Forensic Firearm Investigation: Procedures & Results

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  • 0:03 Forensic Firearm…
  • 0:34 Forensic Examinations
  • 1:14 Comparing Bullets and…
  • 4:02 Other…
  • 4:47 Lesson Summary
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Lesson Transcript
Instructor: Jesse Davis

Jesse has worked in law enforcement for over 10 years in various capacities including patrol and investigations. For five years, his duties included instruction to area schools. He has a Bachelors Degree in Psychology.

Forensic firearm investigations are an important tool when investigating a crime. In this lesson, we'll look at how investigations are conducted. We'll talk about what rifling is, how bullets gain their own 'fingerprints' from being fired, and other forensic investigation tools.

Forensic Firearm Investigations

Let's say that while working in the homicide unit at the local police department, you're following up on a lead related to a shooting that occurred two days before at a gas station. A robbery went wrong, and sadly, the store clerk is dead. Today, a patrol officer calls you, saying a firearm was found in a dumpster not far from where the murder occurred. As you drive to the scene to collect the gun, you wonder if this may be the weapon used in the crime. But how will you connect this gun to the robbery and subsequent killing of the innocent clerk?

Forensic Examinations

A forensic firearm examination is a process in which the characteristics of a firearm and ballistics are studied to link specific bullets, or rounds, to a specific firearm. From there, further investigation attempts to connect the firearm to a specific person. Often, the serial number on the firearm may be obliterated, especially if the gun is stolen, but other pieces of evidence can still exist, such as fingerprints, blood spatter, clothing fibers, and so on. Furthermore, serial numbers that have been scratched off can often still be recovered through a process using etching fluids or gels, which may restore the numbers to a legible condition.

Comparing Bullets and Cartridges

When a round is fired, it travels down the barrel of the gun. The barrel has rifling inside of it. Rifling is a series of land and groove markings inside the barrel of a gun, which cause the round to spin as it exits the gun. Land markings are the high point, and grooves are the low point of the rifling. It's like comparing a sidewalk to a gutter. This spinning action causes the round to fly in a more precise and longer path, like adding spin to a football. Different manufacturers use different amounts or measurements of rifling in their firearms. This allows for forensic examinations to determine specific makes and models of guns.

As the round travels through this rifling, it picks up unique striations, or markings, on the bullet. These markings act as sort of a fingerprint through the various scratches and marks on the round as it exits the firearm. If the firearm used in a crime is recovered, comparative shots may be fired into ballistics gelatin or other materials in an attempt to compare the shots fired in a controlled environment against those found at the crime scene. Ballistics gelatin is a substance used by investigators to determine how a bullet behaves after it is fired. The gelatin is an analogue that mimics human flesh, minus any bone structure.

Let's return to the recovered firearm near your crime scene. With this recovered firearm, you compare bullets fired from this gun to the bullets found at the scene of the crime. This process will either prove or disprove the firearm is the correct one. Bullet A has four land and groove markings on it, and Bullet B has five markings, so this disproves that the gun that fired Bullet B was used in the crime where Bullet A was located.

Casings or cartridges are also an important piece of evidence to consider when matching firearms. Markings from the ejector or extractor and firing pin impressions can show if the gun in evidence is the same one used during a crime.

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