Firearms And Ballistics
Firearm investigation is a specialty of forensic science focusing on the examination of firearms and related subjects. Closely linked to this is ballistics, which relates to the flight path of projectiles, often associated with forensic science during the investigation of firearms. This area of study examines the path of a bullet from when it leaves the firearm up until it strikes the target. During investigations in which the use of firearms is suspected, a number of artefacts may be collected for examination, including firearms, cartridge cases, bullets, live ammunition, trace materials, and any material damaged by a projectile.
Firearms and Ballistics
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The manner in which a firearm is loaded after each shot will vary depending on the type of weapon. Some firearms are bolt action, meaning a bolt ejects the spent cartridge and, when pushing forward, picks up a new cartridge and places it in the chamber, cocking the trigger during this process. Manual action firearms require manual reloading by means of a mechanical device such as lever or pump action. In recoil operated or blow back weapons, pressure generated by the ignited propellant drives back the bolt. Gas operated firearms include a gas port and, until the bullet has passed this point in the barrel, the bolt is locked. An amount of gas leaks into this port and unlocks the bolt, allowing it to move backwards. This ejects the spent cartridge case and loads another. Finally, in a revolving cylinder weapon, pressure on the trigger causes a cylinder containing the cartridges to rotate, positioning a new cartridge so that it may be fired.
Initially, any cartridge cases retrieved from the scene should be accurately measured in all dimensions to aid future comparison. The examination of any cartridge cases found can provide clues as to the firearm used during the shooting. For example, cartridges designed for use in a revolver have projecting base rims, whereas those designed for self-loading firearms do not.
Cartridge cases also bear more distinguishing features that can be used to identify them. The headstamp is an indentation produced at the base of many cartridges during the manufacturing process. These markings can then be used to trace a cartridge back to the manufacturer and determine the make and type of the ammunition. Various sources are available to help identify headstamps. When the firing pin strikes the cartridge case, a characteristic indentation is caused that can be used to link cartridges to specific firearms, similar to the comparison of rifling marks (discussed below). Other markings that should be looked for include ejector, extractor and breech face marks. Firearms often have different firing pin, extractor and ejector designs, therefore the examination and comparison of marks produced by these implements can aid in establishing the firearm used. It should be noted that it is possible for cartridge cases to be reloaded with a new bullet and fresh propellant and primer and reused, in which case the cartridge may bear numerous additional markings. Furthermore, cartridge cases recovered from the shooting scene should be examined for fingerprints and similar forensic evidence.
RiflingDuring the manufacturing process of firearm barrels, a series of spiralling lands and grooves is produced along the inside of the barrel, known as rifling. Land refers to the raised portion of these spiralling indentations, whereas the grooves are the lower portions between these lands. Rifling, which can be one of numerous types, will be cut with either a left or right hand twist, determining whether the bullet will spin clockwise or anticlockwise. As the bullet passes through the barrel, these markings cause the projectile to spin, increasing stability and accuracy whilst leaving characteristic marks on the bullet itself. The grooves present on the surfaces of bullets are unique to the barrel that caused them, making rifling patterns ideal in matching bullets to specific firearms. Bullets are often viewed side by side using a comparison microscope, allowing rifling patterns to be contrasted and any similarities noted. The type and number of spiral grooves, their measurements, and whether they rotate clockwise or anticlockwise can help narrow down the search for the weapon used. Measuring the calibre of a bullet and the angle of grooves can allow for the calculation of twists per metre and thus provide further details of the rifling of a particular firearm.
Serial Number RestorationDuring the manufacturing process, legally produced firearms are stamped with a uniquely identifying serial number, usually on the barrel or action. These numbers are stamped into the firearm, a process which also impresses the digits below the surface of the metal. Even though criminals may attempt to erase these serial numbers to avoid the weapon being traced, it may be possible to restore these serial numbers to a state in which they are legible. Serial numbers are often erased by filing or grinding, which will not necessarily remove the digits below the surface. Alternatively the perpetrator may attempt to change the serial number. Various techniques and reagents have been used to successfully restore these original numbers.
Gunshot ResiduesWhen a firearm is discharged, a cloud of gases and fine particles is released composed of gunshot residues (GSR), sometimes referred to as firearms discharge particles (FDRs) or cartridge discharge residues (CDRs). The mixture will often contain both organic and inorganic particulates, the organic matter consisting of unburned and partially combusted propellant and inorganic matter produced by hot gases acting on the bullet. When released, these fine particles will settle on any nearby surfaces and are easily carried away from the scene. The presence of such residues can provide strong links between suspects or objects and the scene of a shooting, therefore various methods of detecting gunshot and other residues have been developed.
Scanning electron microscopy with energy dispersive X-ray spectroscopy has been successfully utilised in visualising and detecting minute particles associated with firearms. This technique allows for the morphology of the particles to be observed and their chemical compositions established.
Neutron activation analysis (NAA) is a technique primarily used for determining concentrations of elements and has been used in the analysis of residues from firearms. However the use of this technique is very expensive and requires access to a nuclear reactor which is not readily available to all organisations. Flameless atomic absorption spectroscopy (FAAS) largely replaced the use of NAA due to it having various advantages and costing a more reasonable price.
Suspects of shootings incidents may claim that the firearm was unintentionally discharged, either accidentally by the individual or through malfunction of the weapon. Various tests can be conducted on the suspect firearm to help establish the details of the shooting. Trigger pressure relates to the force required to pull a trigger and fire a weapon. In some cases firearms with light trigger pulls may result in accidental discharge, so by calculating the trigger pressure it may be possible to determine the likelihood that the trigger was accidentally pulled. Some firearms allow the user to select either normal trigger pull or light trigger pull (hair trigger); therefore it is also important to discover whether the firearm has this feature and which setting was selected. Firearms are often fitted with numerous safety mechanisms. The examination of the firearm should include the investigation of these mechanisms to conclude whether any of the safety features were malfunctioning. An investigation known as a jarring test may also be performed, in which the firearm is subjected to a series of impacts involving various surfaces and distances to determine whether the action could have resulted in the firearm being discharged.
DatabasesDatabases have been produced to store images of bullets and cartridge cases, allowing comparisons and matches to be made. In the UK, the Forensic Science Service and the Association of Chief Police Officers set up the National Firearms Forensic Intelligence Database, a system allowing information on weapons and firearms to be stored and analysed. This was later replaced by the National Ballistics Intelligence Service database. The FBI sponsors the database Drugfire, an automation nation system combining images input by users with a reference library.
Firearms-related evidence can be used both in relation to the main crime but can also lead to the building of strong parallel criminal cases, such as international firearms trafficking. Sometimes, one piece of evidence can help in both investigations. For example, a ballistic comparison can confirm that a firearm was used in a murder case but also in other crimes committed in another country, which is already an indicator of the routing of that firearm.
Too often, the value of evidence that firearms, ballistics and ammunition provide is overlooked in criminal investigations; enforcement operations stop at the point of seizure or recovery. Yet, seized and recovered firearm-related items may themselves provide critical evidence of a broad range of additional crimes, such as firearms trafficking and illicit manufacturing.
Health and safety procedures are of paramount importance when arriving at crime scenes and should remain a priority throughout the process, to ensure the safety of officers and others present. Accordingly, crime scene examinations must be undertaken with the support, guidance and supervision of suitably trained firearms and other experts, and/or forensic firearms experts, who can help ensure all activity is safe while not compromising potential forensic evidence.
Firearms identification, as detailed in Module 2, is a fundamental step in firearms trafficking investigations because it provides the essential elements for the unique identification and tracing of the firearm. 041b061a72