Forensic firearms casework examination encompasses the following major areas of work:
Examination of firearms, ammunition, and associated items
Examination of firearms and associated items for anything else of forensic interest, e.g., blood, hairs, fibers, glass, fingerprints
Comparison macroscopy of spent bullets and spent cartridge cases
Identification of weapon types from spent cartridge cases and spent bullets
Examination of swabs and clothing from suspects for firearm discharge residues
Examination of clothing and miscellaneous items, e.g., identification of bullet holes, differentiating between entry and exit holes, angles of fire, range of fire
Identification of bullet strike marks, firing points, weapon hides, etc.
Chemical comparison of bullet fragments and propellants
^Serial number restorations ^Examination of scenes of crimes ^Presentation of evidence in courts of laws ^Training of laboratory staff, police, and scene-of-crime officerss
In addition, various research and development projects are undertaken and in Northern Ireland there is also an intelligence-gathering aspect to the work.103,104 Firearm discharge residue work is an important aspect of the overall workload.
firearm Discharge residues
The gases, vapors, and particulate matter formed by the discharge of ammunition in a firearm are collectively known as firearm discharge residue (FDR) or gunshot residue (GSR) (Photograph 16.1). Anything present in the
ammunition may contribute along with a possible contribution from the firearm itself. The residue consists of inorganic and organic constituents.
Inorganic constituents could originate from the primer mixture/cup/sealing disc/anvil, the cartridge case, inorganic additives to the propellant, the bullet core and jacket, metallic containing pigments in colored lacquers/sealers/lubricants, inorganic trace impurities in any component, and also from the chamber/barrel interior/muzzle of the firearm in addition to inorganic debris already present inside the firearm before discharge. The main sources of inorganic discharge residue are the bullet and the primer.
Organic constituents could originate from the primer mixture, the pro-pellant, sealers/lacquers/lubricants from the ammunition, and also from lubricants and organic debris already present inside the firearm before discharge. The main source of organic discharge residue is the propellant.
Firearm discharge residue consists of a complex heterogeneous mixture that is claimed to be mostly particulate in nature.105 Particulate matter can be detected on a suspect, but the possibility of vaporized/gaseous products being adsorbed on to skin or clothing surfaces also exists.
Interior ballistics has been defined as the science that investigates the way the chemical energy stored in the propellant (and to a much lesser extent the primer) is released and converted into the kinetic energy of the projectile.106 Approximately 30% of the chemical energy is converted into kinetic energy; the rest is accommodated in the discharge residue. The discharge of a round of ammunition in a firearm produces high temperature and high pressure for a very short period of time. A typical time period from the hammer or firing pin striking the primer to the bullet or shot exiting from the muzzle is in the region of 0.03 seconds. As a result of the time period and the nature of the discharge process, only partial mixing of the constituents occurs and this accounts for the very heterogeneous nature of firearm discharge residue.
When a round of ammunition is discharged in a firearm, in addition to the projectile(s), firearm discharge residue is emitted, mainly from the muzzle but also from cylinder gaps, ejection ports, and other vents in the firearm. Some of this residue may be deposited on the skin, head hair, and clothing of the firer.
In the vicinity of the muzzle the hot propellant gases ignite and emit light on contact with the surrounding air. This effect is called muzzle flash (the bullet can also cause a flash of light whenever it strikes a target that has a hard abrasive nature107,108). The high pressure propellant gases at the muzzle are released into the air causing considerable turbulence and producing a powerful blast shock. This effect is known as muzzle blast.
Muzzle blast residue can be deposited on the target if the muzzle-to-target distance is less than 3 feet,109,110 that is, a close range shooting. (The term "point blank range" is often incorrectly used in films, TV, and so forth when referring to a contact or close range shooting. The term refers to the distance a projectile will travel before dropping enough to require adjustment of the firearms sights.) Muzzle blast residue nearly always contains unburned or partially burned propellant and soot. If the firearm is fired perpendicular to the target, the resulting residue pattern will be in a roughly circular area around the entrance hole. The diameter of the circle and the density of residue depend on the distance between the muzzle and the target. The pattern size, shape, and density enable range of fire to be determined, give an indication of the angle of fire, and very occasionally the propellant granules can be identified from physical appearance as originating from a specific type of ammunition and enable a tentative identification of weapon type. Chemical analysis of the granules can yield useful information for comparison purposes with suspect ammunition. However, it must be borne in mind that a propellant can change, both in color and chemical composition, because surface coatings may be blown or burned off during discharge and the original granules may not all have a uniform composition.
High-speed photographic techniques have shown that during the discharge process some smoke emerges from the muzzle ahead of the bullet and also that just after the bullet leaves the muzzle it is surrounded by a large cloud of smoke over a short distance.111
The reason(s) for smoke emerging in front of the bullet could be (a) some of the discharge gases escaping in front of the bullet as the cartridge case neck begins to expand just before the bullet starts to move out of the case neck, (b) gases escaping past the bullet before the bullet completely engages the rifling, and (c) obturation not being absolutely complete, allowing gases to escape in front of the bullet through the rifling grooves.
The propellant gases expand rapidly on release into the atmosphere, accelerating to velocities much greater than that of the bullet, and this is why the bullet is surrounded by a large cloud of smoke.
As a result of these effects, FDR is deposited on the surface of the bullet. Whenever the bullet penetrates the target, due to the wiping action of the target material, some of the FDR on the bullet surface is transferred to the perimeter of the entrance hole. This occurs irrespective of the range of fire. On passing through the target, some, if not all, of the lightly adhering residue is removed from the bullet surface and consequently is not present or is present to a much lesser extent on the perimeter of the exit hole. This allows identification of bullet holes and bullet strike marks and differentiation between entrance and exit holes, which can in turn yield useful information about angle of fire, possible firing points, and relative positions of firer and target. The presence of FDR (its location, density, and nature) at a crime scene can often assist in the interpretation of the sequence of events.
Detailed analysis of the perimeter residue often yields useful information, for example, whether the bullet is unjacketed or jacketed and the nature of the jacket material. Primer type may occasionally be inferred and the presence of strontium or magnesium indicates a tracer or incendiary bullet, respectively.
Crimes involving the use of firearms are particularly serious and demand the fullest investigative effort. Thus it can be seen that the detection, identification, and quantification of FDR provide significant evidence in several areas associated with incidents involving the use of firearms.
One of the most important, difficult, and challenging aspects of the work is to connect a suspect to a firearm, or to an involvement with firearms, by the presence of FDR on the suspect's skin, head hair, or clothing, or inside the suspect's dwelling or motor vehicle.
A satisfactory test for the identification of FDR on a suspected firer has been sought by forensic scientists for many years. A satisfactory test would be one that is simple, reliable, fast, inexpensive, and conclusive. Until recently efforts have concentrated mainly on the detection of inorganic components of FDR and encompass qualitative and quantitative methods, culminating in the particle analysis method, which is the most informative method currently available. A brief outline of the most important developments follows.
Development of Firearm Discharge Residue Detection Techniques
Was this article helpful?