Yeah, temperature has effect on external ballistics. Here's a summary of it all:
The primary factors which effect trajectory are: gravity, drag, temperature, and wind. Of these, wind has the greatest effect. The amount of effect depends on the time the bullet is exposed to wind, the direction the wind is blowing, and the velocity of the wind. The shooter must compensate for the effects of wind by sight adjustment or hold off, aiming at a different spot other than center of mass.
Appraising wind velocity and direction at mid-range (half to 2/3rds target distance) is a technique known by experienced shooters to get good results. These knowledgeable shooters understand that reading the wind at mid-range gives an accurate average wind condition from the shooter’s firing point all the way out to the target.
One way to determine wind velocity and direction is by its effects on the environment. These effects should be memorized.
- 0-3 mph - hardly felt, but smoke drifts
- 3-5 mph - lightly felt on face
- 5-8 mph - keeps leaves in constant movement
- 8-12 mph - raises dust and loose paper
- 12-15 mph - causes small trees to sway
Observation of mirage is another method of estimating wind direction and speed. Mirage is the reflection of light through layers of air that have a different temperature than the ground. These layers are blown by the wind and can be monitored by viewing through a spotting scope. To observe mirage at mid-range, focus the spotting scope on the target then turn the focus knob ¼ turn counter clock-wise. The target will be a little fuzzy but the mirage should be clear.
The clock system is used to estimate the value of the wind. Once the direction is determined, the value is assigned to the wind’s velocity.
- Full value is a wind that is blowing perpendicular to the direction that the bullet is traveling. In the clock method, this would be a wind that is blowing from the 3 o’clock or 9 o’clock position.
- Half value is a wind that is blowing from a quartering direction. It only has half the effect on bullet impact. In the clock method, this wind can best be illustrated as a wind that is blowing out of the 1, 5, 7, and 11 o’clock directions.
- No value is a wind that is blowing in the direction of the bullet’s path. The wind comes directly from the front or rear of the shooter. This wind has no effect on the deflection of the bullet. In the clock method, the wind would be blowing from either the 12 or 6 o’clock direction.
After wind has been appraised for value, for example, a 10-mph wind from a quartering direction would be said to have a 5-mph value, a simple formula will allow the shooter to adjust for the distance that the wind displaces the bullet:
Distance to Target in Yards / 100 X Wind Value in MPH / Constant of 7 = Bullet Drift in MOA[A1]
For Example:
600 Yards / 100 X 5 MPH / 7 = 4 MOA Bullet Drift
To get the drift in inches the shooter will take the distance to target in yards / 100 X Bullet Drift in MOA. Shooting at 600 yards, with wind appraised as having a 5-mph value, the formula would look like this:
600 Yards / 100 X 4 MOA = 24 Inches of Drift
In addition to wind, temperature influences the elevation setting required to hit the center of the target.
- A 20-degree Fahrenheit increase in temperature increases the muzzle velocity approximately 50 feet per second.
- A 20-degree Fahrenheit decrease in temperature decreases the muzzle velocity approximately 50 feet per second.
- One minute of adjustment is necessary for every 20-degree temperature change.
- A drop-in temperature requires the shooter to raise elevation.
- An increase in temperature requires the shooter to lower elevation.
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