Did this gun fire this bullet or cartridge case? This is one of the most frequently asked questions asked of the forensic firearms expert. Clearly the answer to this is of vital importance when it comes to investigating gun crime.
The science behind this aspect of firearms identification relies upon the fact that at some microscopic level the profile of all surfaces differ. When a relatively soft bullet passes down a harder steel barrel marks from the rifling, that is the helical set of grooves and lands in the bore that impart spin stability to the projectile, will engrave the bearing surface of the bullet. Similarly marks present on the hard steel internal components of the gun, such as the firing pin, chamber, extractor and ejector can also mark the softer metal cartridge case. Even the roughly sawn off end of a shortened shotgun can uniquely mark the plastic wad found inside a shotgun cartridge. Guns are tools and tools can transfer marks onto the surfaces they come into contact with.
The skill of the forensic firearms expert comes in assessing the significance of these transferred marks. In general the type of marks that can be transferred fall into three categories, i) Class ii) Sub-class and iii) Individual. Class marks are common to a set of tools, for instance the width of the blade of a screwdriver or with firearms the number of rifling grooves and lands together with the direction of twist of the rifling. For instance the maker Colt typically uses six lands and grooves directed to the left. Sub-class marks are unintentional manufacturing marks transferred to a batch of tools. Then there are the individual marks that enable a possible identification to be made, these can arise from damage or wear to the tool or from the manufacturing process itself.
These marks present on bullets, airgun pellets, plastic shotgun cartridge wads and cartridge cases can be compared to marks present on other samples by using a comparison microscope. This piece of equipment, in effect two microscopes joined by an optical bridge, uses a prism to enable two objects to be viewed simultaneously, side by side, at the same magnification. Fig.1 and 2 shows a bullet and a cartridge case comparison, respectively. Figures 3 and 4 show the transferral of sub-class marks from the gun’s breech face to the cartridge case. Unfired cartridges can also be examined to see if there are any tool marks on them that have been transferred from being loaded into a firearm. Consequently it is possible to link live ammunition to a specific gun and to fired cartridge cases.
The principle skill required by the expert is pattern recognition. In order to come to positive conclusion the examiner must be satisfied that the level of agreement exceeds that of any known best non-match. If the level of agreement is sufficient one can conclude that for all practical purposes a single firearm was used. This takes years of experience. Recent research has built on work carried out in the 1950’s to give a statistical foundation to the theory of tool mark identification.
The Theory of Identification indicates that the degree of correspondence which must be exceeded to constitute sufficient agreement for an identification is the best known non-match (by each individual examiner) to have been produced by different firearms. Ideally, the examiner would gain experience in this during their initial training period rather than when they begin to perform actual examinations on their own. This Theory of Identification indicates that, although founded on the scientific method and reproducibility of results, the interpretation is subjective in nature. It is the policy of most laboratories that a second qualified examiner verify the findings of the first examiner. Ultimately, sufficient agreement is the product of the examiner’s personal training, skills, and experience in recognising corresponding patterns of matching striations, recognising corresponding patterns within tool marks, determining the best known non-match in their personal experience, comparing striated and impressed marks. It is incumbent on each examiner to rely on their training and experience to identify and to be able to articulate the process used to determine sufficient agreement and best known non-match.
An evolving concept in striated mark comparisons is the developing study of consecutive matching striae (CMS) as a quantitative method of describing an observed pattern match. CMS is simply a means of articulating the best known non-match described and defined by the Theory of Identification.
CMS was initially proposed in a paper written by Al Biasotti and published in the Journal of Forensic Sciences in 1959 (“A Statistical Study of the Individual Characteristics of Fired Bullets”). In an extensive analysis of 720 known non-match comparisons of land and groove impressions in fired bullets, Biasotti found no instances in which the CMS exceeded four. In 1997, Biasotti and John Murdock jointly published their conservative quantitative criteria for identification as expressed in terms of CMS.
A number of significant studies have been carried out in an effort to further validate these conservative criteria. Although not necessarily practiced by all firearm
examiners, these criteria are of growing importance due to these factors:
– The expectations of more sophisticated jurors
– The need for more objective identification criteria
– The potential for increased credibility for examiners in the courtroom
A five-point scale can be used to explain the findings in a little more depth which allows a little more flexibility and adds weight to the evidence. A five-point scale is considered best practice, although a three-point scale is widely accepted.
Conclusive (Identification)
This is the ‘Highest’ conclusion that can be drawn in a Firearm marks comparative examination. In this situation, the class characteristics match and the individual characteristics are certainly distinctive. This is concluded on the basis of current knowledge and available means, based on the observed similarities in the individual characteristics that the items have. The chance that the observed similarities would have occurred if the items had been fired by different firearms is considered virtually non-existent.
Strong (Could have)
This is the case where the class characteristics are the same but, despite the observed similarities in the individual characteristics, the examination still raised some doubts, e.g. because the striations/imperfections match partly or the similarities are not very convincing and/or because there is some doubt about the distinctive value of the marks. There is more agreement than disagreement.
Inconclusive
This is the case if the items might/might not have been fired by a single firearm. The class characteristics match those of the evidence firearm but no (distinctive) similarities or differences between the individual characteristics could be found. Alternatively, differences in individual characteristics have been observed that can be explained, however, by changes (wear, corrosion, deliberate alteration) in the specific firearm. There is equal agreement and disagreement.
Limited (May not have)
This conclusion is used if the class characteristics match and differences in the individual characteristics have been found. The examination, however, raise some doubt, e.g. because the differences were not so evident and/or because it could not be safely assumed that the differences were not caused by changes (wear,
corrosion, deliberate alteration) in the firearm involved. There is more disagreement than agreement.
Exclusion (Elimination)
This conclusion is drawn if the class characteristics match and the individual characteristics show clear differences. It is all but certain that no changes have occurred in the firearm or machine components (wear, corrosion, deliberate alteration). The chance that the item was still fired by a single firearm is considered virtually non-existent. It has sufficient disagreement.
Any critical comparison microscopy conclusion must be subject to peer review, which ideally should consist of an experienced, trained and competent expert redoing the comparison. It is not uncommon for experts to disagree on the significance of corresponding marks found during firearms related comparison microscopy.
In the UK the police collate ammunition components, recovered from the scenes of unsolved crimes. When a gun, spent bullets and cartridge cases are seized they are screened through this collection to see if there is any previous use of the same gun. This can be a long, labour intensive process, however in recent years there have been huge developments in the automation of the comparison microscopy process. However it is still the case that any potential match must be manually looked at by an expert.
Although underpinned by the scientific method the process of matching guns to bullets and cartridges cases is essentially a subjective one, relying upon the
competence, experience, knowledge and opinion of the expert. In any case involving this type of evidence it is therefore essential that any conclusions concerning a potential match are subjected to rigorous challenge and peer review.
Mark Mastaglio
© The Forensic Firearms Consultancy Ltd 2015