Windage correction

[Jesse1973]

I have a couple questions on windage.
Question 1: All the formulas that I have see give windage adjustment in MOA .Is there a formula for Mils that I haven't come across yet?
Question 2: I have a USMC Marksmanship training guide and they list a wind constant for figuring out correction.It is for .308 though .Would that constant be the same for say a 300 Win Mag or will that change from caliber to caliber ?

 

[Lowlight]

Re: Windage correction

Do yourself a favor and not worry about <span style="font-style: italic">"formulas"</span> and instead go to:

JBM Ballistic Calculator

Then insert <span style="text-decoration: underline">"your"</span> data and print a card out, then laminate it and carry it with you. Set it for a 10MPH for ease, and use that.

Constants do change with caliber, and nobody but a very few who shoot by themselves and are looking for personal entertainment are running <span style="font-style: italic">"formulas"</span> in the field. If you are doing long hand math when you have a target in front of you, you are doing it wrong. And changing the constants will give you mils, or you can simply take your MOA Dope and divide it by 3.43 for Mils.

If you have a 308 and don't know your MV and don't know how to figure it close from drop, then simply use 2650fps for a 175gr bullet. Otherwise get <span style="font-style: italic">"your" </span>data and use that, you'll be much better off in the long run.

 

[louboyd]

Re: Windage correction

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jesse1973</div><div class="ubbcode-body">I have a couple questions on windage.
Question 1: All the formulas that I have see give windage adjustment in MOA .Is there a formula for Mils that I haven't come across yet?
Question 2: I have a USMC Marksmanship training guide and they list a wind constant for figuring out correction.It is for .308 though .Would that constant be the same for say a 300 Win Mag or will that change from caliber to caliber ?
</div></div>

Question 1. If you have windage stated in MOA just multiply MOA it by .29088 to get mils. (0.3 if you're doing it in your head).

The amount of wind deflection is usually calculated in ballistics programs by Didion's equation.

Stated in words
deflection_at_target equals wind_velocity times ((actual_time_of_flight) minus (vacuum_time_of_flight))

If wind velocity is given in feet per second the and the times of flight are in seconds the deflection at the target will be in feet. There is no multiplying coefficient and the answer is exact. This is also known as the lag function.

Equations for variable winds over the trajectory including head and tail winds are given in "Modern Exterior Ballistics" by Robert L McCoy. Few ballistics programs allow handling variable wind over the path.

Vacuum time of flight is simply target distance divided by muzzle velocity, but calculating the actual time of flight involves all the problems of drag computation using drag tables and air density.

So for question #2. No, the equation in the marksman ship guide would not be right for 300 WM. I doubt its as good as the results from a typical ballistics computer.

The practical way to handle wind is to make up drop and windage cards for the ammo you shoot using a ballistics program Maybe more than one for different air density where you shoot. lowlight's advice above is practical and easy to use. Ballistic calculators are another option. The can take into account more information, but are generally slower to use than range cards.

 

[Jesse1973]

Re: Windage correction

Another question.This one dealing with ballistic computer/calculator .If I input a 10mph crosswind would that not through off any correction needed if the wind speed is less than 10mph but more than 0 or if the speed is more than 10 mph?

 

[Compact45]

Re: Windage correction

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DFOOSKING</div><div class="ubbcode-body">No think of it like this....


Distance. Wind (10mph) Full Value

400. 3 mils
500. 4 mils
600. 5 mils


Target is 500yds and wind full value 15mph....half your wind dope and add it on so 6 mils and let'er buck.

Target is 600yds and is full value 20mph....double it, so 10 mils and let'er buck.

Target is 400yds and wind half value 8mph.... 80% of 3 mils is 2.4 and half that for value so 1.2 and let'er buck.

When its too hard to dope an exact speed and direction....fuck it, chuck it. </div></div>



This really simplified the whole wind thing for me too!!! Thanks.

 

[Jesse1973]

Re: Windage correction

Simplified it for me so much now I feel stupid for now thinking of it sooner .LOL