7 Firearms
Not all forensic investigations will involve evidence relating to firearms, but when they do, they will generally be serious crimes. Evidence obtained from the use of firearms includes: 
bullet or pellets 
spent cartridges 
the actual gun 
damage to persons or materials by the bullet 
location of the bullets 
residues from the discharge of the gun which can be found on both the firer and the victim 
markings on the bullets which can identify the actual gun from which they were fired Analysis of these pieces of evidence can then lead to the following: 
what type of gun & bullet 
the actual gun 
who fired the gun 
where they person fired the gun was standing 
where the person hit by the bullet was standing The analytical methods used will include: 
chemical analysis 
microscopy 
database searches 
projectile motion The forensic examination of firearms evidence is often called ballistics, but this is an inaccurate use of the term, which really refers to the study of any projectile as it moves through the air. Since this is the only piece of evidence that wouldn’t be available, what the term really refers to is part of the background theory that is used in forensic reconstruction of what really happened. You need to know about guns before you can make sense of the evidence they generate. So these notes are about the basics of the hardware only. How guns work The first thing to make clear is what goes into a gun is called a cartridge but what comes out the barrel of a gun (other than a shotgun) is the bullet. The diagram to the right shows the construction of a typical cartridge. The bullet is held within the case, and is ejected when the propellant is exploded by the primer. The empty case remains in the gun or is ejected by an internal mechanism (depending on the type of weapon). The primer is activated by mechanical pressure from the firing mechanism of the gun. This is known as the firing pin or hammer and is a spring‐loaded piece of metal which when released hits the primer with enough force to cause it to explode, igniting the propellant, and the gases released by this reaction are what provide the forward motion of the bullet. 7. Firearms As the bullet passes down the barrel of the gun, it picks up speed due to the expanding gases behind it, and it also starts to rotate, due to the rifling (or spiralled grooves) along the barrel. This stabilises the trajectory of the bullet once it emerges from the barrel. It also creates a marking pattern on the bullet which can be unique for that particular gun, and becomes a crucial piece of evidence. Bullets are most often made from lead, but it is a soft metal and has limited penetrating power and in rifles designed for long distance use, the lead gets hot enough to melt inside the barrel. Lead bullets cased in a cupronickel jacket or made from a harder, higher melting metal altogether are used in these specialised situations. Depending on the surface into which a lead bullet penetrates, it may become quite deformed, as shown in the images to the left. Some gun users, wishing to cause more damage to their target, deliberately damage the bullets so that they tumble in flight. Obviously accuracy and velocity suffers, but the impact point is larger. Types of firearms If we exclude large military weapons, such as machine guns and rocket launchers, the basic type of firearms likely to be encountered in criminal investigations are: 
handguns (or pistols) – revolvers, semi‐automatics 
long guns – rifles, shotguns The difference between hand‐ and long guns is obvious, but unless you are knowledgeable in this area, the difference between the weapons within these types may not be. A revolver has a cylindrical chamber above the handgrip which stores a number of cartridges (frequently six). As one is fired, the chamber rotates to bring the next up ready for firing. This is done mechanically by the firer. A semi‐automatic has a spring‐loaded magazine of cartridges in the handgrip. Part of the energy produced by burning cartridge powder is employed to remove the used cartridge from the chamber, cock the hammer and load a new cartridge in the chamber, so the pistol will be ready for the next shot. Both revolvers and semi‐autos have two main "action styles": single and double action. They make no difference to the evidence produced from the gun, so we won’t worry about them here. The long barrel on rifles and shotguns allows two advantages over handguns: 
extra distance since the propellant gases are held behind the bullet for longer, providing greater acceleration 
greater accuracy since the bullet is contact with the rifled barrel for longer Rifle cartridges are basically the same as those for handguns. However, shotgun cartridges are very different. Instead of a single large lead bullet, shotgun cartridges are loaded with a large number of small lead pellets, as shown in the diagram. The inside of the barrel of a shotgun is smooth, unlike that for a rifle, because the small pellets would only be slowed down by spiral grooves, and in any case, would not gain any accuracy due to a lack of complete contact with the barrel. Introduction To Forensic Science IS7.2 7. Firearms The shotgun has several “advantages” when used against still targets. First, it has enormous stopping power at short range, more than nearly all handguns and comparable to most rifle cartridges. The wide spread of shot produced by the gun makes it easier to aim and to be used by inexperienced marksmen. Also, unlike a rifle bullet, each pellet of shot is less likely to penetrate walls and hit bystanders. It is favoured by law enforcement for its low penetration and high stopping power, while many American households use it as a home defence weapon for the same reasons. The typical home defence shot is seldom over ten or fifteen feet. At these relatively short ranges, the shot charge never expands to more than a few inches. Gun Measures Most people will have heard of the terms calibre and gauge applying to guns and cartridges. For example, Clint Eastwood’s character in the movie Dirty Harry is well known for his use of a 44 Magnum, “the most powerful handgun in the world”. The 44 actually is the calibre of the gun – 0.44 inches (11.2 mm) – and this refers to the internal diameter of the barrel. Common handgun and rifle calibres are 0.22 inch (the most common non‐military rifles), 9 mm, 357 (0.357 inch) and 38 (0.38). The bullet fired from a gun must match the size of the barrel. If it is too large, it probably couldn’t be loaded into the chamber in the first place, but if it is too small, then the propellant gases will escape past it, reducing its velocity. Shotgun barrels are measured quite differently, and substantially less obviously, using the gauge measure. The most common shotgun in the world is a 12 gauge: this refers to the number of lead sphere of the diameter of the barrel that weigh one pound (Note – the lead sphere mentioned here has nothing at all to do with the pellets in the cartridge – they are much smaller). The 12 gauge shotgun has a barrel diameter of 18.5 mm (0.73 inch). The shot inside shotgun cartridges is typically 2‐10 mm: smaller shot (2‐5 mm) is called birdshot, and the larger buckshot. Different sizes are numbered, eg #4 birdshot is 3.3 mm, but unfortunately the same numbers are used for buckshot, eg #4 buckshot is 6 mm, and in opposite directions, so that a higher number means a larger birdshot but a smaller buckshot! Introduction To Forensic Science IS7.3