Examination of Firearms Review: 2007 to 2010 - Interpol

another review in this Symposium, this section of my Review will focus mainly on
those aspects more relevant to toolmarks per-se.
A general review of the procedures involved in firearm and toolmark identification
and the validity of the results was published by Bunch and his colleagues (66).
Following a description of the multi-level examinations performed by the examiner,
the scientific foundation of this field and the way conclusions are being drawn,
these authors concluded that the firearm-toolmark discipline is both highly valuable
and highly reliable in its traditional methods. However, additional research is very
beneficial and, depending on its purpose and design, would tend to better address
potential error, identify manufacturing methods that are suspect for comparison
purposes, and further develop machine systems and perhaps probabilistic models.
The Scientific Working Group for Firearms and Toolmarks (SWGGUN) has
recently published guidelines covering several aspects of firearm and toolmark
examinations, like criteria for identification, training and quality assurance (67),
which are relevant for toolmark examinations. These guidelines, like other
SWGGUN documents, are publically-available on the Internet. On the light of
recent challenges on the admissibility of various scientific evidence types in court,
SWGGUN maintains an admissibility resources kit (ARK), assisting firearm and
toolmark examiners prepare better for court appearances and face Frye, Daubert
and other similar challenges.
3.1 Casting and Reproduction Methods
Many cases of toolmark examinations involve the need for duplicating the
impression marks, found at the scene of crime, in order to facilitate their
examination at the laboratory. Several materials had been proposed for this
purpose, like silicone rubber and dental stone.
Today, various suppliers of metallographic, dental and crime scene investigation
products are offering silicone rubbers in easy-to-operate dispensers (dispensing
guns), so avoiding the need for mixing the base with the hardener manually.
Among these may be mentioned, for instance, the Forensic Sil (by Loci Forensic
Products, The Netherlands), the Isomark products (UK), the Struers RepliSet
(Denmark) and the AccuTrans (USA). These dispensing guns (AccuTrans) were
evaluated by Watkins and Brown (68) and by Naccarato and Perersen (69), who
found them to be both practical and accurate for most toolmark applications.
Yo and his colleagues (70) compared five commercially-available casting
materials, including four types of silicone rubber and dental stone, and evaluated
their performance. Class and unique characteristics of the surface in question (a
Canadian two-dollar coin – “Toonie”) and on the casts were examined visually.
Their results demonstrated clearly that all tested silicone rubbers outperformed the
dental stone for this purpose. Despite some minor differences found between the
silicone rubbers, no definite recommendation is given regarding any of these
products.
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