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Section II. BALLISTICS

US. Kffects of the Weather In the case of the highly trained firer, effects of the weather are of primary importance because they can cause an error in the strike of the bullet. The wind, mirage, light, temperature» and humidity all have some effect on the bullet, the firer, or both, a. Wind.

Ill The condition which constantly presents the greatest problem to the firer is the wind. Wind has a considerable effect on the bullet. This effect increases with the range. This is due primarily to the increased time the bullet is exposed to the wind (due to its dropping velocity! per unit distance as the range increases. Wind also has a considerable effect on the firer. The stronger the wind, the more difficulty the firer has in holding the rifle steady. The effect on the firer can be partially offset with good training and conditioning.

<2! Before any sight adjustment can be made to compensate for wind it is necessary to determine its direction and velocity. There are certain indicators which the firer may use to accomplish this. These are range flags, smoke* trees, grass, rain, and the sense of touch. Another important indicator, "mirage/' will be discussed in a later paragraph.

{*) A common method of estimating the velocity of the wind (in training! is based on observation of the range flag. The angle in degrees between the flag and its pole is multiplied by the constant nuin ber .4 (or, the angle is multiplied by 4 and divided by 10). The result gives the approximately velocity in kilometers per hour (fig m:>».

(b) If no flag is visible, a piece of paper, grass, cotton, or some other light material may be dropped from the shoulder. By pointing directly at the point where it lands, the approximate velocity in kilometers per hour (fig 146! is calculated.

(c) If for some reason these methods cannot be used, the following information is helpful in determining velocity: Under 5 kmph (kilometers per hour!, winds can hardly be feltf but may be determined by smoke drift.

At 8—13 kmph, leaves in trees are in constant motion.

At 19—24 kmph, small trees begin to sway.

(31 Since the firer must know how much effect the wind will have on the bullet, he must be able to classify the wind. The universally accepted method is by use of the clock system (fig 1471. A half value wind will affect the strike of the bullet approximately one-lialf as much as full value wind of the same velocity. A wind velocity corrected in this manner is called the "effective wind." The so-called "no value1' wind has a definite effect on the bullet at long ranges if it is not blowing directly from 6 to 12 o'clock. This is the most difficult wind to fire into due to the switching or "fishtail" effect which requires frequent sight changes. Depending on the velocity, this type wind may have a slight effect on the vertical displacement of the bullet.

Figur* 14$. 71« fUf m#iJn>d o/ wind •tfimt^o*.

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