Firearms

Forensic scientists conduct various types of firearms examinations such as function checks, restoring obliterated serial numbers, determining a type of gun that may have fired a particular bullet or cartridge case, effecting firearms identifications and determining muzzle-to-target distance.

Bullet comparison

Much like the spiral a quarterback puts on a football, a bullet must spin as it leaves the barrel of a gun to prevent it from toppling end-over-end allowing for a straight and accurate trajectory.  To force a bullet to spin, gun manufacturers use lands and grooves, or rifling, inside gun barrels.  The lands are raised ridges that twist down the length of the barrel, so when the bullet comes in contact with the raised ridges the lands force the bullet to spin.  The manufacturing process used to produce lands and grooves results in unique microscopic imperfections that ”scratch” the surface of a bullet as it travels through the barrel.  These scratches, called striations, form a unique pattern and are the basis for firearms identification.

A cartridge case, the ammunition component that initially holds the gun powder, can also pick-up markings

 from the inner workings of a firearm.  Newton’s third law of motion states “that for every action, there is an

equal and opposite reaction.”  When

a gun is fired, the “explosion” forces the bullet forward and the cartridge case rearward.  As the cartridge case travels rearward through the gun and comes in contact with numerous surfaces, it is marked by the microscopic imperfections present on those

surfaces.  As with striations on bullets, these markings are used for identification purposes.

A firearm submitted to the laboratory will be test fired multiple times into a recovery tank.  This recovery tank, filled with approximately 200 gallons of water, provides a safe and effective method of retrieving bullets from a fired gun.  The water stops a fired bullet without damaging the bullet so it may be used for microscopical comparison.

Comparison microscope

Comparisons of suspect and evidence samples are conducted using a comparison microscope which is actually two microscopes joined by one optical bridge.  This allows scientists to examine two objects at the same time within the same field of view.  A firearms examiner uses fired standards, bullets and cartridge cases from a gun fired in the laboratory, to compare to fired evidence collected from the crime scene.  Standards produced in the laboratory are first compared to each other to determine if the gun is producing individual characteristics in a unique and consistent pattern.  Once a consistently reoccurring pattern is identified on the standards, the standards can then be compared to the evidence to see if the same pattern of marks exists.

Standards collected from guns fired in the laboratory are not only compared to fired evidence from crime scenes, but are also compared to our Unsolved Case File.  The Gun Recovery and Identification Program, also known as GRIP, consists of a collection of fired evidence samples that are associated with unsolved shootings in DuPage County.  Under GRIP, standards collected from guns submitted to the laboratory are screened against a digitized and catalogued unsolved case file.