Targeting
The range to the target is measured or estimated as precisely as conditions permit and correct range estimation becomes absolutely critical at long ranges, because a bullet travels with a curved trajectory and the sniper must compensate for this by aiming higher at longer distances. If the exact distance is not known the sniper may compensate incorrectly and the bullet path may be too high or low. As an example, for a typical military sniping cartridge such as 7.62x51mm NATO (.308 Winchester) M118 Special Ball round this difference (or “drop”) from 700 to 800 metres (770–870 yd) is 200 millimetres (7.9 in). This means that if the sniper incorrectly estimated the distance as 700 meters when the target was in fact 800 meters away, the bullet will be 200 millimeters lower than expected by the time it reaches the target.
Laser rangefinders may be used, but are not preferred on the battlefield because a laser can be seen by both the sender and the receiver, especially after dark when using night vision equipment. One useful method is comparing the height of the target (or nearby objects) to their size on the mil dot scope, or taking a known distance and using some sort of measure (utility poles, fence posts) to determine the additional distance. The average human head is 150 millimeters (5.9 in) in width, average human shoulders are 500 millimeters (20 in) apart and the average distance from a person's pelvis to the top of their head is 1,000 millimeters (39 in).
To determine the range to a target without a laser rangefinder, the sniper may use the mil dot reticle on a scope to accurately find the range. Mil dots are used like a slide rule to measure the height of a target, and if the height is known, the range can be as well. The height of the target (in yards) ×1000, divided by the height of the target (in mils), gives the range in yards. This is only in general, however, as both scope magnification (7×, 40×) and mil dot spacing change. The USMC standard is that 1 mil (that is, 1 milliradian) equals 3.438 MOA (minute of arc, or, equivalently, minute of angle), while the US Army standard is 3.6 MOA, chosen so as to give a diameter of 1 yard at a distance of 1000 yards (or equivalently, a diameter of 1 meter at a range of 1 kilometer.) Many commercial manufacturers use 3.5, splitting the difference, since it is easier to work with.
Explanation: 1 MIL = 1 milli-radian. That is, 1 MIL = 1x10^-3 radian. But, 10^-3 rad x (360 deg/ (2 x Pi) radians) = 0.0573 degrees. Now, 1 MOA = 1/60 degree = 0.01667 degrees. Hence, there are 0.0573/0.01667 = 3.43775 MOA per MIL, where MIL is defined as a milli-radian. On the other hand, defining a mil-dot by the US Army way, to equate it to 1-yard (1 m) at 1,000 yards (1,000 m), means the Army's mil-dot is approximately 3.6 MOA.
It is important to note that angular mil (mil) is only an approximation of a milliradian and different organizations use different approximations.
At longer ranges, bullet drop plays a significant role in targeting. The effect can be estimated from a chart which may be memorized or taped to the rifle, although some scopes come with Bullet Drop Compensator (BDC) systems that only require the range be dialed in. These are tuned to both a specific class of rifle and specific ammunition. Every bullet type and load will have different ballistics. .308 Federal 175 grain (11.3 g) BTHP match shoots at 2,600 ft/s (790 m/s). Zeroed at 100 yards (100 m), a 16.2 MOA adjustment would have to be made to hit a target at 600 yards (500 m). If the same bullet was shot with 168 grain (10.9 g), a 17.1 MOA adjustment would be necessary.
Shooting uphill or downhill is confusing for many because gravity does not act perpendicular to the direction the bullet is traveling. Thus, gravity must be divided into its component vectors. Only the fraction of gravity equal to the cosine of the angle of fire with respect to the horizon affects the rate of fall of the bullet, with the remained adding or subtracting negligible velocity to the bullet along its trajectory. To find the correct zero, the sniper multiplies the actual distance to the range by this fraction and aims as if the target were that distance away. For example, a sniper who observes a target 500 meters away at a 45-degree angle downhill would multiply the range by the cosine of 45 degrees, which is 0.707. The resulting distance will be 353 meters. This number is equal to the horizontal distance to the target. All other values, such as windage, time-to-target, impact velocity, and energy will be calculated based on the actual range of 500 meters. Recently, a small device known as a cosine indicator has been developed. This device is clamped to the tubular body of the telescopic sight, and gives an indicative readout in numerical form as the rifle is aimed up or down at the target. This is translated into a figure used to compute the horizontal range to the target.
Windage which plays a significant role, the effect increasing with wind speed or the distance of the shot. The slant of visible convections near the ground can be used to estimate crosswinds, and correct the point of aim. All adjustments for range, wind, and elevation can be performed by aiming off the target, called "holding over" or Kentucky windage. Alternatively, the scope can be adjusted so that the point of aim is changed to compensate for these factors, sometimes referred to as "dialing in". The shooter must remember to return the scope to zeroed position. Adjusting the scope allows for more accurate shots, because the cross-hairs can be aligned with the target more accurately, but the sniper must know exactly what differences the changes will have on the point-of-impact at each target range.
For moving targets, the point-of-aim is ahead of the target in the direction of movement. Known as "leading" the target, the amount of "lead" depends on the speed and angle of the target's movement as well as the distance to the target. For this technique, holding over is the preferred method. Anticipating the behavior of the target is necessary to accurately place the shot.
Read more about this topic: Sniper Warfare