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Bullets go ballistic

The science of projectiles

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I'm sure we are all familiar with how dangerous bullets are, especially in light of the recent civil unrest in Bangkok. Weapons and bullets are designed to maim and kill. The mechanics of how deadly projectiles work make for fascinating science. 

A shell casing flies out with a trail of smoke as US Army Pfc Michael Freise fires an M-4 rifle during a reflex firing exercise at the Rodriguez Live Fire Complex, Republic of Korea, on March 23, 2005. The annual, multi-phase exercise is tailored to train, test and demonstrate US and Republic of Korea force projection and deployment capabilities. US DEPARTMENT OF DEFENCE/STAFF SGT SUZANNE M. DAY, US AIR FORCE

Ballistics is the science of projectiles, with emphasis on bullets, bombs, missiles. As far as inventions go, weapons have altered the course of history, arguably more than anything else. Before bombs and bullets, crude weapons were used in warfare, and warriors had to be up close and personal in order to slay the enemy.

Nowadays, a soldier can literally be kilometres away from an unseen target and still cause property damage and kill. Indeed, new-age bullets can be shot around corners, can be guided and can record and transmit information about their surroundings once they stop. This is very valuable in a war zone.

The strategic deployment of bombs, bullets and missiles has changed governments, countries and history over the centuries. Therefore knowing a little about how they work is important.

Bullets and firearms

The word "bullet"comes from the French word boulette, which generally means little ball. Indeed, bullets predate firearms (handguns and rifles) by hundreds of years as the early bullets were, in fact, little round stones. Think slingshot.

And later, in the 1700s to 1800s, bullets were still round stones launched by loose propellant (gunpowder) stuffed in the barrel of a musket or rifle. They quickly became known as "rounds".

Versatile .270 cartridges, from left: 100-grain (6.5g) hollow point; 115-grain (7.5g) FMJBT (full metal jacket boat-tail); 130-grain (8.4g) soft point; 150-grain (9.7g) round nose. SNOWJACKAL

Later, the little stone nuggets were replaced with round balls made of lead. See the picture at above right.

A cartridge, also called a round, comprises four parts: the casing, the primer, gunpowder (or cordite) and the bullet (the actual projectile). See the graphic at far right. The primer is a small charge of an impact-sensitive chemical that "fires" the bullet out of the barrel.

Cartridges come in precise sizes and shapes. For example, a .45-calibre bullet fits a Colt .45 pistol; a .9mm handgun fires .9mm ammunition, and so on.

Bullets generally work in a three-part process. The back of a cartridge casing contains a primer. This is where the initial ignition occurs, but not where the acceleration is produced. The primer is like a fuse, where a smaller reaction starts a bigger reaction.

The primer is ignited when the trigger is pulled and a metal firing pin hits the primer part of the cartridge with great impact. It is the impact of the firing pin on the primer that ignites the propellant.

The propellant then starts the chemical reaction and "fires" or propels the bullet out of the barrel at tremendous velocity.


The exploding gunpowder is what propels the bullet out of the gun. The exploding gunpowder produces an enormous amount of pressure that can go nowhere but down the barrel of the gun. As this happens behind the bullet head, the bullet is forced out of the barrel at incredible speeds of several hundred metres per second.

As the bullet travels to the end of the barrel and exits, it reaches maximum velocity. Due to Newton's third law of motion - which states that every action force has an equal and opposite reaction force - the gun recoils with the same force that moves the bullet forward: the action and reaction.

Due to Newton's second law, which is summarised by the formula force = mass multiplied by acceleration, if you rearrange the formula to become acceleration = force divided by mass, it makes more sense.

Due to the heavy mass of the gun (relative to that of the tiny bullet), the force divided by the mass means that the acceleration of the gun is much less than that of the bullet.

Little bang theory

The sound, bang or pop of a gun firing a bullet is produced by the detonation or pressure wave created when the explosion inside the barrel emerges. It's a bit like when you pop a champagne cork, only with a bullet the speed and pressure are much greater and the pressure wave results in the sound wave that your brain converts into a bang.

The gunpowder is incredibly hot as it exits the gun's barrel, and is what causes powder burns on a victim when shot at close range. A silencer is sometimes used to reduce (but not totally eliminate) the sound of the gunshot.

Inside the silencer are many small chambers that baffle the explosive sound, the expanding gases of the ignited propellant and flash generated when a bullet is fired from a gun.

Cartridge parts. A modern cartridge consists of: 1. the bullet itself, which serves as the projectile; 2. the casing, which holds all the parts together; 3. the propellant, for example, gunpowder or cordite; and 4. the primer, which ignites the propellant. POST GRAPHICS

How bullets maim, kill

Normally, there are no explosives contained inside the bullet part of a cartridge. The intended target is maimed or killed merely by the awesome impact and penetration by the bullet.

A fired bullet travels at high velocity and has a lot of momentum. Momentum is a force that is calculated by multiplying the mass of an object by its velocity. In contrast, a speeding bullet has a much more devastating impact.

The accuracy of a bullet is determined by its gyroscopic and aerodynamic qualities. To help a bullet travel in a straight line, the bullet rotates as it leaves the barrel in a similar fashion to a gyroscope. This is caused by the rifling grooves that are carved inside the gun's barrel.

Rifling grooves are as individually distinct as fingerprints and have helped to solve many murder cases. The rotation gives the bullet gyroscopic inertia, meaning that it resists forces acting to alter its motion, such as wind motion and air resistance.

When a bullet hits a target, such as a person, the sudden impact - called strike velocity - decreases its high momentum very abruptly, which produces a huge amount of kinetic energy. The bullet's kinetic energy is transferred to, for example, human tissue.

The damage inflicted by a bullet is determined by the distance between the target and the gun, as well as the size, design and speed of the bullet upon impact.

Bullets penetrate flesh easily due to their conical tip. Taking into account that pressure is worked out by taking the force divided by the area. As the area of a bullet tip is tiny, and due to the force with which the human target is hit, bullets easily penetrate flesh and bone.

Bullets can puncture and collapse lungs, penetrate brains, and if shot into the torso, can spin tip over base and shred organs, causing internal bleeding. Many impacts result in death.

If a person is only wounded in the arms or legs, they can bleed to death if the blood flow is not stemmed.

Bullets shot into the air

When a bullet is shot out of a gun, the speed at which it travels is far greater than the terminal velocity of a bullet, that is, the maximum speed at which a bullet falls to earth due solely to gravity when dropped from, say, a helicopter.

Basically, due to the mass of the bullet and air resistance acting upward against a falling bullet, the speed at which a bullet can vertically fall reaches a maximum velocity.

Realistically, this speed, multiplied by the bullet's mass, would probably not be enough to kill a person, although it would certainly hurt if you were hit by even a falling bullet. Similarly, this can happen when a bullet is fired straight up into the air and begins falling back to earth due to gravity.

At a shallower angle, however, the gyroscopic motion still causes the bullet to maintain a high speed, meaning that even as a bullet falls toward the earth, it is still travelling extremely fast and still has huge momentum, which can kill.

The point of this article is to help you to understand the physics involved in ballistics and to appreciate just how dangerous handguns and their bullets are. I hope you agree with me that weapons of all kinds should be outlawed.

Dave Canavan has an MSc in Behavioural Ecology and is the Head of Secondary at Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! Contact Dave at davidc@gardenbangkok.com.

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