Читать книгу How to Estimate Range and Wind - Pseudonym Sniper - Страница 4

Ballistics

The Basics

Before I explain to you the techniques for range estimation and how to make allowances for environmental factors, I need to cover off with you the basics of ballistics.

I say the “basics” because that is exactly what this will be…a basic introduction to some concepts.

The subject of Ballistics could be the subject of an entire book (in fact there are several books that have been published on the subject) and so, if after reading the following section of this book, you feel the need to know more, feel free to do further reading.

If you are an experienced shooter, you may choose to skip this section and head straight to the range estimation section of this book. For the less experienced, stick with me and I'll ensure you finish up with a basic understanding of ballistics and how it can affect your fall of shot.

For those of you still with me, ballistics can be broken down into three sections or terms: Internal, External and Terminal.

Internal ballistics refers to the interior workings of a particular weapon system and how that weapon system’s ammunition functions.

External ballistics refers to the flight of the bullet once it leaves the muzzle of that particular weapon system, until such time as it reaches the target.

Terminal ballistics refers to what happens to the bullet after it hits the target.

Terminology

To grasp a basic understanding of ballistics, the shooter needs to be familiar with terminology used by other shooters and other experts. The following list contains both the term and its definition:

Muzzle velocity refers to the speed of the bullet as it leaves the muzzle (the end of the barrel) and is measured in feet per second (fps). It can fluctuate due to outside influences, such as ammunition type and batch/lot number and can also be affected due to environmental factors, such as heat and humidity, which we will cover off later in this book.

Line of Sight refers to the imaginary straight line that can be drawn from the eye, through the aiming device or scope, to the point of aim.

Line of Departure refers to the imaginary straight line that can be drawn from the bore of the rifle and indicates the bullet’s flight path if it was not influenced by gravity, drag, etc.

Trajectory refers to the actual flight path of the bullet as it travels from the muzzle of the rifle to the intended target. As you would expect, knowing the trajectory of the bullet fired from your rifle is critically important when shooting at long ranges.

Midrange or Maximum Trajectory (aka Maximum Ordinance) refers to the highest point in the bullet’s trajectory/flight path on route to the target. As the trajectory/flight path is an arc, the shooter must be aware of this point as he/she may be required to shoot under an obstacle to engage the target. Failure to know the midrange trajectory may lead to the bullet striking the overhead obstacle rather than the intended target.

In Commonwealth Military States, this is often referred to as the Culminating Point.

Bullet drop refers to how far the bullet drops from the line of departure (see above) to the point at which it impacts the target.

Time of flight refers to the time taken for the bullet to travel from the rifle to the target.

Retained velocity refers to the speed of the bullet at the point of impact.

Environmental Factors Influencing Trajectory

The trajectory (see above for definition) of your bullet can be influenced by a number of Environmental factors and you must be aware of these and how they affect your bullet’s trajectory to ensure that your first round is on target and effective.

The most obvious factors are gravity and drag.

Gravity is a constant factor and the result is that as soon as your bullet leaves the muzzle, gravity begins to pull it towards the earth. This requires the shooter to use the elevation adjustment drum or hold over techniques to engage long range targets. At extended ranges, this may mean that the line of departure actually intersects with the light of sight, allowing the bullet to travel in an arc, with gravity bringing the bullet back down onto the intended target.

Drag is essentially the effect that the atmosphere has on your bullet during its flight, causing it to slow. Factors influencing drag are temperature, altitude, humidity and wind. I will discuss wind later in the book.

In regards to temperature, the higher the temperature, the less dense the air. As the air is less dense, the drag on the bullet is less. This can result in a higher point of impact.

A change in altitude can affect your trajectory also.

For example, an increase in altitude results in a reduction in air pressure and air density. This increases the bullet’s efficiency, as there is less drag, which in turn results in a Point of Impact (POI) that is higher than the Point of Aim (POA).

Humidity is another factor that can influence trajectory.

Humidity varies along with the temperature and altitude and affects the trajectory in the following manner: when the humidity increases, the air is less dense, and so the Point of Impact is higher; when the humidity decreases, the air becomes more dense and so the Point of Impact drops.

Q.) With all these environmental factors in play, how can you best estimate your bullet’s trajectory?

A.) Density Altitude.

Density Altitude

Density Altitude is the most accurate method of judging a potential shift in trajectory.

Wikipedia defines Density altitude as“the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground. ‘Density Altitude’ is the pressure altitude adjusted for non-standard temperature.

Both an increase in temperature and, to a much lesser degree, humidity will cause an increase in density altitude. Thus, in hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude”.

So, essentially, Density Altitude is the actual density of the air at your location. It is a direct representation of the environmental conditions that the bullet “feels” like it is flying through.

A Kestrel Weather Gauge is a great tool for getting this reading as it takes into account the temperature (both true temperature and ISA standard sea level temperature), the atmospheric pressure (both true atmospheric pressure and ISA standard sea level atmospheric pressure) and the humidity.

Once you have the Density Altitude figure, you can enter it straight into your Ballistic Software or use a Ballistic table that charts Density Altitude to get your firing solution.

You can also calculate the Density Altitude manually; however, I won't be covering how to do so in this book. The calculation does not take into account humidity, and as such, it is not as accurate as using a Kestrel Weather Gauge to determine the Density Altitude.

If you would like to know how to calculate Density Altitude manually, take a look at the following article by Linden B. Sisk:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/ManualDA/

Linden also describes how to create a Ballistic table using Density Altitude in the following article:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

I suggest doing this initially to give yourself a generic Ballistic table that will give you a good indication of the change in Trajectory based on any Density Altitude changes.

Once you have collected enough data of your own, use it to produce your own Density Altitude Ballistic table as it will be more accurate than the "generic" one as the data is specific to your rifle and ammunition.

I suggest checking all of your data tables if you change ammunition lot numbers or type.

As a shooter, it is your responsibility to create and maintain a data book in which you record the temperature, altitude, humidity and density altitude readings, in addition to the usual data.

In the following chapters, you will encounter numerous mathematical equations that are used by Military/LE Snipers and Long Range shooters. But don’t worry, as I have explained them all in detail and provided examples to help you to understand them.

Towards the back of this book, there is an Equation Quick Reference Guide page to help you find the equation you are looking for quickly and easily.