Toolmarks

Step #2: Understanding Toolmarks

Now that you know the basics of firearms, it is time to learn how crime scene investigators apply this knowledge to help solve firearm-related mysteries. Read through the information below and answer the questions to demonstrate your understanding of the core concepts.

What is a Toolmark?

In criminalistics, the impressions or indentations left on surfaces by tools or other objects are called toolmarks. If an object (the tool) contacts the surface of another object with enough force, the pattern of the tool can be permanently reproduced onto that surface. These indentations left behind are often barely noticeable to the unaided eye so investigators must rely on microscopes to determine if there is a link between a tool and a toolmark.

Toolmarks are permanent impressions left on a surface by a tool.

Class Characteristics versus Individual Characteristics

Often, the toolmarks left behind on a surface may appear to be consistent with a group of tools. This is known as having class characteristics. For example, the four types of screwdrivers shown below would all leave similar but different markings on a surface. The are all flat-head screwdrivers but they have different features.

While toolmark class characteristics can help investigators narrow down the type of tool they are looking for, they would not provide enough evidence to convict somebody even if they were caught holding the tool in question. This is because there could be hundreds or even thousands of screwdrivers with the same features. Instead, investigators will rely on microscopy to try to find unique imperfections on the surface of the tool. These unique imperfections or individual characteristics are typically caused by variations in the manufacturing process, differences in the use of a tool, imperfections caused by the abuse of a tool, and rust. In many cases, these microscopic imperfections are transferred to a surface when the tool is used, essentially creating a unique fingerprint linking the tool and the toolmark together.

A comparison microscope

Comparison Microscopes

As mentioned before, toolmarks left on a surface are often too small to see without the aid of a microscope. One such microscope is the comparison microscope, which is able to show two objects side by side simultaneously. Magnification is usually adjustable from 4x to 40x. The markings on the surface of the two objects on the right most likely came from the same tool.

The toolmarks on the left match the toolmarks on the right.

Examining the Evidence

Fired cartridge cases or shotgun shells are commonly found at crime scenes involving semiautomatic and fully automatic firearms. Fired bullets are commonly found, as well. Often these fired bullets are found in walls, ceilings, floors, furniture, mattresses and appliances. If an autopsy is performed on any of the shooting victims, it is the responsibility of the medical examiner to collect those bullets and properly package them for further investigation.

As evidence, bullets are individually wrapped in unused wrapping paper and placed in a small container. Either the bullet or the container is labelled, depending on the protocols of that jurisdiction. While collecting and handling evidence, investigators must be careful to not add or alter any toolmarks so that the bullets or casings can be used for comparison.

A test fire water tank used for bullet recovery

There are three possibilities that investigators face when it comes to collecting firearm evidence.

The firearm is recovered without any related evidence (no fired bullets or casings). In this scenario, investigators will perform what is called a test fire, which typically involves firing the bullet into a tank of water so that it can be easily retrieved. Once investigators recognize the identifying toolmarks the firearm leaves on the test bullets and casings, they will be able to match them to any bullets or casings that may be found in the future or that were found in previous crime scenes.
The firearm is collected along with fired bullets and casings. In this scenario, investigators will perform test fires to confirm that the bullets and casings found at the crime scene do, in fact, belong to the retrieved firearm.
Fired bullets and casings are found but no firearm. In this scenario, investigators will use a comparison microscope to determine if the fired bullets and casings came from the same firearm.

NIBIN

With the advancements of digital technology, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) maintains a database called NIBIN, which is a national database of digital images of spent bullets and cartridge cases that were found at crime scenes or test-fired from confiscated weapons. NIBIN allows investigators to submit photographs of collected bullets and casings and the database automatically tries to find a match. More information about the database can be found here: ATF and NIBIN.

Firearm Related Toolmarks

When it comes to firearms, there are several opportunities for the firearm to leave toolmarks on on a bullet or a cartridges. These include:

Striations (bullet)
Breechface marks (cartridge)
Firing pin impressions (cartridge)
Extractor marks (cartridge)
Ejector marks (cartridge)
Chamber marks (cartridge)

A cartridge case (left) and bullet (right)

Striations

Striations are markings that are left on bullets by the machining process and cutting of the rifling in the barrel. When the bullet travels through the barrel of the firearm, the direction of the rifling and size and number of lands and grooves of the barrel are transferred to the bullet. This information can be used to compare bullets to determine if they came from the same firearm.

The first step in comparing fired bullets is comparing the direction of the rifling and the widths of the lands and grooves. These characteristics, along with the caliber and the type of bullet are considered the class characteristics of a bullet. If the class characteristics match, then investigators can proceed to examine the individual characteristics of the bullets. The individual characteristics would include unique markings left on the bullet created by the manufacturing process of the firearm.

Same direction of rifling but he lands and grooves of the two bullets do not match

The striations of the two bullets are a perfect match

Agreement of both class and individual characteristics

Breechface Marks

The marks on the breechface are caused by the manufacturing process. These marks are individual to that firearm.
Different markings will be made by different manufacturing techniques and tool surfaces
The marks from the breechface are transferred to the cartridge case during the firing process.

Microscopic breechface comparison

Parallel breechface marks

Circular breechface marks

Arched breechface marks

Firing Pin Impressions

Firing pins have different shapes which result in different markings on cartridges
Firing pins are machined items and typically have markings from that machining process

Rectangle firing pin and impression

Hemispherical firing pin and impression

Comparing firing pin impressions can reveal individual characteristics

Extractor Marks

The extractor is a piece of metal with a hook on the end designed to pull the cartridge from the barrel after firing.
The interaction of the extractor with the cartridge case results in markings both on and inside the rim of the cartridge case.
The marks made by the contact of the extractor with the rim of the cartridge case can be used for comparison.

Extractor marks can be seen on the outer rim of the casing or inside the groove.

Marks on the casing made by the extractor can be used for comparison.

Ejector Marks

The ejector is designed to eject the fired cartridge case from the firearm.
The ejector comes in contact with the cartridge as the slide moves rearward. When the ejector hits the cartridge, it is “kicked” from the slide and out of the firearm.
The contact between the cartridge case and the ejector results in an ejector mark.
Not all firearms have ejectors or leave ejector marks.

Ejector marks can be used to compare cartridge cases under the comparison microscope

Chamber Marks

Chamber marks are caused by imperfections in the barrel chamber. The marks are passed on to the cartridge case when it expands and is removed.

Toolmarks on the chamber...

Can be transferred to the casing.

Step #2: Understanding Toolmarks

Now that you know the basics of firearms, it is time to learn how crime scene investigators apply this knowledge to help solve firearm-related mysteries. Now that you have read through the basics of firearm-related toolmarks and evidence, let's check your understanding. Be sure you are logged into your FCS Google account so you can take the quiz below.

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