MECHANISM OF FORMATION OF GUNSHOT RESIDUES.

Mechanisms of formation of gunshot residues by Forensic Yard

Gunshot residue, also known as firearms discharge residue, is the residue that gets deposited on the hands and clothing of the victim when a gun is fired.

When a gun is fired, especially a revolver, powder residue from the propellant and primer may be blown around the cylinder. Traces of such residues are sometimes transferred to the hands of the shooter.

Their location and identification play a critical role in the investigation trials. The absence of GSR can also provide valuable information.

GUNSHOT RESIDUES

GSR is the unburnt particles, gases, and non-volatile substances coming from primer, propellant, bullet, and firearm.

Gunshot residue consists of organic as well as inorganic components. Organic compounds present are mainly from the primer and the propellant such as nitrocellulose, lubricants used for lubrication of bullets, nitro-glycerine in the case of double-base propellants, and organic materials such as plasticizers.

Inorganic components from the primer and propellant include lead, barium antimony, nitrite, nitrates, copper, zinc, carbon, lead, etc.

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FORMATION OF GUNSHOT RESIDUE

When a gun is fired, a high volume of incandescent gaseous material is produced. These gaseous materials are mainly produced as the combustion products from all the propellants and consist of carbon dioxide, carbon monoxide, water as steam, and nitrogen oxides. This vast cloud of gases is also partially burnt and unburnt propellant particles and combustion products from the priming compound.

The moment when the firing pin strikes the primer, the priming compounds explode violently. At this moment, the temperature of the priming cap rises approximately to 2500°c. The metallic compounds present in the residue are volatilized and emerge from the primer pocket as vapor. This cloud of vaporized metallic components rapidly condenses, forming exceedingly small spherical and spheroidal particles in the size range 0.1-5µ. 

In case of close-range firing, dust ring, burning, blackening, and tattooing appear as GSR on the target. While in that of distant firing, no powder pattern is available, tiny metallic particles, which have been cut during traveling through the barrel, appear on the edges of the entry wound on the targets.

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Conclusion

The presence of GSR at the expected regions of hands, arms, face, and clothes of the suspect also helps to ascertain whether he or she had fired the projectile or not.

The firing of a weapon propels residue toward the target and blows gun powder and primer residue towards the shooter. Based on the location of the GSR found, it can be determined whether it was a suicide or a homicide.

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