Measurement, Standards, and Accuracy
The examination of physical evidence involves the comparison of questioned items to known standards in order to establish (if possible) whether or not they have a common source, or could have a common source. This requires the observation and documentation of physical characteristics using a variety of instruments.
These instruments are typically used by firearm and toolmark examiners to measure
- weight, such as the weight of a bullet, or the range of weights required to squeeze the trigger and fire a particular firearm (trigger pull),
- dimensions, such as the width of land and groove impressions in a fired bullet or the width of a toolmark,
- velocity of a fired bullet (in rare cases).
Any measurements taken should be documented in the contemporaneous notes kept by the examiner throughout the examination. Laboratory protocols determine which measurements are appropriate to be taken and used in an evaluation of evidence. There are many valid, reliable, and correct approaches for obtaining measurements of the physical features of firearm and toolmark evidence. Instrumentation available in the laboratory and the knowledge and experience of the examiner determine how measurements will be taken.
It’s important to note that the ability of a firearms examiner to take accurate measurements has multiple effects, including
- accurate results,
- certification of the firearm examiner (e.g., competency and proficiency tests),
- laboratory accreditation of the firearms section.