Angular measurements
Instead of measuring hold-over in terms of linear distance (inches or cm), it would be helpful to
translate those linear distances into units of angular measure. The concept of angular measure is
that an angle of 1 degree demarcates 1.7 yards at 100 yards, or 3.5 yards at 200 yards. Everyone
with a basic understanding of geometry should understand how angles work.
There are two units of angular measurement commonly used in rifle scopes. The first is the "minute of angle." Dividing a circle into 360 degrees, then each degree contains 60 minutes. One MOA demarcates 1.0472" per 100 yards of distance. The second is the "mil". One mil is one part transverse per 1000 parts distance. In units we understand, 1 mil is 3.6" per 100 yards (ie, 100 yards is 3600", one thousandth of which is 3.6"). Consequently it's also 1 yard at 1000 yards. Alternatively, in metric, 1 mil is 10cm per 100 meters, or 1m at 1000 meters.
Wind
Just like the atmosphere pushes on the bullet as it moves forward, slowing it down, any winds
present in the bullet's path can affect its trajectory. The most common effect is the cross wind. A
10mph cross wind will move a typical 308 bullet about 6" at 300 yards. The following graph
demonstrates the wind deflection as range increases for a left or right 10mph wind.
RANGE DRIFT for 10mph cross 100 0.6" 200 2.6" 300 6.0" 400 11.0" 500 17.8" 600 26.5" 700 37.5" 800 50.9" Lead
For moving targets, the shooter must aim in front of the target a distance which depends on the
target distance and speed. This is called "lead." We'll generate a table for some standard target
speed and add it to our table.
Both the "drift" and "drop" values in the tables can be translated to use angular measurements (MOA or mils) instead of linear measurements (inches or cm) to aid utility. Typical Data Card
The shooter might end up with a data card that looks something like this. The first line describes
the load so he can keep straight what the data-card describes. The second line reminds him what
each column means.
155 LAP: 2825fps 100yd 0' RANGE elev wind 4mph->(MOA) 25 4.00 0.25 6 moa 50 0.75 0.25 6 moa 75 0.00 0.50 6 moa 100 0.00 0.50 6 moa 125 0.25 0.75 6 moa 150 0.50 1.00 6 moa 175 1.00 1.00 6 moa 200 1.50 1.25 7 moa 225 2.00 1.50 7 moa 250 2.50 1.50 7 moa 275 3.00 1.75 7 moa 300 3.75 2.00 7 moa 325 4.25 2.00 7 moa 350 5.00 2.25 7 moa 375 5.75 2.50 7 moa 400 6.50 2.50 7 moa 425 7.25 2.75 7 moa 450 8.00 3.00 7 moa 475 8.75 3.25 7 moa 500 9.50 3.50 7 moa 525 10.25 3.50 7 moa 550 11.25 3.75 7 moa 575 12.00 4.00 7 moa 600 13.00 4.25 8 moa 625 13.75 4.50 8 moa 650 14.75 4.75 8 moa 675 15.75 5.00 8 moa 700 16.75 5.00 8 moa 725 17.75 5.25 8 moa 750 18.75 5.50 8 moa 775 20.00 5.75 8 moa 800 21.00 6.00 8 moaColumns: 1. Range 2. elevation for #1's target distance, in MOA 3. wind for #1's target distance, in MOA 4. lead for #1's target distance, in MOA for a target traveling at 4mph (a medium walking pace) All the trajectory values can be calculated using one of the modern small-arms ballistics calculator programs, such as Sierra Ballistic Explorer, Exbal, QuickTarget, Agtrans, etc. Several parameters are critical to their accuracy: (1) bullet ballistic coefficient (BC) values, (2) accurate measured muzzle velocity from a chronograph, (3) solid zero distance, and (4) accurate environmental conditions including station pressure, temperature, or density altitude. Want a set of cards from DEMIGOD LLC? Click here
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