Objective Evaluation of Subclass Characteristics on Breech Face Marks.

Area of Science:

• Forensic Science
• Ballistics
• Firearm Examination

Background:

• Subclass characteristics on breech faces of spent cartridge cases can lead to false firearm identifications.
• Empirical studies have focused on recognizing and understanding the conditions of subclass characteristic production.
• A probabilistic approach to the influence of subclass characteristics on identification has been lacking.

Purpose of the Study:

• To quantitatively measure the effect of subclass characteristics on the strength of association in firearm examinations.
• To assess the probabilistic impact of subclass characteristics on likelihood ratio (LR) calculations.

Main Methods:

• Utilized a 3D automatic comparison system to quantify similarities between cartridge case specimens.
• Computed likelihood ratios (LRs) for specimens fired from thirteen S&W .40S&W Sigma pistols.
• Analyzed the influence of subclass characteristics on LR values.

Main Results:

• The influence of subclass characteristics on likelihood ratios was found to be limited.
• Even when subclass features are prevalent among potential sources, their impact on LR remains constrained.
• A significant effect requires over 40% of the potential firearm pool to share subclass characteristics.

Conclusions:

• Subclass characteristics have a minimal probabilistic effect on firearm identification outcomes.
• The prevalence of subclass characteristics needs to exceed 40% within a suspect pool to significantly alter identification strength.
• This study provides a probabilistic framework for evaluating subclass characteristics in forensic ballistics.