(This was my first paying published article.)
Adapting the Marine Corps Windage Formula
for Rounds Other than the M118
by Donald Plunkett
The United States Marine Corps scout
sniper manual (FMFM1-3b) contains the following formula for figuring windage;
R X W
---------
= Windage change in minutes of angle (MOA)
C
Where R= the range to target in hundreds
of yards, W = the wind velocity in miles per hour, and C = a constant.
The Marines use the constant 15 out to 500
yards. Beyond 500 yards the constant changes
so that;
C = 14 at 600 yards
13 at 700 to 800 yards
12 at 900 yards
and 11 at 1000 yards.
This formula is simpler to remember than a
full set of windage tables but it is an approximation based on the M118 match
173 grain bullet at a muzzle velocity of 2550 feet per second fired at sea
level. Any changes in these variables
throws the formula further and further off until its usefulness is virtually
eliminated. By starting with a set of
windage tables established for your particular rifle, load and conditions it is
possible to back calculate and come up with a new set of constants that make
this formula precise enough for most military sniping.
The first step is to obtain a full set of
windage tables tailored to your rifle.
The best way to do this is to actually fire at measured ranges under a
variety of precisely measured wind conditions.
As you record the required sight corrections in your data book you will
develop a complete real world picture of what your rifle is doing under all
conditions. If you are unable to fire at
the actual ranges you can come up with a pretty good approximation by using a
chronograph and a good ballistics program.
If you use a computer generated windage table you will need to know the
ballistic coefficient (BC) of the bullet
you are using. Remember that the BC
changes as the velocity changes so make sure you are using the correct BC for
your muzzle velocity. Be sure your BC
matches the lot of bullets you are using. For example Sierra has recently
changed the meplat on their 168 grain .308 matchking bullet and updated the BC
for it. You will also want to plug in
the average temperature and altitude where you expect to be shooting. Remember that changes in altitude can throw
your elevation and windage data off. If
your data is set up for sea level and you are planning to shoot at 5000 feet
you will want to calculate a new set of data and rezero when you reach the new
altitude. Temperature effects on
trajectory are less predictable and should be established by checking your data
book. Calculate your windage for a
minimum of three wind speeds at each range.
I will generally run from 5 to 35 miles per hour in 5 mph
increments. When you are finished check
the table against your data book numbers.
If there are any major discrepancies you will have to scrap the table
and rework it from real world range data.
Here’s the factory wind drift chart for
Federal .308 168 grain BTHP match with a muzzle velocity of 2600 fps fired at
sea level. Drift is given in inches.
Wind speed in MPH
5 10 15 20 30
Range in yards
100 0.4 0.8 1.2 1.6
2.4
200 1.5 3.1 4.6 6.2 9.3
300 3.7 7.4 11.1 14.8 22.2
400 6.8 13.6 20.4 27.2
40.8
500 11.1 22.2 33.3 44.4 66.6
600 16.1 33.3 49.9 66.6 99.9
700 23.5 47.1 70.6 94.2 141.
800 32.0 64.1 96.1 128. 192.
900 42.1 84.2 126. 168. 253.
1000 53.8 107. 161.
215. 322.
Convert the windage in inches to MOA for
each range and wind velocity. Divide the
inches by the range in hundreds of yards (600 yards = 6) to get MOA. Round your minutes off to the nearest quarter
minute for scopes that have quarter minute windage resolution.
For 600 yards the data looks like this;
Wind speed in MPH 5 10 15 20 30
Range in yards 600 2.75 5.5 8.25 11.0 16.75
Now plug your MOA into the formula;
R X W
Range times
wind velocity divided by minutes equals
--------- = C your constant.
MOA
Wind speed in MPH 5 10 15 20 30
Range in yards 600 10.9 10.9 10.9 10.9 10.7
Now for each range add the constants
together and divide them by the number of wind speeds your table uses. Federal used 5 different wind speeds so I
would divide the total by 5. Round this
number off to the nearest whole number.
You can round to the nearest half, quarter, or tenth to reduce the error
factor but it will make the constants harder to memorize.
54.3 / 5 = 10.86 (11) for 600 yards
For federal .308 168 grain BTHP match with
a muzzle velocity of 2600 fps fired at sea level the numbers come up thus:
C = 13 for 100 to 200 yards
12 for 300 to 400 yards
11 for 500 to 600 yards
10 for 700 to 900 yards
and
9 for 1000 yards
Plug your numbers into the Marine formula
with the new constants and compare them to your established tables. You’ll find the error is smaller than you can
hold under most field conditions. For
absolute precision you’ll still want to refer to your tables. Now instead of having to memorize 50 or more
wind drift values you can memorize a simple progression of a half dozen or so
values. For fast acquisition of targets
of opportunity it’s the way to go.
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