Physical Evidence Handbook

Firearm and Toolmark Unit

The Firearm/Toolmark unit performs a variety of examinations applied to all aspects of firearms and toolmark evidence, but most often focused on determining whether a particular firearm or tool was used in the commission of a crime. Gunpowder patterning, restoration of obliterated serial numbers, trajectory reconstruction and the application of automated computer technology to unsolved firearm cases represent additional examination services offered by the unit.

Firearm Evidence
Crimes committed with a firearm normally result in the collection of evidence that may include revolvers, semiautomatic pistols, rifles shotguns and homemade firearms. Semiautomatic firearms may have been modified to fire in an automatic mode. Both commercially manufactured and homemade silencers may be encountered and submitted to the laboratory for examination.

Firearm Examples
Revolver, pistols, shotgun (with circular magazine and folding stock),
and assault rifle and gun with sound suppressors

A firearm discharge assures the need to recover and examine bullets, shot pellets, shotshell components and other projectiles. Fired cartridge cases and shotshells can also be connected to a suspect weapon.

Every firearm submitted to the laboratory is given a thorough examination to evaluate and document its mechanical operability, including possible damage or alteration, trigger pull and physical features. The gun's potential for accidental discharge or faulty safety function is carefully documented. Finally, each submitted firearm is test fired in order to obtain standard bullets and cartridge cases and to demonstrate its function.

Firing Bullet into Recovery Box
Firing into bullet recovery box

The question asked most frequently by investigators is whether a suspect gun was the one that fired bullets or cartridge cases recovered in a shooting investigation. Using a forensic comparison microscope to compare patterns of markings on a bullet surface to the same kind of markings on a test-fired bullet can produce a bullet identification. Similarly, comparing unique markings created on a cartridge case by firing in a gun to markings on a test-fired cartridge case can result in a cartridge case identification. Such identifications prove that a specific gun fired the recovered evidence.

Comparison Microscope
Bullet and cartridge case identification through comparison microscope

Shooting scenes at which damage has been caused by projectiles passing through vehicles, walls, windows and other objects offer the opportunity to define bullet caliber, bullet direction, number of shots fired and bullet paths. With this information shooter and victim location and movement can often be determined to aid investigators in shooting reconstruction or witness corroboration. Frequently, shooting reconstruction will require an on-scene evaluation by the firearm examiner.

IBIS Automated Computer Technology
In response to the proliferation of violent firearm crimes, forensic scientists have incorporated computer technology within an automated forensic firearms identification system known as IBIS. Using specific markings created by a gun when a cartridge case is fired, the IBIS system is employed to create a database of marking images that can be compared to each other with the computer software. The power of the computer allows large numbers of images to be compared, effectively allowing the examiner to determine whether a specific suspect gun was used in unsolved shootings in numerous jurisdictions.

Computerized Firearms Comparison System

Imagine shooting investigations taking place in two different cities several weeks apart. Investigators eventually submit cartridge cases recovered at both scenes to the crime laboratory for examination. In the course of working the cases, images of markings on the cartridge cases are entered into the IBIS database and a correlation request is made. The firearms examiner reviews the correlation images on the computer screen, along with correlation rankings of likely matches, and determines that cartridge cases from the two cases exhibit the same patterns of markings. The actual cartridge cases will be compared on the comparison microscope and an identification is verified. The same gun was used in both shooting crimes. Since test fires from submitted guns are entered into the IBIS database where appropriate, a gun recovered in a third incident can be compared to these IBIS-linked cases and demonstrated to be the specific gun used, now possibly linking a suspect to the shootings as well. The various investigations need have no known link to each other until the IBIS linkage is determined.

The ability to compare hundreds of cases to each other in only a few minutes is an obvious practical benefit of IBIS computer technology. Comparing cases on a regional basis magnifies this advantage. Finally, IBIS technology doesn't require the transfer of bullets and cartridge cases between agency and laboratory after initial lab examination until identification is likely.

Powder Pattern Analysis
When a gun is fired in close proximity to a target, gunpowder residues and gases discharged from the gun muzzle create physical effects that can assist in estimating the distance between the muzzle and the target. Burnt and partially burnt gunpowder residues can be deposited on a shirt, creating a pattern that can be revealed with visual, microscopic, or chemical examination. Stippling patterns on skin can be documented with appropriate photography. In both cases the dimensions of the pattern and the density of the residues in the pattern can be used to estimate muzzle to victim distance.

Once the pattern has been defined as clearly as possible, a suspect weapon can be test fired at various distances to create test patterns for comparison. Pattern duplication is most precise when the same ammunition and gun can be fired to create the test patterns. Generally, distance estimates can be made when the gun is three feet or less from the victim or target at discharge.

Shotgun Blast
Muzzle blast of a shotgun

Blast Pattern

Toolmark Evidence:
ToolsToolmarks are striations or impressions left by forceful contact between two objects of differing hardnesses. For example, a firearm barrel (the harder object) engraves microscopic striae on a bullet (the softer object) as the bullet passes through the barrel. We encounter this principle in action whenever a prying tool is used to force a door or lock, a bolt cutter is used to cut a padlock or in the nearly infinite combination of tools and materials encountered in the investigation of criminal activities. The manufacturing processes used to make the tools create unique imperfections on the tool surfaces, and that uniqueness is reflected in the toolmarks created when the tools are used.

A positive identification of a toolmark will require submission of the suspect tool and the creation of test toolmarks in the laboratory with the tool. Comparison of the test marks to the toolmarks recovered at the scene will be conducted to determine whether the submitted tool was used. It is always preferable to submit the actual toolmark from the crime scene, but when size of the object does not permit toolmark recovery, the toolmark can be cast. Laboratory comparison of the properly recovered cast to the test marks can be made. Scaled photography of the toolmarks at the scene provide important (sometimes essential) information used in deciding how to make test marks in the laboratory.

Impression marks are a useful type of toolmark and can also record the unique characteristics of a tool. A gun's firing pin striking the softer primer material of a cartridge creates a firing pin impression that is an example of this type of toolmark. Chisels, hammers, vise grips and many other tools can create impression toolmarks. Even a car bumper striking a bicycle frame in a hit and run incident can create a reproducible toolmark. The investigator who views toolmarks as any marking created by one object on another will be rewarded with more possible links of a suspect to a crime than the investigator with the narrower view that limits toolmarks to tools.

Obliterated Serial Number Restoration
Many items such as firearms, electronic equipment, farm equipment, industrial machinery, trucks, motorcycles and other motor vehicles will have identifying serial numbers or markings stamped on the metal surfaces. Even though federal and state laws make it a violation to alter or remove identifying data from some of these items, numerous cases involving this type of evidence are still encountered. Criminal activity involving theft or illegal use of these items may cause an offender to remove identifying data so that ownership cannot be traced. When numbers have been over-stamped or obliterated by grinding, sanding or other methods, the numbers may no longer be identifiable. Through use of chemical and physical restoration techniques, the numbers can sometimes be made readable again.

Serial Number Restoration
Serial Number Restoration