Firearms
One of the most important tools forensic scientists have in today's battle against evil doers is the ability to analyze firearms and all of the actions that make up firing a bullet. The term used to identify this field is Firearms Identification. Simply put it means: the identification of fired bullets, cartridge cases or other ammunition components as having been fired from a specific firearm. A common misconception is that this field can be called ballistics. Ballistics refers to study of the dynamics of projectiles whereas firearms identification is an all-encompassing field.
The evidence typically submitted to a crime lab will include: a firearm, fired bullets, fired cartridge cases, live ammunition, clothing, and other objects related to the case in question. The basis of firearms identification is that no two firearms produce the exact same marks on a fired bullet or cartridge. Through the manufacturing process and use of the firearm a characteristic fingerprint is left on the bullet and cartridge. These characteristics, although they may be similar, cannot be reproduced on anther firearm.
The first step in a firearm identification is to look at what are called class characteristics. These are characteristics common to a particular group or family of items. In the case of a firearm these would be the caliber and rifling pattern of the barrel. The caliber of the firearms refers to the size of the bullet that is fired. Obviously a bullet that is 0.30" in diameter could not be discharged from a firearm that has an inside barrel diameter of 0.15". The rifling pattern on the inside of a barrel is often unique to a specific firearm manufacturer. This pattern is the spiral staircase pattern that can be seen when looking down the barrel of a firearm.
Different Bullet Calibers
Spiral Pattern of Rifling (Looking Down Barrel)
Class characteristics can also be used in identifying breech marks, firing pin impressions, extractor marks, ejector marks, etc. These characteristics will be explained further down in more detail.
Often a firearm identification process can stop right now if enough discrepancies are found but if not, individual characteristics must be examined. Individual characteristics are those characteristics unique to an item.
Bullet
Identification (Left: Microscope
Used in Bullet Identification)
The rifling patterns mentioned above leave marks on the bullet as it travels down the length of the barrel. A rifling pattern can be easily seen by the naked eye by looking down the barrel of the gun. Between the ridges that make up the rifling pattern are microscopic scratches that also leave marks on the bullet. It is these marks that prove useful in firearms identification. Bullets discharged from the firearm in question will be compared with bullets obtained from samples obtained in the forensics laboratory. Samples are obtained by discharging a bullet into a tank of water. Enough friction is created between the bullet and water that the bullet does not travel the entire length of the tank. Several of these samples are obtained and compared to the bullet(s) in question. Using a pretty cool microscope like the one seen above scientists are able to compare the two samples as they appear in a common viewing window to the scientist. Each sample is on a platform that can be rotated in almost any direction thereby allowing a perfect match to be made if possible.
Bullets Matched Using the Microscatches
Becoming an expert in bullet identification requires a great deal of experience and it is this experience that proves beneficial when decisions need to be made concerning the overall uniqueness of the striations present on the bullets. One of the biggest problems in bullet identification is that most bullets that are in question are rarely received intact. This is usually not a problem because enough identifying are present on the bullet to draw a conclusion.
Damaged Bullets
Cartridge Case Identification
A process similar to the one use in bullet identification is used in identifying cartridge cases. Cartridge cases are the cases that house the bullet in the firearms before it is discharged. Some of the marks used to individualize a cartridge are shown below.
Marks left on Cartridge Case Near Firing Pin
Striated Action Marks
These are marks common to a cartridge that has been passed through an auto loading or repeating firearm. The cartridge experiences a multitude of contact points as it is loaded, fired, and unloaded in a firearm. Chamber marks are made as the bullet is inserted into the chamber of a firearm. The chamber is the holding place for the bullet when it is fired. When the bullet is fired a large increase in pressure is experienced in the cartridge case thereby pressing it into the walls of the chamber. This pressing into the chamber walls is what creates the pattern on the cartridge case. .
Chamber Scratch Marks on 22 Caliber Rim Fire Cartridge
Firing Pin Drag Marks
This are marks made by the firing pin when it strikes the primer of the bullet and remains there for a fraction of a second. The primer is like the detonator for the bullet. The fraction of a second that the pin remains in contact with the primer of the bullet in conjunction with the recoil of the gun creates what are called firing pin drag marks. The recoil of the firearms forces the slide down thereby scratching the cartridge of the gun against the firing pin. Some handguns, such as the Glock, have a very unique pattern and are easily identified.
Firing Pin Drag Marks
Extractor Marks
Once a bullet is fired it must be removed from the chamber of the firearm. In many cases a part that looks like a hook grabs the rim of the bullet and pulls it out of the chamber. This contact is unique to each firearm.
Extractor Marks on Cartridge
Firing Pin Impressions
A bullet is struck from behind by a firing pin to detonate the primer. Every firing pin is unique in the pattern it leaves on the bullet. A firing pin may not strike the bullet in the direct center, it may have a unique shape to it or it may have slight imperfections or defects present on it. Each of these items is looked at when an individual match is trying to be made.
Positive ID on Firing Pin Impression
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