It requires years of experience and examination of hundreds of cases to competently review and interpret the GSR findings in the full range of criminal cases involving guns. All of the methods used should be accredited and stand up to scrutiny and full anti-contamination measures should be in place. The interpretation of what constitutes a GSR particle is not undertaken to the same standard in all laboratories and this is based on the level of expertise of the scientist. The findings should also be subject to a full interpretation based on the case circumstances and any other relevant information that could have a bearing on the presence of GSR.
This is particularly important in cases where only single particles or low levels of GSR are found or where there is a possibility of cross-contamination. PFS offers this expertise and have been instructed in the full range of types of cases from terrorism, murder and armed robbery to other serious and organised crime throughout the UK, the Caribbean and other jurisdictions.
Considerations when examining gunshot residue:
Are the particles GSR or is there a source of similar non-firearms related particles?
- have they been correctly identified?
- are the particles from fireworks, nail guns or brake pad linings?
If the particles are GSR how were they deposited?
- adventitious transfer from the general environment?
- cross-transfer from a contaminated source e.g. another defendant or armed police officers involved in arrest?
How many particles were found?
- Little interpretation can be provided for low level findings (1-3 particles)*.
*  EWCA Crim 2722;  EWCA Crim 2507
Have documented, validated laboratory procedures been followed?
- have quality control samples been taken and analysed; appropriate PPE been worn?
- were exhibits opened prior to being sent to the laboratory?
Has sufficient information been presented to fully interpret the GSR findings?
- is there any other potential source of GSR?
Have particles indicative of GSR been reported?
- these are usually of little significance and on their own do not normally influence the interpretation of results.