By Gianni Bauce This is the fifth in a series of articles on firearms with specific…
By Gianni Bauce
This is the fourth in a series of articles on firearms with specific reference to Firearms syllabus of the Learner Hunters/Guides exams. The topics to be covered in separate posts are:
Part 1 – Safety and Firearms
Part 2 – Types of Firearm & the Parts of a Firearm
Part 3 – Ammunition
Part 4 – Ballistics
Part 5 – The Use and Care of Firearms
Part 6 – Firearms & the Law
Ballistics is the field of mechanics concerned with the launching, flight behaviour and impact effects of projectiles, especially ranged weapon munitions such as bullets or rockets. (N.B. a projectile is an object that has been thrown or launched – a stone, an arrow and a bullet can all be projectiles once they are flying through the air.) To put it another way ballistics is the study of the natural laws that govern the performance of projectiles and the use of these laws to predict their performance. It tells you what is happening to a projectile after you pull the trigger and predicts the performance of the projectile by applying the basic natural laws of energy, gravity and friction.
It is a very broad field of study and it is broken down into three categories:
1) Interior or Internal ballistics – what happens inside the firearm’s chamber and barrel, from the instant the trigger is squeezed to the moment projectile exits the muzzle.
2) Exterior or External ballistics – what happens to the bullet after it exits the muzzle to the point just prior to hitting the target.
3) Terminal ballistics – what happens to the bullet when it hits its target, the effect and performance on striking and entering for example an animal.
We will now look in more detail at each of these aspects of ballistics.
INTERIOR OR INTERNAL BALLISTICS
This concerns what happens inside the firearm’s chamber and barrel, from the instant the trigger is squeezed to the moment projectile exits the muzzle.
Aspects of Internal Ballistics
- Lock time: the time interval between pulling the trigger and exploding the primer, or between the release of the sear and the impact of the striker on the percussion cap.
- Ignition time: the rate at which the powder burns.
- Barrel time: the time taken for the bullet to travel through the barrel.
EXTERIOR OR EXTERNAL BALLISTICS
This concerns what happens to the bullet after it exits the muzzle to the point just prior to hitting the target. External ballistics is the study of the natural laws governing the performance of a projectile in flight and the use of these laws to predict the projectile’s performance and point of impact.
Factors affecting the external ballistics are:
- Calibre of the bullet
- Sectional density of the bullet,
- Ballistic coefficient of the bullet,
- Velocity and energy at the muzzle,
- Trajectory and Angle of Departure,
- Type of bullet, shape and material the bullet is made of,
- Spinning velocity,
- Range of target,
- External factors such as wind
We will now examine each of these in turn.
The calibre of the bullet is the diameter of the bullet head, measured either in millimetres or in inches. A bullet which is described as .458 has a diameter of 0.458 inches; a 9 mm bullet has a diameter of 9 mm or 0.355 inch, but this last is usually expressed in millimetres. It is one of the most important factors to take in account when considering “stopping power” because the larger the calibre the larger the wound it creates. The minimum calibres recommended for areas of operation where thick-skin animals and large carnivores occur is .375 H&H.
The Sectional density of the bullet is a mathematical factor generally expressed as a three-place decimal figure representing the ratio of bullet mass to its cross-sectional area.
|TYPE OF GAME
|.200 – .230
|.270 – .280
* Recommendation from Mike Lagrange for very large game – .350 lb/inch
The ballistic coefficient of the bullet is a number given to a bullet to express its ability to overcome the resistance of air as a result of its shape, length, weight, diameter and nose design all of which affect its ability, velocity and range against air resistance.
Velocity or projectile speed is measured in “feet per second” or “centimetre per metre” at the muzzle. This velocity is calculated by using a chronograph, an instrument to measure the time required by a bullet to pass between two precisely measured points.
Muzzle velocity – the velocity (speed) of the projectile when it exits the muzzle.
As opposed to Remaining velocity – speed of the projectile in a given point of its trajectory.
Muzzle energy – the amount of work capable of being done by a projectile at the muzzle of afirearm (the kinetic energy possessed by the projectile as it exits the barrel).
Trajectory – the location of a projectile above or below the line of sight, that is the actual path followed by a bullet or projectile in motion (sometimes called “bullet path”.
The bullet begins decelerating and falling towards the earth the instant it leaves the muzzle. The rate of deceleration and fall are continually increasing and follows a parabolic curve. Trajectory is generally measured in two ways,
1) Mid-Range Trajectory Height (MRTH) – the widest distance between the line of sight and the trajectory. In other words, the distance between the line of sight and the Culminating Point.
2) Drop – the measurement taken at right angles to the extended centre line of the bore down to the path of the bullet at any given point.
External ballistics involves the measurement of a wide variety of variables, almost all of which are represented by initials that you will find in the diagram below and explained below that.
- AOD – Angle Of Departure – the angle formed between the LOD and the base line
- AOE – Angle Of Elevation – angle formed by the Line Of Departure (LOD) and the Line Of Sight (LOS).
- CP – Culminating Point – highest point attained by a bullet in flight, beyond when it starts to dip
- LOD – Line Of Departure – the particular or actual line followed by a projectile when it leaves the muzzle of a firearm.
- LOS – Line Of Sight – an imaginary line from the eye through the sight (rear and fore) to the target or point of aim at a given range.
- MRT or MRTH – Middle Range Trajectory (Height) – the highest vertical distance of a bullet above the line of sight at a point approximately halfway from muzzle to target or point of impact or the maximum height above LOS that a bullet would impact on its way to a given zero range
- POI – Point Of Impact – point where the bullet first hits the target or the ground
- PPOI – Where the trajectory and LOS cross-meet. If the aim is correct, it coincides with the point the bullet hits the target
Bullet shape & material
External Ballistics are affected by the type and shape and material of a bullet. These matters are explored at great length in Section 3 of this series, under Ammunition.
(N.B. Bullet mass is expressed in terms of Grains and not Grammes.
1 Grain = 0.065 g and 1 Pound = 7000 Grains
Spinning Velocity (owing to rifling) can result in drift, the deviation of a projectile due to the rotation or spinning.
Range of target
Range of target is the distance between the exit point of the bullet to the impact point
Zeroing of a firearm is performed according to a certain range of target. Shooting a target located at a different range, means to get different result on the point of impact. In other words, if the range of the target is not the one considered in zeroing the firearm, the shot may result too high or too low depending on the actual range of target.
The diagram shows that, at a range of target ranging from the muzzle to just before the point blank, the shot will be low (target A).
At a range of target coincident with the point blank (target B) and at the zeroing range of target (target E) the shot will be perfectly in the centre of the target, since LOS and trajectory coincide.
At a range of target between the point blank and the zeroing target (targets C and D), shot will be high, with the higher shot at the range of the culminating point of the trajectory (target C).
The only difference between shots B and E is the angle of impact of the bullet, as shown in the diagram below:
External factors such as wind operate of external ballistics. The most ob vious example is wind drift – the deviation of a projectile from the line of sight due to the wind force.
This concerns what happens to the bullet when it hits its target, the effect and performance on striking and entering, for example, an animal and transferring its energy into the flesh of the animal.
Factors Affecting Terminal Ballistics
Bullet design and the velocity of impact are the factors which mostly influence the terminal ballistic.
Bullet performance on or after impact varies with the difference in animal species, animal size and the condition under which it is hunted. For this reason, a wide range of calibres and bullets are manufactured each designed for a specific purpose.
The Knock-Down or Stopping Power of a Bullet
This refers to the ability and stop a determined charging dangerous animal, to kill it effectively. Calibre is the single most important factor in providing stopping power (provided that the bullet has enough momentum to ensure maximum penetration). Given similar energy value, the larger the calibre of a weapon, the greater the stopping power delivered. Generally, when shooting thick skinned animals, large calibres (particularly with solid bullets) should be used, in order to deliver the greatest blow at the point of impact, apart from also increasing wound size.
Hydrostatic Shock Theory
This refers to the violent expansion of an animal’s body fluids and tissues when struck by a bullet at high velocity, causing massive haemorrhage and shock to the animal. According to this theory, the knock-down power is due to more than just the foot-pounds of energy released, because the hydrostatic shock needs to be added to this.
Furthermore, this theory implies that the rotational speed of a spinning bullet (which can be in excess of 200 000 rpm) due to the rifling twist is also energy that literally explodes the body tissue and fluids on impact.
(Discussed at length in the previous Firearms section on Ammunition)
Soft-nosed bullets have the ability to expand on or after impact, deforming to take the shape of a mushroom. The exact shape of the “mushroom” is influenced by the shape and material of a bullet and by the velocity of the bullet.
A bullet that is changing shape in this way is causing maximum damage in a confined area to a target and therefore having greater stopping power
Firearms are tested by manufacturers and by firearm dealers as well. Here some important information about firearms testing.
Minute Of Angle (MOA)
MOA is the measurement depicting the accuracy of a firearm. Technically it is to is 1/60 of a degree, subtended at 100 yards. A gun that is capable of shooting 3 bullets into a groups smaller than 1 inch across at 100 yards is said to shoot “minute of angle” groups. This is most desirable for precision and good hunting shooting. In reality. If a firearm cannot do this, it is not worth keeping.
This is a method of proving that a weapon is safe to use, in accordance with the firearms factory’s specifications. The test is done by using 30% to 60% overcharged cartridges in a controlled environment. If the weapon is OK after subsequent firing and inspection, then it is stamped with the word NITRO PROVED (N.P.) on the barrel. (This test is not done on every weapon.)
We have looked above at a number of aspects of the science of ballistics and its role in helping us to understand the performance of firearms and projectiles. In every case there is far more to be learnt but we have examined the important distinctions that will operate on our use of firearms