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There are four different conclusions that firearms examiners use to report the results of their microscopic examinations:
- The bullet/cartridge case was fired by the firearm
- The bullet/cartridge case was not fired by the firearm
- There are insufficient characteristics present to identify or eliminate the bullet/cartridge case as having been fired by the firearm
- The bullet/cartridge case is not suitable for comparison
The biggest limitation would be the condition of the evidence. If the evidence (bullets and cartridge cases) is too damaged or mutilated to reveal sufficient individual characteristics, then no comparison can be made.
The lack of a suspected firearm also presents limitations for the examiner’s conclusions. However, if bullets and/or cartridge cases are obtained that are in fairly good condition, they can be used to determine the type of weapon used and can potentially be compared to evidence from other crimes in order to help track the shooter.
To ensure the most accurate analysis of evidence, the management of forensic laboratories puts in place policies and procedures that govern facilities and equipment, methods and procedures, and analyst qualifications and training. Depending on the state in which it operates, a crime laboratory may be required to achieve accreditation to verify that it meets quality standards. There are two internationally recognized accrediting programs focused on forensic laboratories: The American Society of Crime Laboratory Directors Laboratory Accreditation Board and ANSI-ASQ National Accreditation Board / FQS.
In disciplines such as firearms and tool marks where testing requires analysts to compare specific details of two samples, quality control is achieved through technical review and verification of conclusions. This involves an expert or peer who reviews the test data, methodology and results to validate or refute the outcome. This review encompasses the microscopic work, bench notes and written reports. The percentage of cases that undergo verification may vary depending on the experience of the analyst. In addition, defense attorneys may hire independent firearms examiners to review and reexamine questioned evidence to ensure accuracy of the findings.
The Scientific Working Group for Firearms and Toolmarks (SWGGUN) website publishes Quality Assurance Guidelines for this discipline on their website.
Firearms reports are usually brief and will list the evidence examined and the results of that examination: The bullet/cartridge case was fired by the firearm in question; it was not fired by the firearm in question; there are insufficient characteristics present to identify or eliminate it; or it is not suitable for comparison.
Are there any common misconceptions about the area of firearms examination or any other information that might be important to the non-scientist?
Firearms are part of the American fabric of life. Most individuals have been exposed to them either directly or through the media. As is often the case, many myths and pieces of incorrect information become part of that exposure, often because they are seen in movies or on television. Examples include a person who is shot being thrown back through a door instead of dropping to the ground or the idea that somehow a loaded gun will discharge without anyone being near it. Because of these misconceptions, people can sometimes become confused when presented with scientific findings. It is always best to seek information on firearms evidence from a qualified firearms examiner to ensure complete understanding of what the examination results really mean.