Criminalistics Laboratory - Firearms and Toolmark Section

Firearm firing

Firearms Identification

Firearms Identification, sometimes incorrectly referred to as ballistics can be defined as, "A discipline of forensic science which has as its primary concern to determine if a bullet, cartridge case or other ammunition component was fired in a particular firearm".

Studies have shown that no two firearms, even those of the same make and model, will produce the same unique marks on fired bullets and cartridge cases. Manufacturing processes, use, and abuse leave surface characteristics within the firearm that cannot be exactly reproduced in other firearms.

Under normal conditions, firearms do not change much over time. This allows a firearm recovered months or even years after a shooting to be identified or eliminated as having been used to fire a specific bullet or cartridge case.

Firearms Identification is a comparative examination, where the ammunition components of unknown origin (from the scene of the shooting or body) are compared with bullets, cartridge cases, and shotshells of known origin that have been produced in the laboratory by test firing the suspect firearms. The known and unknown items are compared microscopically using a comparison microscope.

Unique Markings Comparison microscope
Unique markings

 

Comparison Microscope

 

 

Distance Determination

During the investigation of a shooting there are normally several different perceptions of what actually occurred. Such things as struggling over the firearm or where the victim and shooter were standing in relationship to one another are questions that need unbiased answers.

Firearm examiners routinely examine a shooting victim's clothing for bullet holes and other evidence that may allow for a determination of the distance from the muzzle of the firearm to the clothing. Determinations are made as to which holes are bullet entrance holes and which are bullet exit holes. The area around bullet entry holes can be examined for patterns of gunshot residues. If residue patterns are detected they are then compared with test targets produced by firing the firearm in question at various known distances allowing the examiner to approximate the distance from the object that was shot to the muzzle end of the firearm. When ammunition containing multiple projectiles is used in a shooting, the pellet pattern can be used for determining the distance.

Firearms Function Testing

It is not uncommon in a shooting investigation for a suspect or victim to claim that a firearm for some reason accidentally discharged during a scuffle or handling. For this reason every firearm that is received into the laboratory as being suspect in a shooting, must be examined and function tested.

The function testing is an examination of a firearm concerning its mechanical condition and operation. This would include testing all of the safety features in the firearm to determine if they function as the manufacturer intended. To determine the amount of trigger pull that is required to cause the firearm to discharge. And also determine if it is capable of accidentally discharging without the trigger being pulled, or if the firearm is capable of being fired at all.

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National Intergrated Ballistic Information Network (NIBIN)

The use of firearms, especially semiautomatic firearms has increased significantly over the last few years. Because of the growing number of gang shootings, drug related shootings, and rapid interstate movement, the likelihood that a specific firearm will be used in multiple crimes is increasing.

When a cartridge case is recovered as evidence in a shooting, technicians enter digital images of the breach face and firing pin impression on the cartridge case into the computers that make up NIBIN. NIBIN gives the firearm examiner the ability to search test fired cartridge cases from a known firearm, and firearms evidence from crime scenes for possible matches. This allows the operator to link unrelated crimes committed with the same firearm and also to match suspect guns to shootings.

NIBIN Criteria
The NIBIN firearms computer database is used to determine whether a firearm has been discharged in multiple shooting incidents.  

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*As of December, 2010, only fired cartridge cases, deposited by firearms such as semiautomatic pistols and rifles will be entered into NIBIN.  

Toolmarks

A tool mark is considered to be any impression, cut, gouge, or abrasion caused by a tool coming into contact with another object. Most often, tool marks are encountered at burglary scenes that involve forcible entry into a building, but also are encountered in homicide cases where an axe or knife strikes bone. Generally, these marks occur in the form of indented impressions into a softer surface or as abrasion marks caused by the tool cutting or sliding against another object.

Just as firearms possess and leave individual characteristics from their manufacture and use, so do tools such as pry bars, chisels, axes, knives, etc. leave marks that can be used to positively identify the use of a particular tool. Tool marks examinations are a comparative examination, where a tool mark or cast of a tool mark are compared with known tool marks produced in the laboratory by the suspect tools. The known and unknown marks are compared microscopically using a comparison microscope.

Cross section of toolmark on metal wire