Introduction

Recall from the earlier unit on impressions and prints that shoes, tires, and fingers can leave marks behind which can be used to link an item or suspect to the crime scene. These markings have both individual attributed to a specific source with a high degree of certainty and class attributed to a group characteristics.

Similarly, firearms leave impressions on bullets. In this lesson you will learn about the impression evidence used to link a bullet to a firearm.

A fired bullet.
A fired bullet with rifling impressions from the barrel of a gun. Courtesy of Robert M. Thompson, NIST.

Lesson Objectives

Following successful completion of this lesson, students will be able to...

  • Distinguish between class and individual characteristics of firearms.
  • Describe how striation mark evidence is collected and analyzed.
  • Compare evidence from firearms to identify similarities or differences in striation patterns.

Essential Questions

  • How are the unique characteristics of firearms important to criminal investigators?
  • When a firearm is involved in a crime, what pieces of information can be collected from the scene?

Enduring Understandings

  • An exchange of material will occur whenever someone is at a crime scene (Locard's Exchange Principle). Materials left behind (or carried away) from the crime scene as well as markings and impressions from those items can be linked to an individual or item.
  • Evidence must be collected in a specific and strategic manner, as well as systematically documented, to ensure that no tampering or contamination occurs.

The above objectives correspond with the Alabama Course of Study: Forensic Science and Crime Scene Investigation standards: 4, 22, 25, 29.