Fundamentals of Firearms ID

Studies have shown that no two firearms, even those of the same make and model, will produce the same unique marks on fired bullets and cartridge cases.  Manufacturing processes, use, and abuse leave surface characteristics within the firearm that cannot be exactly reproduced in other firearms.

Firearms do not normally change much over time.  This allows for firearms recovered months or even years after a shooting to be identified as having fired a specific bullet or cartridge case.  Tests have been conducted that found that even after firing several hundred rounds through a firearm the last bullet fired could still be identified to the first.

It should be noted that not all firearms leave consistent reproducible marks.  But overall it has been my experience that around eighty percent of the firearms that I examine produce what is sometimes called a "mechanical fingerprint" on the bullets and cartridge cases that pass through them.

All cases that involve firearms identification start with preliminary examinations of the evidence for similar class characteristics.  Class characteristics can be defined as:

Intentional or design characteristics that would be common to a particular group or family of items.

PencilsA very basic example would be that several no. 2 pencils in a box are yellow and have pink erasers. The color and eraser type is a common class characteristic to all of the pencils. 

When it comes to firearms and ammunition it is not quite so simple.

The class characteristics of firearms that relate to the bullets fired from them includes the caliber of the firearm and the rifling pattern contained in the barrel of the firearm.

Cartridges and Cartridge cases on the other hand are examined for class similarities in what are called breech marks, firing pin impressions, extractor marks, ejector marks and others.

If dissimilarities in class characteristics are found or if a general lack of good class characteristics are present no further comparisons may be necessary.

When similar class characteristics are identified the examinations progress to a final stage where an attempt is made to find a "match" in what are called individual characteristics.  The AFTE Glossary defines Individual characteristics as: 

 marks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or caused by use, corrosion, or damage. They are unique to that tool and distinguish it from all other tools.

The transfer of individual characteristics from a firearm to the ammunition components passing through it is what makes firearms identification possible.

Click the Next button below learn more about bullet identification.


Home | Top | Next