Physical Characteristics

Groove impression striations on .38 special
caliber lead bullets fired from a
Rohm revolver (individual characteristics)

Courtesy of FirearmsID.com

The three categories of marks found on a fired bullet surface are typically class, subclass, and individual characteristics as defined below:

Class characteristics – Measurable features of a specimen that indicate a restricted group source. They result from design factors and are therefore determined prior to manufacture. In the context of this module, general rifling characteristics of the barrel of a firearm are the class characteristics of interest.
Individual characteristics – Marks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or are caused by use, corrosion, or damage. They are unique to a particular tool and distinguish it from all other tools. In the context of this module, the tool is the interior of the barrel of a rifled firearm.

Note: Accidental characteristics is a term formerly used to mean individual characteristics.
Subclass characteristics – Discernable surface features of an object that are more restrictive than class characteristics in that they are
- produced incidental to manufacture,
- significant because they relate to a smaller group source, that is, a subset of the class to which they belong,

identifiable within a time frame since manufacturing processes change over time.

In the context of this module, subclass characteristics relate to limited resemblances that could appear in consecutively manufactured barrels that are rifled using the same tool. These subclass characteristics could persist for a period of time even though the overall individual characteristics of the tool (rifling, broach, etc.) change with each barrel produced. Subclass characteristics should not be confused with individual characteristics. Subclass characteristics typically appear on the groove impressions rather than the land impressions of fired bullets. They may be misinterpreted, especially in cases with very limited microscopic marks of value, or in instances in which no firearm is submitted.