[MUSIC] This lecture's about firearms. In some countries firearms are very rare and those that do exist are strictly controlled. And gun crime is rare. In other countries, ownership of firearms is much more widespread, control is laxer and gun crime is more common. When we talk about the forensic science involved with firearms, it's really divided into three parts. There's the internal ballistics, which is about what happens while the projectile is still within the barrel of the gun. There's the external ballistics, which is what happens when the projectile is in flight between the gun and the target. And there's the terminal ballistics about what happens when the projectile hits the target. So what we're going to talk about in this lecture is the internal ballistics. What happens in the barrel, and how this affects the use of firearms in evidence? So let's take a quick look at guns starting with the original guns, the early guns, such as this one here, the Tsar Cannon in the Kremlin in Moscow. Early guns are what we call muzzleloaders. That is, everything that has to go into the gun has to be shoved through the muzzle and down the length of the barrel to the sealed end. What muzzleloaders needed, was some explosive and a projectile. A fuse would then be lit. The fuse would burn down into the explosive. That would detonate, and then the energy released, the pressure and the temperature from the explosion of that material, will cause the projectile to shoot down the barrel at high speed. Guns come in all sorts of sizes, from very, very small, to very, very big. Here's an example of what we might consider the first of the superguns. And this a gun known as the Great Turkish Bombard. And several of these were built for the Siege of Constantinople by Mehmed the Conqueror back in 1453. And these were enormous guns, five meters long with a 760 millimeter internal diameter, made out of bronze. They fired cannon balls which were carved out of granite and weighed 680 kilos. And because of the size of the gun, it took a lot of men and a lot of oxen to move the thing around. So obviously, you couldn't fire this thing very often, and they were able to fire it seven times a day. Nevertheless, it was very important in smashing down the walls of Constantinople in 1453. And 300 years later, these guns were still in use by the Turks to protect their city. Since then, engineering has improved. We no longer make guns out of bronze. They're made out of steel nowadays. This means that we can make the barrels much stronger. Which means that we can release a lot more energy inside the barrel without the barrel bursting, ie, we can get much higher temperatures and pressures and therefore we can fire projectiles much further and much faster. So this is the muzzle loader. Not only did materials improve, but also the design of guns improved. So these muzzleloaders were replaced in the 19th century with the breech loaders. So in a muzzle loader, everything has to be put down the barrel. In the breech loader, it's loaded front of the breech. This is much more efficient and means that the firing rate can be increased. And nowadays of course, the loading of a new projectile into the barrel is automatic. A third improvement is the invention of the cartridge. The early guns like that Tsar Cannon we saw, the explosive charge and the projectile were separate and had to be loaded into the barrel separately. The cartridge combines both the projectile, ie, the bullet, and the explosive, which provides the energy into a single object. And clearly, loading a single object into the barrel is much more efficient. Well let's look at these cartridges, okay?. What is the detailed structure of a cartridge? You have to have a bullet. Then behind the bullet is the propellant which provides the energy for the bullet, and this is contained within the cartridge case. Now the propellant provides the hot, expanding gases which gives the kinetic energy to the projectile, and they will be materials such as black powder or nitrocellulose. These do not ignite on their own. There has to be something to make the propellant ignite. And this is a chemical built into the base of the cartridge and this is called the primer. Now the primers are shock-sensitive compounds. These detonate when the experience a mechanical shock. The detonation of the primer then ignites the propellant and causes the gun to fire. Typical primers that are used are chemicals such as lead styphnate shown on the slide. So now we know what the cartridge is made of. What happens to each part of the cartridge? Well, the bullet, of course, is shot towards the target, so it will typically be far away from where the firing took place. What about the cartridge case? When the gun is fired, the cartridge case is ejected from the breech as we saw in the photograph just now. This means that the cartridge case will be lying at the site of the firing unless of course, the gunman takes specific precautions. Either he collects the cartridge case after firing, or uses another technique that we've seen a few times now, in that he's firing the gun from inside a sack so that the cartridge cases are collected inside the sack. Usually, however, the cartridge case is at the scene of the firing. Finally, the propellant. What happens to the propellant when it ignites? Well this is messy. It's turned into gases and particles. And we'll talk about these gases and particles in the next section. Now, we said that the primer detonates, and it's the primer detonation that causes the propellant to ignite. What is the mechanical shock that causes the primer to detonate? It is the impact of the firing pin on the end of the cartridge. When the trigger of the gun is pulled, that releases the firing pin to hit the end of the cartridge, and that causes the gun to fire. The base of the cartridge is made of metal, so when the firing pin hits it, it will leave an impression. And this is the first piece of forensic evidence from firing a gun. The impression of the firing pin on the base of the cartridge. Firing pins are going to be individual. They are going to become worn in particular ways. So no two firing pins are going to leave exactly the same mark. So, suppose there is a crime scene where a cartridge case is found. A suspect is apprehended who possesses a gun. The question is, was that gun used to fire the shot at the crime scene? The way this is investigated is the gun is loaded and a test bullet is fired, and the test cartridge case is obtained and compared using a comparison microscope to the cartridge case found at the crime scene. If those firing pin marks on the two cartridge are identical, then it can be concluded that that gun indeed was used. Another technical improvement that was made in guns is rifling. Early guns, such as the Tsar Cannon that we saw, have smooth barrels. Rifling is the cutting of grooves along the barrel. These grooves are not straight, but they curve. When the bullet or the shell, is fired, it's gripped by those grooves, and that gives it some spin. And this spin, just like the bowler in cricket giving the ball some spin, improves accuracy, and the stability of the firing. Most guns are rifled. Some guns are not, for instance shot guns are not rifled. They fire pellets. And typically home-made weapons are not rifled either. The gun in the picture is a seven inch gun from Singapore's Labrador Park, and the breech is missing, so you can look down the gun and you can see the pattern of the rifling. What's the forensic consequence of this rifling? The forensic consequence of the rifling is that the groves and the lands, when they grip the projectile moving down the barrel, leaves scratch marks on the side of that projectile. These scratch marks are known as striations. Now just as every firing pin will be individual, each barrel will also be individual, and will leave characteristic striations on the bullet. The procedure is the same. If you have a bullet from a crime scene, you have a suspect gun. You once again reload the gun, fire it, this time you collect the bullet. You compare the test bullet with the bullet from the crime scene and of the comparison microscope. If the scratch marks match up, you can conclude that that bullet was indeed fired from that gun.
