Originally Posted by Bravo 4
I will see about another pic or two, maybe a frontal shot for Wildrose. My buddy had taken some whilest I went for the truck.
With all the debate over spin drift, coriolis and how to correct for them (along with the wind) I just think I took the easy way and took a sighter shot. I did figure on .4mils for spin drift, and at my location hidden in the trees I was getting a full value of 12mph per my wind meter. The forcast had called for 20mph winds and had been the same for three days. However, beeing in mountainous terrain and shooting across a valley it can always be different. The two days prior I had been pretty consistent until I shot at something over half way across the field. I think the wind was swirling in the corner as I couldn’t hardly hit crap, always left or right. So I made the decision to impose the limit I did. What about the coriolis? These were the things I had been thinking about for those past few days. I was shooting due WEST. From all the reading I’ve done on the subject I think it should make the impact of my rounds strike a little low…??? It may have because my impacts were a little lower then what exbal told me. But then again, I don’t trust those to be 100%. What I trusted was what I had confirmed already, even the wind.
So to answer your question about the wind, I had 4mils dialed in “left” to compensate for everything affecting the flight of my bullet at the time. And “cheated” with a sighter.
That’s what the hesitation was all about.
Thanks for the additional info. Always glad to know I'm not alone in the slight anxiety caused when attempting a long shot. Here's some info I can offer, maybe it will help the next time you're ready to make another long shot....
For making a long shot like the one you took, once you have taken into consideration the atmospheric condition (temp, barometric pressure, altitude) there are five factors to consider. They are:
- ballistic program will provide this data.
- ballistic program will also tell you how much windage compensation is required.
- while I understand coriolis, I have never quite developed a formula for calculating it.
- with a right-twist barrel, it will cause your point of impact to move to the right. many ballistic programs can calculate spin drift.
when the bullet is spinning threw the air (assuming a right-twist), the air pressure on the right side of the bullet is lower than the left. As a result, with a 9 o'clock wind, the bullet will move down and to the right. With a 3 o'clock wind, it will move the bullet up and to the left. The correction for magnus effect is 15% of your spin drift + windage compensation. So for instance, with your 1425yard shot, you probably needed 1.5moa left windage to correct for spin drift and 9moa left wind for a 12mph 9 o'clock wind (~.75moa per mph) for a total 10.5moa left windage. As a result, you would need an additional 1.575moa additional 'up' (10.5 x .15=1.575).
If the wind were blowing in the opposite direction, You would still need 1.5moa 'left' for spin drift, and 9moa 'right' for wind for a total 7.5moa 'right'. As a result, 1.125moa 'down' to compensate for magnus effect (7.5 x .15 = 1.125)
Also, this caclulation assumes no up or down angle.