Firearms and toolmarks
Forensic scientists in the Firearm and Toolmark Section primarily determine if bullets, cartridge cases, shotshells, wadding or other ammunition components were fired in a particular firearm. Additionally, their other duties include function testing of firearms, muzzle-to-target distance determinations, toolmark examinations and maintaining the various reference collections.
Firearms identification
Forensic scientists determine if a bullet, cartride case or other ammunition component was fired by a particular firearm. Firearm examiners benefit from knowledge and study of ballistics in their work. Ballistics is the science of projectiles in motion. Examiners consider the movement within the firearm (interior ballistics) when evaluating a firearm or fired bullets in their examinations. They may also rely on their knowledge of the effects of a projectile's impact on a target (terminal ballistics) when evaluating evidence for proximity determinations. Although firearm examiners consider the motion of projectiles in their work, ballistics constitutes only a portion of their focus within the entire field of firearms identification.
Creating test specimens
If a firearm is submitted to the laboratory, a firearm examiner will test fire the firearm to collect fired bullets, fired cartridge cases, or both. They compare the test fired specimens to the bullets and cartridge cases collected at a scene. Creating quality test specimens is important to successful microscopic comparative examinations. By selecting proper ammunition, examiners can improve their ability to recognize matching features on the specimens. The Firearm and Toolmark Section has a supply of ammunition in a variety of calibers, cartridge types and bullet styles and weights for handguns, rifles and shotguns. Firearm examiners must recover fired bullets for comparison to evidence bullets. The section has a water trap and cotton trap for recovering fired bullets. The examiner will consider many factors including bullet velocity and construction when choosing the appropriate recovery method.
Comparative examinations
Law enforcement agencies submit firearms, bullets or cartridge cases to the laboratory. Firearm examiners compare bullets and cartridge cases to determine if they were fired in the same firearm or firearms. Firearm examiners use a comparison microscope to simultaneously view and compare two cartridge cases or bullets. The microscope magnifies the specimens allowing examiners to see very small marks left on their surfaces during the firing process. Examiners look for corresponding features on the bullets and cartridge cases to determine if they “match."
Function testing
Firearm examiners perform testing to assess the functioning ability of a firearm. They can test the firearm to determine if it is capable of discharging a cartridge or shotshell. They can also evaluate the mechanical operation of a firearm when there are allegations or concerns that the firearm discharged unintentionally. Examiners may evaluate and test a firearm’s mechanical condition and observe if any modifications were made to the firearm. For example, examiners can recognize a semi-automatic rifle that contains machine gun parts. This modification causes the rifle to fire full-automatic. Such modification has the potential to create an unsafe condition. Automatic firearms are also regulated under state statutes. Firearm examiners may look at reference firearms when diagnosing function problems on an evidence firearm. The examiners can also use the reference firearms to supply missing or damaged parts to non-functional evidence firearms so they can create test specimens.
General rifling characteristics
General Rifling Characteristics (GRC) databases can be used as an aid to identify possible manufacturers or brand names of firearms that share class characteristics observed on suitable bullets and bullet fragments and, in some instances, cartridge cases. GRC databases are typically used in cases where bullets are submitted to the laboratory, but no firearm is available for comparison, or the submitted firearm(s) has been eliminated as having fired the submitted evidence. While typically used for relatively intact bullets, there are times when the observable class characteristics on evidence bullet fragments or cartridge cases are sufficient to make a search potentially informative.
Distance determination
Firearm examiners can evaluate items to determine the distance between the muzzle of a firearm and an object when it was shot. Although almost any object can be the target of gunfire, the laboratory most frequently receives requests for proximity determination examinations on victim’s clothing. When a firearm is discharged it emits smoke, soot and other residues, primarily from its muzzle. The residues exit the muzzle generally in a cone shape and can land on the target object surrounding the bullet hole. The size and density of the pattern of residues on the object can vary with distance, allowing a firearm examiner to determine the proximity of the shooter to the target when the gun was fired. Examiners visually examine and chemically process evidence for the presence of gunshot residues. They use chemical tests that react with nitrites (a burned component of gunpowder) and lead (from contact with the bullet or the vaporous lead from the primer, or ignition source of a cartridge). Examiners test fire the firearm used in the crime and same or similar ammunition at a variety of muzzle to target distances. This allows the examiner to reproduce the pattern of nitrites and the presence of lead residues. Comparison of the test targets to the evidence allows the examiner to determine a distance range. Similarly, a firearm examiner can determine the approximate distance the muzzle of a shotgun was from a target by reproducing the size of the pellet pattern. Examiners test fire the shotgun at different distances and compare the pellet pattern to the pattern on the evidence.
Toolmark examination
Tools can be used in the commission of crimes. They can aid criminals in gaining entry or damaging property. When the tools are used, they can leave behind marks on the surfaces they contact. Law enforcement agencies will submit tools and the items bearing toolmarks to the laboratory. Examiners in the Firearms and Toolmarks Section can help solve a crime by linking a toolmark to a particular tool. Toolmark examiners evaluate tools to determine that they are capable of producing the questioned mark. They will then use the tool to make test toolmarks, usually in soft lead. Silicone-based casting materials are useful for capturing marks on items that are too large to transport to the laboratory or to fit under a microscope. The examiner will also cast the test toolmark for comparison. Similar to bullets and cartridge cases, the examiner will conduct comparative examinations on toolmarks using the comparison microscope. The toolmark examiner will compare the microscopic features of the test mark and the toolmark on the evidence. If the marks “match,” the examiner can determine that the tool used to create the test toolmark also made the mark on the evidence item.
Reference collections
The firearm reference collections at the BCA Forensic Science Services (FSS) laboratories in St. Paul and Bemidji have a variety of pistols, revolvers, rifles and shotguns. Firearm examiners use reference firearms in function testing to evaluate the condition of similar evidence firearms. Examiners also use the reference firearms to supply missing or damaged parts to non-functional evidence firearms so they can create test specimens. The reference firearms also help the Forensic Intelligence Response Support and Testing (FIRST) Section in serial number restorations. The firearms show the serial number locations and numbering conventions used by the manufacturers, allowing firearm examiners or FIRST technicians to identify partially obliterated or restored characters in a firearm’s serial number. Additionally, firearm examiners will use reference firearms for exemplars in court proceedings, for training exercises and to conduct research experiments. The Firearm Section’s firearms collection consists of seized firearms donated by local law enforcement agencies. The BCA FSS laboratories in St. Paul and Bemidji have a large supply of ammunition including cartridges and shotshells for pistols, revolvers, rifles and shotguns. This ammunition can be used to create test specimens similar to the evidence ammunition. The laboratories also have a standard ammunition file, a cataloged reference collection of ammunition of known brands and types, that can be used to help examiners determine information such as manufacturer, cartridge type or bullet style. The Firearm Section’s ammunition collection consists of purchased ammunition or boxed ammunition donated by local law enforcement agencies.
