Friday, December 21, 2012

What is a Forensic Firearm and Tool Mark Examiner? Part 3

What is a Forensic Firearm and Tool Mark Examiner?

Comparing Ballistics


By: Jerry Petillo

This is Part 3 of a 3 Part Series
So far, we have talked about the basic examination of each object (gun, bullet, cartridge case) submitted to the crime lab. One of the most notable results that firearm examiners can give is when they link crime scene to crime scene and/or firearm to crime scene. This is accomplished by the comparison of the microscopic individual characteristics found on fired bullets an cartridge cases.

The comparison microscope is the instrument that firearm examiners use to compare fired bullets and cartridge cases. The comparison microscope is essentially two microscopes connected by an optical bridge that allows the simultaneous viewing of two separate objects under similar magnification. It allows the bullet or cartridge case to be manipulated in different positions under a variety of reflected light sources. During the comparison firearm examiners evaluate the correspondence of the individual characteristics.

Comparison Microscope
The process of microscopically comparing fired bullets and cartridge cases to each other or to a known specimen from a suspect firearm can have four possible outcomes.

  1. Identification (individualization)- This means the two fired ammunition components came from the same source (same gun).
  2. Elimination- This means the two fired ammunition components came from different sources (different guns). 



Microscopic comparison of two bullets fired from the same firearm.
Note the vertical line down the middle of the image.
This line separates the image from the
left stage and the image from the right stage

Microscopic comparison of two cartridge cases fired 
from the same firearm. Note the vertical line down the middle
 of the image. This line separates the image
 from the left stage and the image from the right stage.
Microscopic comparison of two cartridge cases fired from
different firearms. Note the vertical line down the middle of the image.
Note the differences in firing pin shape as well as surrounding marks.


    3.  Inconclusive- This means there is not enough information on the fired
        ammunition component to make a conclusion. This is synonymous with
        saying “I don’t know.”

Although these two cartridge cases have the same 
general shape they do not display any significant individual
 characteristics. Can they be from the same gun / different 
gun? I don't know.
    4.  Unsuitable- This means that there is not enough class characteristics
         necessary to continue with the microscopic examination and comparison. 

A fragment possibly from the core of a bullet. No 
discernible class or individual characteristics. Unsuitable. 


The above outcomes are referred to as the range of conclusions.

As mentioned earlier, In a case in which the responsible firearm has not been recovered, the characteristics of the lands and grooves can be valuable information to a firearm examiner. The number and measurement of the lands and grooves can be entered and queried into a different database known as the general rifling characteristic database (GRC). The result of this query can generate a list of all of the firearm manufacturers that produce firearm barrels with the same class characteristics as the evidence bullet. Firearm Examiners can use the results of the GRC database query to generate possible leads for criminal investigators to follow. Investigators can remain alert for all firearms of those manufacturer that come into police custody and request comparison of the crime scene evidence to that suspect firearm.   

An image capturing the measurement of a land impression 
on a fired bullet

An image capturing the measurement of a groove
 impression on a fired bullet
This is a very basic and shortened depiction of what a Forensic Firearm Examiner does in the laboratory, the examination process, and the possible out comes of the evidence examination.

Consider the following scenarios:

  • In multiple shooting cases where there is a crime scene in each case and investigators have no reason to believe there is a connection between the two shootings, the tool mark evidence from the fired bullets and cartridge cases can link the scenes and give investigators a new direction to look in the course of solving this crime.
  • In a case where informant Information suggests multiple shooting incidents are related, the comparison of the firearm evidence from the scenes can potentially corroborate this informants information.
  • In a case where a person is arrested with an illegal firearm, comparison of the "knowns" from the firearm to the unknowns from a scene may link the firearm and thus give new leads to investigators. 
These are just a few examples of the value of firearm evidence and the tool mark examination conduced in the Lab.

Any questions please contact the author at gp@advanced-forensics.com

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