Important note: This whole section has been compiled from books and websites, with occasional help from people working in this field. Like the rest of Deep Background (of which this is only a small part), this information is designed for writers of X-Files fiction, and covers only those things that they might be expected to need. It is not a comprehensive guide to the subject.
Types of gun:
Whatever the type of gun, cartridges are much the same. They consist of:
Airguns can also be used. They used compressed air to fire the pellet, so don't make a loud noise.
Evidence collected:The Firearms Section is usually sent a firearm or firearms, fired bullets, spent cartridge cases, spent shotshells, shot, shotshell wadding, live ammunition and clothing.
Aim: The aim is usually to determine if the bullet inside a victim matches a particular gun (as we see the FBI Ballistics expert doing in "Anasazi") To do this they first check the "class characteristics":
If the bullet and the gun have the same calibre and rifling, then the bullet is from the same type of gun. However, the expert then has to go on and see if it is likely to come from the same specific gun.
The theory is that the bullet is made of rather soft metal and gets marked as it passes down the barrel of the gun. Any peculiarity of the gun will therefore show up on the bullet. No two guns are the same due to tiny differences at the manufacturing stage and from subsequent use. The particular characteristics of a gun are sometimes called the "mechanical fingerprint."
When testing, the gun is fired several times so the appearance of the fired bullets can be studied. This is done through a comparison microscope - essentially two microscopes in one, designed so two samples can be examined at the same time.
The examination can show up breech marks - marks made on the cartridge when it is forced back against the breech block, which will be marked in an unique way with scratches, marks of tools etc - and striated marks - tiny scratches on the bullet itself as if travelled down the barrel.
If there is no gun recovered at the scene: If a bullet is found, the above tests can be carried out to narrow down the possibilities of the type of gun used. By examining the rifling it is possible to come up with a shortlist of possible manufacturers who could have made the gun. The FBI maintains a database of all known manufacturers of guns and their characteristic rifling patterns.
The webpage cited above gives a nice description of a case in which this method told investigators what type of gun was used in a crime. They then searched all the local small ads until they found a gun like this being advertised and found out that the man who had advertised it had sold it to someone who matched the description of the suspect.
If there is no bullet but only a cartridge, it is harder and not very much information can be obtained.
Gunshot residue: This can be found on clothing, at the scene of the crime etc. By exmaning the pattern of this residue it is possible to find out how far away the gun was when it was fired, especially if they already know the type of gun used.
Gunshot residue, made up of gunpowder and bits of lead, will only travel about 3 to 5 feet.
Tests for gunshot residue:
A gun fired when in contact with an object will normally deposit a very intense ring of residue right around the edge of the bullet hole and the heat may melt synthetic clothing.
A bullet fired from close range (ie within a foot) tend to result in a very concentrated deposit of residue around the bullet entrance hole and will include a sooty deposit of visible residue.
A bullet fired from between 1 and 3 feet away tends to deposit a lot of residue, but most of it invisible to the naked eye.
When there is no residue it could mean that the gun was fired from a long way away, or that there was something between the gun and the target.
Computers: Large databases of firearms records, crime scene information etc are kept on a system called "DRUGFIRE" sponsored by the FBI, and another one called IBIS maintained by Forensic Technology.
See the medical section for information on the medical treatment of gunshot wounds. This section is merely the forensic side of the subject.
Determining position of gun:
This is job both for the firearms expert and the pathologist
Intermediate range: If the gun was still close enough to leave residue:
Long-range No powder marks. The only mark is the bullet hole.
Entry and Exit Wounds
The entrance wound will always be smaller and more precise than the exit wound. The entrance wound is always surrounded by a ring of abraded skin.
At very close range, all the little shots are concentrated in one place. This leads to huge wounds. When someone kills themselves with a shotgun to the head, a lot of the head is destroyed.
At fairly close range the shot begins to expand. As we get to about 2 feet away, the wound begins to look like a large central hole with a few little holes around the edge. Beyond 4 or 5 feet the shot disperse more and more and is more likely to make lots of little wounds, not likely to be fatal. As a rough rule, the distance of the gun in feet will be diamater of the wound (from the extremes of the little shot wounds) in inches minus one. Thus a wound that is five inches across was done at four yards.
Accident? Surprisingly few gunshot fatalities are accidental. People who own guns are usually very careful with them. Most accidents happen if someone else gets hold of the gun.
Suicide? First, check to see if it is physically possible that the wound could be self-inflicted. If the gun was fired more than 3 feet away, then it can't be. A wound in the back of the head is more likely to be homicide than suicide.
Some of these indicators are obvious at the scene. Some require post-mortem exams - eg to determine the trajectory of the bullet.
Only about a quarter of suicides leave notes.
With a handgun (like Mulder and Scully's), statistics show that 83 percent of suicides shoot themselves in the head. Most of the rest go for the chest, and a tiny minority go for the abdomen. Only 50 percent of shotgun suicides are in the head. 36 percent are in the chest.
Homicide? Gunshots wounds are the most common method of murder in America. Usually it is fairly obvious, by the trajectory, position of gun and other circumstances - eg armed robbery. All materials recovered at the scene or at the autopsy will be sent to the ballistics section.