What is a Forensic Firearm and Tool Mark Examiner? Part 1

*Thank you, Joe Giacalone for letting me share the article.*



Ballistics and Tool Mark comparisons 


This is Part 1 of a 3 part series

By: Gerard Petillo - retired NYPD Sergeant and Independent Forensic Firearm Examiner


How can they tell a bullet or cartridge case was fired from or in a gun?


The author conducting testing


A forensic lab or “crime lab” has many different specialized departments or sections.  Each section performs a different area of analysis on evidence submitted from crime scenes. Working inside each of these sections are scientists with specialized backgrounds and training specific to the section they are working in. There is no single person in a crime lab that can perform analysis in every section.


In the firearm and tool mark section, scientists have specialized training in (but not limited to) the modern development and manufacturing process of firearms and ammunition, The operation of the various types of firearms, firearm mechanisms, the proper ammunition they use, how to disassemble and trouble shoot firearms, How to recognize and classify fired ammunition components, the use of microscopes, recognizing class and individual characteristics, the residues produced during discharge, serial number restoration, as well as courtroom testimony.


What is Tool Mark Identification?


Lets start with what is a “tool”. The scientific definition of a tool is the harder of two objects when brought in contact will mark the softer object.  Tool mark identification is a discipline of forensic science that has its primary purpose to determine if a particular mark was made by a particular tool. 


Forensic Firearm Identification is a discipline of forensic science, which has its primary purpose to determine whether a bullet, cartridge case, or other fired ammunition component was fired from a particular firearm. Because the firearm is harder than the ammunition that it fires, the firearm is considered a tool and the marks on fired ammunition components left by the firearm are considered Tool Marks.


Firearm Identification is therefore a subset of Tool Mark Identification.


Often times firearm identification is incorrectly referred to as ballistics. Ballistics is the study of projectiles in motion. Ballistics can be divided into three (3) sub categories:

Image of shock waves produced by
firearm. An example of External


  1. Internal ballistics- the study of the projectile during its travel through the barrel of the firearm 
  2. External ballistics- the study of the projectile when it leaves the barrel during its travel to the target 
  3. Terminal ballistics- The study of the projectile as it strikes the target. 


Although information regarding ballistics is important to a forensic firearm examiner it is not what we do in the crime lab.  


Where do Firearms Get Their Unique Marks From?


Briefly stated, during the manufacturing process the tools used to manufacture the various parts of the firearm (drill bits, broaches, buttons etc) experience microscopic wear during this process. Each part they make (barrel, breech, firing pin etc.) may be slightly different (unique) than the one before it or the one after it. After the firearm is sold and used by the owner, the firearm can then experience use, abuse, and or neglect which can add to the individuality of the firearm part and subsequently the marks left on the bullet or cartridge case. 


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