Principles of Ballistics and Projectile Motion
- 1. Introduction to Ballistics Callibre, Internal Ballistics Lecture by: Dr. Attia Razzaq
- 2. BALLISTICS CALLIBRE True calibre is a measure of the internal bore of a weapon. However, in common usage, calibre refers to the type of cartridge a gun is designed to fire. An example of a common pistol calibre is the 9 mm Parabellum, the most common pistol calibre in the world. Confusingly, there are often several different designations for the same calibre, and so 9 mm Parabellum is also called 9 mm Luger, 9 mm P, 9 mm NATO or 9 x 9 mm. In addition, there are other 9 mm calibres such as 9 mm Makarov and 9 mm Short. These cannot be discharged in a 9 mm Parabellum pistol nor are they compatible with one another.
- 4. BALLISTICS CALLIBRE For this reason, forensic scientists should always be specific in their statements and will normally comment on the compatibility between any weapon and ammunition examined. Calibres with metric dimensions are usually European in origin whereas those with imperial dimensions are usually North American or British in origin. Obsolete calibres are calibres for which ammunition is no longer commercially available, and weapons chambered in these calibres are often regarded as antiques under UK law.
- 6. BALLISTICS INTERNAL BALLISTICS The subject of internal ballistics covers the time from when the primer is struck until the projectile exits the barrel. When the trigger is pulled, the firing pin will strike the primer at the base of the cartridge. This causes a shower of sparks to ignite the propellant powder in the cartridge case. The propellant powder burns at a very high rate, creating a large volume of gas and a substantial increase in pressure. The pressure is contained by the breech block at the rear of the cartridge and the barrel surrounding the cartridge, so that the pressure will act on the projectile (or the wad in a shotgun), driving it down the barrel.
- 7. BALLISTICS INTERNAL BALLISTICS The rate at which the propellant burns is calculated to ensure that the pressure continues to rise so that the projectile travels down the barrel. One might expect, therefore, that the powder in a pistol cartridge would burn more rapidly than the powder in a rifle cartridge, the slower burning of the rifle cartridge ensuring constant acceleration of the projectile down the longer barrel. It also explains why the same projectile fired from the same cartridge but from a weapon with a shorter barrel will produce a lower velocity than from a long barrel. Similarly, projectiles from a ‘sawn-off’ shotgun or rifle will produce lower velocities.
- 8. BALLISTICS EXTERNAL BALLISTICS The subject of external ballistics deals with the behavior of the projectile after its exit from the barrel, during its flight and then when it makes contact with a target – this is the trajectory. Many factors combine to influence the trajectory of the projectile. When in flight, the main forces acting on the projectile are gravity and air resistance (which can take the form of both drag and wind deflection). When looking at small arms external ballistics, gravity imparts a downward acceleration on the projectile, causing it to drop from the line of sight. Drag or air resistance decelerates the projectile with a force proportional to the square of the velocity. Wind makes the projectile deviate from its trajectory.
- 9. BALLISTICS EXTERNAL BALLISTICS As a result of gravity, a projectile will follow a parabolic trajectory. To ensure that the projectile has an impact on a distant target, the barrel must be inclined to a positive elevation relative to the target line. This is known as sighting the weapon and explains why a weapon has to be sighted at different ranges. To give a practical example, a projectile fired from a rifle sighted to hit a target at 150 metres might also strike the point of aim at 50 metres but will shoot high at 100 metres and low at 200 metres.
- 10. BALLISTICS EXTERNAL BALLISTICS A. An unsighted rifle will miss the aim point at 150 metres owing to the effects of gravity and deceleration on the projectile’s trajectory. B. A sighted rifle will compensate for these effects with a parabolic trajectory and hit the aim point at the 150 metre mark.
- 12. BALLISTICS EXTERNAL BALLISTICS Projectiles discharged from a rifled barrel are spin-stabilised. This is the spin created by rifling as the projectile passes through the barrel. The spin gives the projectile gyroscopic stability, preventing it from tumbling in flight. Without this spin being imparted to the projectile, it quickly becomes unstable and accurate shooting is impossible. Ballistic tables predict the path of a bullet by considering the many external factors above.
- 13. BALLISTICS EXTERNAL BALLISTICS If the ballistic coefficient (BC) of a projectile is known – it combines the air resistance of the bullet shape (the drag coefficient) and its sectional density (a function of mass and bullet diameter) - all parameters relating to ballistic flight can be calculated. These are sometimes used by forensic scientists to determine, for example, the maximum distance a projectile fired from a particular weapon could travel. It should be noted, however, that use of this type of information is rare, as most criminal shootings take place at very close range, rarely exceeding a few metres.
- 14. BALLISTICS TERMINAL BALLISTICS Terminal ballistics includes the study of wound ballistics, and generally relates to the behaviour and effects of a projectile when it has an impact on a target and transfers its energy thereto. Bullet design and the velocity of impact largely determine how effective the contact is. ‘Terminal ballistics’ covers the impact of any projectile striking any target, but often concentrates on the effects of small arms ammunition on a live target, human or animal, and the ability of the projectile to incapacitate or kill. Significant factors are o bullet weight, o composition, o velocity and o shape.
- 16. BALLISTICS TERMINAL BALLISTICS Projectiles are designed either for maximum accuracy or for penetration of a target while avoiding over-penetration. They thus cause maximum damage to the intended target, but minimise the risk of peripheral unintended damage. Frangible bullets are designed to disintegrate when they impact a target, particularly a hard target. This reduces the risk of over-penetration and contact with unintended targets. They are often used for training or for law enforcement in densely populated areas. Frangible bullets are intended to disintegrate into tiny particles upon target impact to minimize their penetration of other objects. Small particles are slowed more rapidly by air resistance, and are less likely to cause injury or damage to persons and objects distant from the point of bullet impact.
- 17. BALLISTICS TERMINAL BALLISTICS Expanding bullets are designed to expand or fragment shortly after impact. This causes a rapid transfer of the bullet’s energy, thereby increasing tissue disruption, speeding incapacitation and increasing the likelihood of death. It also reduces the chance of over-penetration (where the projectile leaves the intended target and may accidentally make contact with a secondary target). Expanding bullets, also known as dumdum bullets, are projectiles designed to expand on impact. For this reason, they are used for hunting and by most police departments, but are generally prohibited for use in war. Two typical designs are the hollow-point bullet and
- 19. BALLISTICS TERMINAL BALLISTICS Armour-piercing bullets are designed to defeat hard targets. They have a mild steel or hardened steel core and designed to stay intact on impact to aid penetration power. Hard targets include body armour and armour used to protect vehicles.