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The paper discusses the interpretation of gunshot residue (GSR) evidence and how it can provide valuable insights into activities associated with the use of a firearm and connect them to a person of interest in a criminal investigation. The paper also outlines the general principles of the interpretation of analytic results and frameworks used by forensic practitioners. It is argued that there are two approaches to the interpretation of GSR evidence irrespective of its origin or methods adopted: formal and case-by-case.

The latter approach is commonly applied by forensic scientists because it helps them to take into consideration specific circumstances of a case. The case-by-case approach to the interpretation of GSR evidence is rooted in the use of the Bayes’ theorem, which allows calculating the likelihood ratios of competing hypotheses. The paper also discusses the details of People v. Robert Blake to show the application of competing propositions at the source and activity levels in the interpretation process.


The interpretation of the evidence is at the core of forensic science; therefore, aspiring forensic experts have to utilise coherent, logical frameworks for ascribing meaning to and making inferences about single or multiple items of evidence. It is of utter importance to have a deep understanding of the theory of interpretation to make informed choices of different interpretations pertinent to a particular field of activity. The understanding of competing interpretation frameworks is especially important for judging the validity of firearm discharge residue (FDR) evidence or gunshot residue (GSR) evidence interpretation due to the stochastic process of residues formation (Ditrich 2012).

The aim of this paper is to discuss the interpretation frameworks and issues associated with GSR evidence. The paper will also discuss general principles of evidence interpretation and how they help to arrive at a balanced expert opinion that can be used to inform parties in legal proceedings.


Forensic specialists recognise that interpretations of forensic evidence are prone to the introduction of errors, which can have substantial consequences in legal contexts. It follows that practitioners’ interpretation should be performed within logically-consistent frameworks irrespective of adopted methods (Morrison 2014). The importance of this assertion is emphasised by the fact that the analytical dimension is only one sphere of forensic experts’ interest. The other one is concerned with judicial and investigative aims of the trade, which necessitates the cooperation with numerous stakeholders in the legal process.

The modern forensic science relies heavily on the principles of Bayesian reasoning. These principles allow to reason under uncertainty and communicate results of the reasoning in an effective manner. A forensic practitioner, in the process of interpretation, is guided by the rule derived from the calculation of the likelihood ratio based on the Bayesian model according to which the process cannot be divorced from a framework of circumstances. The framework is formed by the elements of interpretation such as time, actions, and location, among others (Evett et al. 2000). The more certain elements in the framework, the more accurate the interpretation.

The second principle is that the interpretation can only be considered valid if at least two competing hypotheses are proposed and addressed. The third principle calls for considering the probability of evidence at hand with respect to the proposed propositions. The application of these principles should be universal and has to start at the moment when a case is presented to the specialist (Evett et al. 2000).

The evidential interpretation must be impartial; therefore, it is not sufficient to direct analytical lenses at proposition only. To promote clarity and logical reasoning, the hierarchy of propositions must also be considered. Furthermore, the stratification of competing hypotheses helps forensic practitioners to understand two opposite positions—that of prosecution and that of defence (Evett et al. 2000). At the highest level of the hierarchy, a proposition can come in the following form: Mr. Lambert murdered Ms. Smith. A competing proposition can be articulated as follows: some other man murdered Ms. Smith.

At the activity level, forensic experts should consider circumstances of a case in order to form the two opposite hypotheses: Mr. Lambert discharged a murder weapon, and some other man discharged the weapon. At the lowest level of the hierarchy of proposition, it is necessary to consider the following two hypotheses: the trace recovered from the suspect, weapon, or the crime scene are GRS, and the trace is not GRS. Forensic specialists with extensive experience recognise that the creation of propositions is the most challenging part of the evidential interpretation process.

When interpreting evidence, it is not sufficient to provide verbal support to one of two hypotheses; rather, it is necessary to quantify their likelihood ratios. The magnitude of the ratios reflects the support for each proposition. Such numerical values, which are calculated with the help of the Bayesian equation, are extremely important for reporting evidence’ weight (Morelato et al. 2012). The equation, its role in forensic science, and its application to the evaluation of competing hypotheses will be discussed at length in the following section of the paper.

Interpretation of GSR Evidence

GSR Evidence

GSR refers to discharge materials produced by the explosion of a cartridge during a firearm discharge (Chang et al. 2013). GSR is comprised of discrete burnt and unburnt products arising from the propellant, the primer, the cartridge case, the bullet, and the firearm (Chang et al. 2013). GSR produced by the primer is referred to as inorganic GSR or IGSR; GSR stemming from the propellant is called organic GSR or OGSR. Given the extreme scale of the force with which volatile, gaseous particles exit the firearm’s muzzle, the majority of them is deposited on the target.

However, since GSR also escapes other openings of the weapon such as the ejection pot and breach area, it can be detected on other surfaces that include, but are not limited to, the shooter’s hands close, and hairs as well as other objects in the vicinity of the discharge (Morelato et al. 2012). The distribution of GSR is affected by numerous factors the key of which are the location, barrel length, distance from the culprit, time ager shooting, the ammunition, and weapon type (Ditrich 2012).

When it comes to the interpretation of GSR evidence, the primary focus of forensic specialists is to classify it as IGSR. The sub-source level classification deals with the uncertainty of origin attribution regardless of the case under consideration. It has to do with the fact that the residue can be attributed to environmental sources. Another level of the forensic expert’s task concerns the persistence of the particles and the risk of secondary transfer (Charles & Geusens 2012). Thus, the role of the practitioner is to arrive at expert opinion on GSR evidence by taking one of two interpretative routes: formal approach and case-specific approach.

Formal Approach

The formal approach to the interpretation of GSR evidence was developed in 1979 by Wolten and associates who classified particle composition and morphology (Maitre et al. 2017). The seminal research conducted by the scholars resulted in the emergence of formal classification standards such as those produced by the American Society for Testing and Materials (ASTM). Forensic practitioners used to compare their results to classifications of characteristic particles described in standard guides to make expert conclusions. Given that each case’s specific conditions were not considered during the interpretation, this approach was dubbed as formal by Romolo and Margot (2001).

Under the formal framework, the characterisation of GSR is performed through scanning electron microscopy (SEM) with the help of an attached energy dispersive X-ray spectrometer (EDX). The analytical focus of SEM/EDX investigations is on special surface layers of GSR, which allowed to make judgements about its morphology and microstructure (Sturm, Schartel & Braun 2012). The interpretation of GSR evidence with the help of ASTM guidelines revolves around the “difference between the court questioning and the conclusions reached by the forensic scientists” (Maitre et al. 2017, p. 3). Whereas the courts interested in the activities before, during, and after the discharge of the firearm, forensic specialists taking the formal approach to the interpretation focus on the source of GSR.

Taking into consideration the fact that evidence related to a specific case is not compared to all amount of recovered residue in order to prove that more than several particles fall under the criteria specified in classifications, the interpretation can be misleading. The lack of OGSR information also affects the interpretation of the evidence. Benito et al. (2015) argue that the use of lead-free ammunition further complicates GSR interpretation through the formal approach. Therefore, an alternative, case-by-case approach has emerged.

Case-by-Case Approach

The approach was proposed by Romolo and Margot (2001), who took an issue with the formal framework for its inability to interpret GSR evidence in the context of a particular case’s circumstances. The case-by-case approach presupposes the comparison of residue particles with the specific ammunition with respect to conditions of a case. The approach is rooted in the framework of Bayes’ theorem; therefore, it is also referred to as Bayesian approach.

To achieve impartiality of the evidential interpretation of GSR, the theorem’s requirement to use at least two mutually exclusive hypotheses is used: two samples of GSR are derived from the same source, and two samples of GSR are derived from different sources (Hannigan et al. 2015). The comparison of residues presupposes that their chemical composition consists of several classes, which accounts for substantial variabilities.

The following equation shows the application of Bayes’ theorem to the interpretation of evidence.


The application of the theorem to the interpretation of GSR evidence necessitates the calculation of “prior probabilities that concern the first degree of belief of stakeholders about each proposition” (Maitre et al. 2017, p. 3). The two hypotheses in the equation are Hp (prosecution) and Hd (defence). E represents evidence; I represents circumstances of a particular case.

The calculation of the likelihood ratio (LR) allows determining to what degree the evidence corresponds to one of the two hypotheses and measures its ability to discriminate between them. By applying the ratio, it is possible to translate the jury opinion into their final conviction with respect to a case. When conducting LR assessment, the forensic specialist has to consider relevant circumstances (I). Details about these circumstances can be derived from the person of interest, police, witnesses, or investigator, among others.

Information about the circumstances of a case can influence evidential interpretation; therefore, whenever new details emerge, the LR assessment has to be changed correspondingly. The generation of the two hypotheses prior to the completion of the results of an analysis is another principle that guides forensic practitioners working under the case-by-case framework. The assessment also involves the application of analytical analysis that produces evidence (E) with respect to the two propositions.

Hierarchy of propositions

The interpretation of GSR evidence involves the development of several competing propositions that can be classified into three levels: the source level, the activity level, and the offence level (Maitre et al. 2017). The offence level represents the highest order of questioning. It necessitates the assessment of information that is the most cases is not available to forensic practitioners and falls under the remit of triers of fact; therefore, its discussion is not included in the paper.

At the source level, the simplest question that should be asked is whether the recovered trace represents GSR. Thus, the aim of the evidential interpretation with respect to Hp is to assess the degree of compatibility between the recovered material and what is considered GSR. From this vantage point, it is clear that the competing proposition should claim that the material is not connected with the discharge under consideration.

Taking into consideration the fact that GSR or particles that resemble them can be produced by a variety of legal activities and environmental sources, it is important to have Hd proposition (Morelato et al. 2012). If the firearm and ammunition have been found during the investigatory process, it is necessary to consider whether or not the recovered residue corresponds to the GSR produced by the same ammunition and weapon as a reference (Morelato et al. 2012). Thus, the source level hypotheses deal with alternative sources of GSR.

Activity level propositions concern actions taken by the suspect prior to, during, and after the event under consideration. It is clear that the second level propositions represent the chronological dimension of the interpretive process. To take into account this dimension, the forensic expert has to assess the transfer and persistence of GSR. To form competing propositions, the specialist should have additional information about the deposition of the residue, its retention, and the time of the event (Morelato et al. 2012).

Secondary transfer is the parameter that should be considered by the forensic practitioner during the process of evidentiary evaluation because the presence of the trace is only indicative of the fact that the suspect handled the firearm. However, the presence itself does not confirm the fact because it can be explained by accidental contamination. Even though the probability of such a scenario is extremely small (0.02 for each of 3 particles (Pb-Ba-Sb)), it should not be disregarded by practitioners (Hannigan et al. 2015). Currently, there is no evidence of the secondary transfer of OGSR (Hofstetter et al. 2017).

When it comes to the persistence of GSR, it is important to understand whether or not circumstances of a case corresponding to the results. Specifically, it is important to established temporal connections between the alleged activity of the suspect and the sampling. Therefore, the second component that has to be considered by the practitioner is the persistence of GSR. By accounting for the component, it is possible to establish whether the trace has been left prior to or after the alleged activity of the person of interest. The persistence and variability of IGSR particles are used under the Bayesian approach because their characteristics tend to change with time. For example, the number of particles diminishes rapidly in the first 30 minutes after the discharge (Morelato et al. 2012). It follows that the inclusion of the component in LR calculations is of utter importance for proper evidential interpretation. The persistence of OGSR has not been sufficiently studied (Gallidabino et al. 2013).

People v. Robert Blake

The case involved Robert Blake, who was accused of discharging his weapon twice at his wife Bonnie Lee Bakley (Sweetingham 2005). The shots were fired from outside of the victim’s car. The suspect claimed that he was sitting next to his wife when the event occurred. The police did not take any precautions to ensure that the suspect’s hands were not contaminated when he was in their custody. His clothing was also open to contamination. At the time of the homicide, Blake carried a firearm other than the murder weapon (Burnett 2014). There were no witnesses of the event.

Murder Weapon

The murder weapon was a vintage 9 mm Walther pistol (Burnett 2014). At the source level, it was necessary to determine whether or not the weapon left the GSR on Blake’s hand. The investigators failed to recognise that it was not sufficient to show that the match of samples from the hand and clothes of the suspect indicate his involvement in the homicide. The process of evidential interpretation was conducted without the analysis of the elemental composition of the residue produced by the combination of the murder weapon and ammunition found near the scene. Instead, the investigators concentrated on “whether or not it produced breech GSR” (Burnett 2014, p. 125).

Prior to test firing, the weapon was cleaned with isopropyl alcohol, which was followed by the firing of lead-free ammunition (Burnett 2014). Therefore, the possibility of the sampling of the particle’s composition before the test was precluded. The casings from the crime scene were not sampled. It is clear that at the source level, it was necessary to show that there was a connection between the trace recovered from the suspect’s hands and the casings.

GSR from Hands and the Vehicle

The vehicle in which the homicide had been committed was sampled; the samples were examined with the help of SEM/EDS analysis. GSR samples from the suspect’s right hand showed five consistent characteristics, whereas the left hand revealed only one GSR particle (Burnett 2014). The samples revealed that GSR’s main features were aluminium, lead, antimony, and barium. However, aluminium could not be produced by the cartridges found at the crime scene, which suggested that it was an ingredient from ammunition that had been previously fired from the murder weapon.

Police Environment

Blake was apprehended by the police and delivered to the police station. Unfortunately, the police officers did not take the necessary precautions to ensure that he was not exposed to contamination in the police car or the station itself before the sampling was done. In addition, more than three hours elapsed between the shooting and the sampling.

Contamination plays an important role in the evidential interpretation of GSR. It has to do with the fact that particles of lead, barium, and antimony can be produced by numerous sources. For example, there are many sources of environmental pollution that can produce IGSR-like particles: welding processes, paints, and varnishes, among others (Morelato et al. 2012). A study conducted by Grima et al. (2012) explored the possibility of GSR contamination from fireworks. The findings of the study suggested that “some particles if treated individually were found to be indistinguishable from GSR” (Grima et al. 2012, p. 49).

The resemblances in morphology and shape between the two types of residue are attributed to the similarities in firing conditions. This fact has given rise to numerous independent lines of investigation challenging the evidential value of IGSR. A corollary is that the use of ASTM GSR criteria in the process of evidential interpretation should be abolished in favour of the case-by-case approach. Furthermore, it follows that the reference GSR population from the crime scene is absolutely necessary to determine the source of residue (Grima et al. 2012).

Automobiles are also known to produce particles similar to IGSR. Specifically, Pb-Ba-Sb is associated with automobile-related activities (Morelato et al. 2012). Brake pads can produce Pb-Ba-Sb particles; however, their morphology is usually angular. Airbag explosions can also deposit particles that can be confused with IGSR by forensic practitioners without sufficient experience. The findings of a recent study by Brozek-Mucha (2015) suggest that welding fume particles stemming from steel and aluminium alloy welding resemble IGSR. However, the study suggests that even though single particles containing aluminium, lead, and titanium are similar to those produced by the discharge of a firearm, the presence of iron and iron oxide particles in a large population can indicate their origin (Brozek-Mucha 2015). It means that forensic specialists should be cognizant of this fact while evaluating the evidential value of one or several particles.


The amount of the residue on the suspect’s hands was twenty times lower than could have been expected, given that no washing of hand was involved (Burnett 2014). Furthermore, the suspect was apprehended in only three hours after the shooting, which means that the lack of residue cannot be ascribed to deterioration. It shows that when interpreting the evidence at the activity level, forensic specialists had to recognise a lack of correspondence to the circumstances of the case. Burnett (2014) argues that even though several IGSR particles were recovered from Blake’s hands, their number should have approached 97. Therefore, the persistence of IGSR played a prominent role in the interpretation of the evidence in the case.


The paper has discussed the evidential interpretation frameworks and principles applicable to GSR. General principles of the interpretation of the analytical results of a forensic investigation have also been outlined in the paper. It has been argued that a case-by-case or Bayesian approach is superior to the formal framework used for GSR interpretation because it helps to align the process with both circumstantial elements of a case and judicial aims. To exemplify the contribution of GSR evidence to the investigative and interpretation processes, People v. Robert Blake was used.

Reference List

Benito, S, Abrego, Z, Sanchez, A, Unceta, N, Goicolea, MA & Barrio, RJ 2015, ‘Characterization of organic gunshot residues in lead-free ammunition using a new sample collection device for liquid chromatography-quadrupole time-of-flight mass spectrometry’, Forensic Science International, vol. 246, pp. 79-85.

Burnett, B 2014, ‘The gunshot residue evidence of People v. Robert Blake’, in Scanning Microscopies conference proceedings, Monterey, California, CA, pp. 121-134.

Chang, KH, Jayaprakash, PH, Yew, CH & Abdullah, AFL 2013, ‘Gunshot residue analysis and its evidential values: a review’, Australian Journal of Forensic Sciences, vol. 45, no. 1, pp. 3-23.

Charles, S & Geusens, N 2012, ‘A study of the potential risk of gunshot residue transfer from special units of the police to arrested suspects’, Forensic Science International, vol. 216, pp. 78-81.

Ditrich, H 2012, ‘Distribution of gunshot residues—the influence of weapon type’, Forensic Science International, vol. 220, pp. 85-90.

Evett, IW, Jackson, G, Lambert, JA & McCrossan, S 2000, ‘The impact of the principles of evidence interpretation on the structure and content of statements’, Science & Justice, vol. 40, no. 4, pp. 233-239.

Gallidabino, M, Weyermann, C, Romolo, FS & Taroni, F 2013, ‘Estimating the time since discharge of spent cartridges: a logical approach for interpreting the evidence’, Science & Justice, vol. 53, no. 1, pp. 41–48.

Grima, M, Butler, M, Hanson, R & Mohameden, A 2012, ‘Firework displays as sources of particles similar to gunshot residue’, Journal of the Charted Society of Forensic Sciences, vol. 52, no. 1, pp. 49-57.

Hannigan, TJ, McDermott, SD, Greaney, CM, O’Shaughnessy, J & O’Brien, CM 2015, ‘Evaluation of gunshot residue (GSR) evidence: surveys of prevalence of GSR on clothing and frequency of residue types’, Forensic Science International, vol. 257, pp. 177-181.

Hofstetter, C, Maitre, M, Beavis, A, Roux, CP, Weyermann, C & Gassner, AL 2017, ‘A study of transfer and prevalence of organic gunshot residues,’ Forensic Science International, vol. 277, pp. 241-251.

Maitre, M, Kirkbride, KP, Horder, M, Roux, C & Beavis, A 2017, ‘Current perspectives in the interpretation of gunshot residues in forensic science: a review,’ Forensic Science International, vol. 270, pp. 1-11.

Morelato, M, Beavis, A, Ogle, A, Doble, P, Kirkbride, P & Roux, C 2012, ‘Screening of gunshot residue using desorption electrospray ionisation-mass spectrometry (DESI-MS)’, Forensic Science International, vol. 217, no. 103, pp. 101-106.

Morrison, GS 2014, ‘Distinguishing between forensic science and forensic pseudoscience: testing of validity and reliability, and approaches to forensic voice comparison’, Science and Justice, vol. 54, pp. 245-256.

Romolo, FS & Margot, P 2001, ‘Identification of gunshot residue: a critical review’, Forensic Science International, vol. 119, pp. 195-211.

Sturm, H, Schartel, B & Braun, WU 2012, ‘SEM/EDX: advanced investigation of structured fire residues and residue formation’, Polymer Testing, vol. 31, pp. 606-619.

Sweetingham, L 2005, ‘’, CNN. Web.

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