOA out of target center. I believe he already solved his issue but it is buried in a bunch of off topic rhetoric able slope angle compensation bs so I don't even remember what he discovered.