Monday, December 17, 2012

What is a Forensic Firearm and Tool Mark Examiner? Part 2

The Forensic Firearm Examination Process

Part 2

By: Jerry Petillo

If you missed Part 1

Generally speaking, During the examination process, Firearm examiners will identify and classify the type of evidence, i.e.: bullet, cartridge case, shot shell component, etc. Next they will attempt to identify the class and individual characteristics of the particular piece  of evidence. Next they will attempt to individualize the evidence to a specific and unique firearm.

 Class characteristics can be referred to as family characteristics. Simply stated, class characteristics are capable of putting an item in a group or groups. All items in a particular group share the same class characteristics. Class characteristics are also capable of eliminating an item from a particular group. In the case of firearm identification, the rifling characteristics (lands and grooves) on an evidence bullet are an example of class characteristics. Class characteristics can not individualize an item to one particular or unique source.

Individual characteristics are characteristics that are unique to one particular item. In Firearm Identification, striated tool marks on a fired bullet are examples of individual characteristics created by the firearm. Impressed and striated tool marks on a fired cartridge case are examples of individual characteristics created by the firearm. Individual characteristics (in sufficient quantities) can individualize an item to a particular source or firearm.

Firearm and related evidence is sent to the crime crime lab for analysis. Firearms come into police custody from many different sources including a suspect or a crime scene. Bullets are recovered from a victim or crime scene. Cartridge cases are recovered from a crime scene. It is the function of the firearm examiner to apply their knowledge, skills, and abilities to determine (when possible) and report what the item is, what it does, and or where it came from.

Ballistic Evidence
In the case of a firearm, the goal of the examination is to determine make, model, caliber, and does it shoot or not as well as determine if it can be linked to an incident through the tool marks that it makes on bullet or cartridge cases. Ultimately the firearm will be test fired under controlled circumstances. It will be loaded and fired into a bullet recovery medium usually a large water tank. The fired bullets and cartridge cases recovered will be packaged and stored as known exemplars from that particular firearm. The result of the examination, does it shoot or not will be stated in the final report. This result may have legal implications to a person who may have possessed the firearm.  

The tool marks from the knowns may then be imaged and captured into a forensic firearm database known as IBIS/NIBIN. The database is capable of capturing the unique marks using a proprietary process and compare them to the rest of the database. If a link or match is suggested the knowns will be physically compared to the unknown by a human firearm examiner to confirm the link in the database.
Rusted Firearm restored so it can
be test fired.

In the case of a fired cartridge case recovered from a crime scene, this type of evidence is considered an unknown. The goal is to classify it and then see if can be linked to an incident based on the tool marks that the firearm produced. It will be examined to determine caliber, and manufacturer. The tool marks on the fired cartridge case may be examined to determine class characteristics. It will also be examined to see if it displays any individual characteristics. The cartridge case may then be imaged and captured into the IBIS/NIBIN database and compared to the rest of the database. 

Fired cartridge case with firing
pin impression.

Before we can talk about bullet examination it must be mentioned that spiral grooves are manufactured into the barrel of a firearm. The spiral grooves are designed to impart spin on the fired bullet as it travels down the barrel and this spin continues during the bullets flight. This spin is similar to the spin that we see on a properly thrown football. The purpose of the spin is to give the bullet distance, stability, and accuracy during flight. The raised areas inside the barrel between the grooves are referred to as the lands. Lands and grooves are considered class characteristics. The characteristics of the lands and grooves can be valuable information to a firearm examiner in a case where no firearm has been submitted. 

In the case of a fired bullets recovered from a victim or a crime scene, the bullet is also referred to as an unknown. The goal is to classify the bullet and then see if it can be linked to a gun or incident.

Spiral Grooves in Barrel of Firearm
Hatcher, Jury, Weller 1959
Bullet examination can be a little more complicated than cartridge case examination. The bullet leaves the barrel at a very fast rate of speed. When it impacts an object or objects it can become damaged. The extent of the damage will determine how it is examined and how much information can be gleaned by the firearm examiner. It is not uncommon for the class and individual characteristics that a firearm examiner is looking for to become completely obliterated due to impact and therefore no conclusions can be made.

Lead bullet significantly damaged
Expanded "Mushroomed" hollow point

The first step of bullet examination is to determine caliber. Generally, the firearm examiner will measure the base of the bullet in an attempt to determine a nominal caliber. For instance a 9mm luger caliber bullet should measure approximately .355/1000 of an inch (+ or -). If the base of the bullet is damaged this type of measurement may not be possible. 

Next, the lands and grooves on the bullet created by the barrel of the firearm will be evaluated for class and individual characteristics. The class characteristics will include counting and measurement of the lands and groves as well as the direction (left or right) of the lands and grooves.

.45 cal-left twist
.45 cal -right twist

Individual characteristics exist within the lands and grooves and are used during comparison to determine individualization to another unknown bullet or known exemplar produced from test fired firearm. Finally, the bullet may then be imaged and captured into the IBIS/NIBIN database and compared to the rest of the database.

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