Bullet Flight: The Basics of Sniper Business
It may seem to the naked eye that the rifle scope is parallel to the barrel. This is actually not the case.
The axis of the barrel and the optical axis of the sight form an angle called the aiming angle. And the trajectory of the bullet, of course, is not direct, and the target is not always on the same level as the rifle - often you have to shoot with significant elevation or declination angles.
In flight, the bullet is affected by gravity and various aerodynamic forces that must be considered when aiming.
The effect of gravity on the flight of a bullet is estimated quite simply. For a certain time (or at a certain distance), the bullet is reduced, and this decrease depending on the firing distance can be calculated using a ballistic calculator or according to the tables, and then make corrections using the corresponding flywheel of the sight.
As a rule, flywheels are graduated in angles - in the West angular minutes (minutes of angle, MOA) are accepted, in Russia - thousandths of a distance, or milliradians (lateral size of 1 m at a distance of 1000 m, 1 mrad = 3.43 MOA) .
To facilitate the task, the flywheels are sometimes graduated in meters of distance (such a correction will work for a particular munition under standard conditions).
The correct determination of the distance is extremely important for accurate shooting.
There are many methods for this - from using a laser range finder or comparing the reticle with known sizes of objects to basic signs such as “the movements of a person’s arms and legs are distinguishable from 500-600 m”.
There are also a number of situations that make it difficult to correctly visualize distances. The greater the firing distance, the greater the error in determining the distance will affect the final result - the possibility of hitting a target.
Air resistance slows down a flying bullet, and this must be taken into account when calculating corrections (especially in non-standard situations - for example, in mountainous areas, when air is rarefied). Humidity and air temperature also affect. But much more important is the aerodynamic drift of a bullet from a side wind.
The fact is that when shooting at long distances (several hundred meters) along the flight path of a bullet, the wind can change several times - both in strength and in direction. High-rise buildings in the city create powerful air currents that seriously impede the work of police snipers during special operations.
The speed and direction of the wind must be determined by the fluctuations of the ascending air flows - mirages - or even completely predicted. The sniper proverb says: “Beginners study ballistic tables, and experienced snipers study the wind.”
The point of application of gravity to the pool (center of mass) does not coincide with the point of application of aerodynamic forces (center of pressure, located in front of the center of mass). As a result of the action of these forces, a tilting moment arises in the plane of the trajectory.
But since the bullet rotates and is a gyroscope, its axis of rotation deviates perpendicular to the plane. That is, if the bullet rotates to the right, a deviation to the right occurs and a precession occurs - oscillations of the axis of rotation of the bullet. The axis of this precession will be deflected to the right, aerodynamic forces deflect the bullet in the same direction.
This phenomenon is called derivation. It depends on the speed of the bullet and the speed of its rotation, mass and shape. Usually, this effect begins to affect the accuracy of shooting only at sufficiently large distances (where it is likely to "get lost" against the background of much more significant drift by the wind).
As a separate case, it is worth considering shooting adjusted for the elevation angle of the target. This situation occurs in the mountains or in the city, where snipers equip positions on the roofs of buildings.
When shooting at a target located above or below the shooter, it is imperative to make a correction, which depends on the elevation angle, but does not depend on whether it is a positive angle or a negative one - in both cases, with the introduction of the usual correction, the bullet will go above the target. Shooting at an angle to the horizon requires an amendment less than usual.
The fact is that an absolute decrease in the trajectory of a bullet to the line of the barrel is always perpendicular to the horizon, and a relative decrease (trajectory of a bullet to the line of sight) is perpendicular to the line of sight.
Through the glass
Quite often, police snipers have to deal with a situation where a terrorist who has taken hostages is behind a transparent barrier - glass. You can aim at it, but can you get there? It would seem that glass is a fragile material, but it can significantly affect the shooting result. In this regard, snipers have several thoughts.
Firstly, it all depends on the thickness and material of the glass. Secondly, do not use expansive bullets, which tend to change trajectory in unpredictable ways when passing through solid obstacles. Thirdly, a shot perpendicular to the glass affects the trajectory of the bullet less.
Some types of glass give a lot of sharp fragments that can harm not only the terrorist, but also the hostages. The method is often used when one of the police snipers breaks the glass with a shot, and his colleagues hit the target almost without a pause.
How to shoot?
The properties of a cartridge and a bullet mean no less in sniper business than the advantages of a barrel or sight. Therefore, speaking of high-precision small arms, we mean the rifle-ammunition system. There are many varieties of ammunition for sniper weapons, differing in caliber, cartridge length, bullet design and powder charge characteristics, however, the .308 Winchester caliber cartridge, also known as 7.62 NATO, should be recognized as a real workhorse.
We are talking about the cartridges of the “match” class, manufactured on precision equipment with minimal tolerances. Bullets used in this type of ammunition are denoted by the English abbreviation BTHP (Boat-Tail Hollow Point). The term boat-tail (“boat feed”) refers to the characteristic conical tail of the bullet. The tail cone, reducing the leading part of the bullet, thereby improves its aerodynamic characteristics, reduces speed loss and increases lateral wind resistance.
The bullet also has a cavity in the head part (hollow point) - this enhances the lethal effect. 168 grenade .308 Winchester sniper ammunition with a BTHP bullet is available from Remington, Hornady, Lapua, Norma, Federal and others. Sniping also uses .223, .300 and even half-inch .50 caliber cartridges (for large-caliber rifles).
Usually snipers use ammunition of a special design of the “match” class. But sometimes you have to work with other types of bullets, the properties of which are important to consider when shooting. Bullets of a "fragile" type, when touched with a target, are scattered into small fragments, the penetration ability of which is very small. Almost 100% of the bullet’s energy is transferred to the target.
Such bullets were created specifically for security agents accompanying flights. Hitting a terrorist, a "fragile" bullet does not pass right through and if accidentally hit it is not able to break through the fuselage of an aircraft or injure others.
Expanding bullets when hit by the target “open”, giving up to 70% of energy, which also reduces the risk of damage right through. A bullet from a general-purpose cartridge, due to its shape, gives off the least amount of energy (about 50%). The risk of passing through in this case is quite large.
1. The barrel must not come into contact with anything! 2. The trigger should be pulled by the most sensitive part of the index finger pad. 3. To set the position of the bed, put a bag of sand under the butt. By pressing on it with a hand free from shooting, you can make fine adjustment in height. 4. It is necessary to precisely maintain such a distance between the eye and the eyepiece in order to see the entire visual field completely and without distortion. Usually this value is 7 × 10 cm. 5. If time permits, it is necessary to fix the position of the rifle with a belt. 6. Avoid stalling weapons. 7. As an emphasis, it is better to use not a standard bipod, but a bag of sand.
Ammunition of the “match” class To maximize the capabilities of high-precision sniper rifles, ammunition of high accuracy, the so-called cartridges of the match class, is required. They are manufactured on precision equipment with minimal tolerances. Particular attention should be paid to the design of the bullet. It is distinguished by a conical tail in the form of a “boat stern” and a cavity not filled with lead in the bow of the shell.
Breathe and shoot. When breathing, it is important not to disturb the stable position of the rifle. Therefore, it is best to perform a shot on exhalation, with empty lungs, when a sniper can “freeze” for several seconds. To extend the pause between breaths, the shooter must take a deep breath twice before shooting to saturate the blood with oxygen. However, when the count goes for seconds, the sniper may simply not have enough time for two deep breaths. Then the “hardening” technique is applied - for lungs filled half or three quarters.
The firing pattern at an angle to the horizon The dashed line shows the absolute reduction of the bullet at a given distance. Without special amendments, the bullet will fall above the target.
According to the ballistic curve, Gravity acts on a flying bullet in the same way as on any falling object. During the flight, the bullet is significantly reduced, which can lead to a miss. The reduction can be calculated from the tables or using a ballistic calculator, but you need to accurately determine the distance.
Train your eye! Good old "thumb method" allows you to quickly assess the distance
There are a number of situations where the visual determination of distance even with a trained eye can give an error. In some cases, objects may seem closer: in a depression hidden behind hills, viewed from top to bottom, along long straight landmarks such as rails, or against a contrasting uniform background like snow or sand. In other cases, objects may seem farther than they are actually located: against the background of large objects and structures, when viewed from the bottom up, in a narrow space or in a visible lowland.