Firearms Identification

1
Firearms Identification
2
Forensic firearms examinations

• identification of a bullet, cartridge case, or
  other ammunition component as having been fired
  by or in a particular firearm.
• the possibility for such singular identification
  can be attributed to specific machining processes
  used in the manufacture of firearms.

3
Rifling

• Helical grooves (rifling), cut into the bore of a
  barrel of a firearm during production, increase
  the accuracy of that firearm
• Effects
• Spin
• Stabilized
• Marks on the bullet (lands and grooves)

4
A rifled barrel

• will contain scratches, scrapes, and other minute
  nicks and flaws
• exacerbated through the subsequent use and
  discharge of the firearm
• or as a result of natural wearing processes such
  as rusting and corrosion
• bullet will have unique microscopic markings as a
  result

5
Firing pin impressions

• During the discharge of a firearm, the firing pin
  strikes the primer of a cartridge, creating
  microscopic contact marks and unique
  indentations.

6
Breechface impressions

• As the powder within the cartridge begins
  burning, the cartridge case is propelled
  backwards against the breechface with enough
  force to be impressed with the characteristic
  microscopic features of that surface.

7
When no firearm is recovered

• rifling parameters taken from the recovered
  bullets can be used to determine the possible
  manufacturers of the firearm from which the
  bullets were fired.
• General rifling characteristics or GRC are the
  identified rifling pattern (i.e. 8/right) and the
  diameters of the individual lands and grooves. 
• Using these parameters, firearm examiners can
  search through a database of known rifling data. 

8
A typical GRC search

• a 9mm LUGER bullet, fired from a 6/right rifled
  barrel, with a land width of .055 and a groove
  width of .125. 

9
Results of Search
CARTRIDGE MANUFACTURER TWIST LG LAND GROOVE
9MM LUGER AA ARMS INC R 6 .055 .120
9MM LUGER ASTRA R 6 .053 .128
9MM LUGER BERETTA R 6 .055 .130
9MM LUGER HI-POINT FIREARMS R 6 .055 .120
9MM LUGER INTERDYNAMIC R 6 .055 .124
9MM LUGER LLAMA R 6 .054 .120
9MM LUGER MAUSER R 6 .054 .128
9MM LUGER SMITH WESSON R 6 .056 .122
9MM LUGER STAR R 6 .054 .126
9MM LUGER SWD INC R 6 .055 .120
10
FBI Database

• The database used by firearm examiners all over
  the world is compiled and maintained by the FBI.
• The FBI has been recording rifling data from
  firearms for over 20 years.
• As new firearms are manufactured, the rifling
  data is included in the database.

11
Case Study

• Many years ago, a double murder occurred in our
  community. An elderly couple was found shot to
  death in their home by their son.
• It appeared from the scene that there had been a
  break-in but the son was immediately a suspect.
  Investigators had a feeling that the son may have
  committed the crime but could not determine if he
  had owned or had access to a firearm.
• Bullets from the scene were examined and it was
  determined that they were of 38 caliber and were
  fired from an 8/right rifled barrel.

12
Case Study

• A list of manufacturers was compiled and given to
  the investigators.
• Using this list, the investigators searched a
  weekly "Bargain Mart" classified ad publication.
  By calling the ads for firearms included on the
  list of possibilities, the investigators found
  one individual who said he had sold his revolver
  to an individual who matched the description of
  the son.
• The son was then picked out of a police line-up
  by the individual who had sold him the gun. When
  confronted with the evidence the son confessed to
  the killings.