Wind classification Wind poses the biggest problem for the sniper. The effect that wind has on the bullet increases with range. This is due mainly to the slowing of the bullet's velocity combined with a longer flight time. This allows the wind to have a greater effect on the round as distance increases. The result is a loss of stability.
Since the Sniper must know how much effect the wind will have on the bullet, he must be able to classify the wind. The best method is to use the clock system. With the Sniper at the center of the clock and the target at 12 o'clock, the wind is assigned the values: full, half, and no value. Full value means that the force of the wind will have a full effect on the flight of the bullet. These winds come from 3 and 9 o'clock. Half value means that a wind at the same speed, but from 1,2,4,5,7,8,10, and 11 o'clock will move the bullet only half as much as a full-value wind. No value means that a wind from 6 or 12 o'clock will have little or no effect on the flight of the bullet.
A common method of estimating the velocity of the wind during training is to watch the range flag. The Sniper determines the angle between the flag and pole, in degrees, then divides by the constant number 4. The result gives the approximate velocity in miles per hour. If no flag is visible, the Sniper can use paper grass, or some other light material at shoulder level, then drops it. He then points directly at the spot where it lands and divides the angle between his body and arm by the constant number 4.
Conversion of wind velocity to minutes of angle All telescopic sights have windage adjustments that are graduated in minutes of angle or fractions thereof. A minute of angle is 1/60th of a degree. This equals 1 inch at 100 meters (1.145 inches). Snipers use min. of angle (MOA) to determine and adjust the elevation and windage needed on the weapon's scope. After finding wind direction and velocity in MPH, the Sniper must then convert it into MOA using the following formula. This formula is a rule of thumb only and is used as a starting point. Practice and keeping a good data book are the ultimate goals and tools.
RANGE (1st digit) x VELOCITY (MPH) = MOA (for a full value wind)
CONSTANT ( for a half value wind divide this answer by 2)
The constant depends on the targets range:
100-500 meters "c"=15
600 meters "c" =14
700-800 meters "c"= 13
900 meters "c"= 12
1000 meters "c"=11 Effects of light Light does not affect the trajectory of the bullet; however, it does affect the way the Sniper sees the target through the scope. This can be helped by practice and a good data book.
Effects of temperature Temperature affects the firer, ammunition, and air density. When ammunition sits in direct sunlight, the burn rate of powder is increased, resulting in greater muzzle velocity and a higher point of impact. A general rule is that when a rifle is zeroed, a 20 degree increase in temperature will raise the point of impact by 1 MOA. A 20 degree decrease in temperature will drop the bullet 1 MOA. The key is consistency in shooting environments or knowing how the changes effect your shots when the consistency is off.
Effects of humidity Humidity varies along with the altitude and temperature. The Sniper can encounter problems if drastic humidity changes occur in his area of operation. If humidity goes up, then the impact of the bullet goes down due to the drag effect and "weight" that the extra water adds to a bullet in flight. The book suggests a 20 % rule of thumb here. Meaning that if your humidity raises 20% from the day you zeroed your rifle, then the bullet will strike 1MOA low. ( this author recommends using 40% changes instead, but your experience and data will determine the exact number for you and your rifle)