Firearm toolmark identification from "summary" of Fundamentals of Forensic Science by Max M. Houck,Jay A. Siegel

Firearm toolmark identification is an area of forensic science that deals with the comparison of the unique markings found on bullets and cartridge casings.

This process of firearm toolmark identification begins with comprehensive microscopy of the test and known bullets or cartridges allowing for comparison of all their individual characteristics.

In this type of investigation, a forensic examiner will attempt to match an evidence bullet or casing to the barrel of a specific gun by examining and comparing microscopic striations, marks, scratches, and gouges left when a firearm fires a projectile.

Firearm toolmark identification also requires the use of special digital imaging programs such as MicroControl, Forensic Imaging System, or Virtual Image Preservation System that can magnify certain objects and identify details.

Only upon careful analysis, including matching and exclusionary stretching of likelihood can any reliable classification of identity between two ballistic specimens be made.

Due to the large amount of variation in bullets, even from a single gun barrel, this method of identification has been criticized in court due to questions about its scientific credibility.

Exclusionary stretching means that a toolmark has enough individual and unique identifications so that it can be distinguished from other similar markings and compared differently.

These individual characteristics may include lands, grooves, depth of impressment in the tools used, class characteristics, and other features of importance in the comparison.

Furthermore, the image must be correctly lit and shadowed in order to prevent apparent impressions which would otherwise mislead interpretations.

For a conclusion to be accepted with veracity, extreme careful study should be carried out before speaking to the specifics of any case.