Reading the Weather
One person's efforts at obtaining a good sitting position, use this as a sort of guide for developing yours.
- Wind poses the biggest problem. 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 shooter 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 shooter 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 1,2,4,5,7,8,1 0, and 1 1 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.
- Before adjusting the sight to compensate for wind, the shooter must determine wind direction and velocity. He may use certain indicators to accomplish this. These are range flags, smoke, trees, grass, rain and the sense of feel. However, the preferred method of determining wind direction and velocity is reading mirage. In most cases, wind direction can be determined simply by observing the indicators.
- A common method of estimating the velocity of the wind during training is to watch the range flag. The shooter 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 shooter 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 arrn by the constant number 4.
- The most desired method of determining wind is with mirage. A mirage is a reflection of the heat through layers of air at different temperatures and density. A shooter can see the mirage as long as there is a difference in ground and air temperatures. In order to see this mirage the shooter must focus his spotting scope on an object at midrange, then place the scope back onto the target without read using the focus. As observed through the telescope, the mirage appears to move with the same velocity as the wind, except when blowing straight into or away from the scope. Then the mirage gives the appearance of moving straight upward with no lateral movement. This is called a " boiling mirage." In general, changes in wind, up to 12 mph can be readily determined by observing the mirage.
Minute of Angle
- A Minute of Angle (MOA) is a term of measurement which equals 1 inch at 100yds. This thin angle spreads out with distance. So that same MOA equates to 2 inches at 200yds, 3 inches at 300yds & so on. This makes MOA, as a term of measurement, convenient for shooting because we describe bullet trajectories in inches & distances in yards. Most quality scopes use windage & elevation increment adjustments of 1/4 MOA per click at 100yds. (check your scopes specifications to be sure) Some manufacturers don't usually tell you the MOA's in their advertisements, although they're saying the same thing when their specifications state "1 click equals 1/4 inch at 100yds." Shot placement, particularly at long distances, is now just a question of learning how much to apply these incremental adjustments at various distances.
Elevation & Windage Formulas
- Let's say you've fired a shot at a 300yd target & it hit 15 inches low. How do you adjust your elevation? Simple, here's a formula to determine scope correction in MOA.
- On a scope with 1/4 MOA adjustment, we achieve this 5 MOA adjustment by raising the elevation 20 clicks. After all, 5 MOA divided by the 1/4 MOA (.25) adjustment on the scope equals 20. But you probably already new that.
Conversion of wind velocity to minutes of angle
- After finding wind direction and velocity in MPH, the shooter 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 dope book are the ultimate goals and tools.
- Range(100's of yds) x Windspeed(MPH)=MOA (for a full value wind)/"c"
- "c" or constant depends on the targets range
- 100-500 meters=15
- 600 meters=14
- 700-800 meters=13
- 900 meters=12
- 1000 meters=11
Effects of Light
Effects of Temperature
Effects of Humidty
- Effects of light Light does not affect the trajectory of the bullet; however, it does affect the way the shooter sees the target through the scope. This can be helped by practice and a good data book.
Effects of Temperature
- 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 Humidty
- Effects of humidity Humidity varies along with the altitude and temperature. The shooter 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).