Memory effect in firearms that is, the possibility for a weapon to release inorganic particles whose elemental composition depends on its entire shooting history, is responsible for most of the interpretation difficulties encountered in forensic gunshot residue analysis. The presence of residues chemically inconsistent with the last discharged round, the creation of particles having unusual elemental profiles, and the dependence of residue population composition on the collection point are all manifestations of memory effect. The experimental results reported in this paper highlight the ineffectiveness of a wide number of gun cleaning procedures in reducing memory effect. Moreover, the common alternative of discharging batches of rounds having a "new" primer mixture does not fully eliminate the possibility to recover "old" residues at least from the shooter's hands. Two brand new pistols and ammunition having lead-based, leadless and heavy metal free primers were used. Specimens, collected both from the shooters' hands and from cotton targets set nearby the gun muzzle, were analyzed by SEM-EDS and by ICP-OES. After discharging 10's of new ammunitions, the number of old residues ejected from the gun muzzle indeed showed an asymptotic decrease to zero. In spite of this, the number of old residues recovered from the shooter's hands did not follow any predictable trend. These different behaviors suggest that all internal components of a gun, and not just the barrel, play a role in memory effect.
Keywords: firearms; inductively coupled plasma optical emission spectrometry (ICP‐OES); inorganic gunshot residues (IGSR); memory effect; scanning electron microscope energy dispersive spectroscopy (SEM‐EDS); weapon cleaning procedures.
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