Congruent Matching Cells (CMC)
Definition
An objective, measurement-based metric for cartridge-case comparison developed at NIST, quantifying the fraction of surface topography cells showing high correlation between questioned and known items, complementing subjective visual comparison.
- Developer
- NIST
- Applies to
- Cartridge-case comparison
- Type
- Objective, measurement-based metric
- Measures
- Fraction of surface cells with high correlation
- Complements
- Subjective visual comparison
Common questions
Why was CMC developed when firearms examiners already compare cartridge cases visually?+
Traditional visual comparison relies on an examiner's subjective judgment of pattern similarity, which is harder to validate statistically and has drawn scrutiny in admissibility challenges. CMC divides the surface into cells and computes a quantifiable correlation score for each, giving a numeric, reproducible measure that supports the examiner's conclusion rather than replacing it.
Does a high CMC score alone prove two cartridge cases were fired from the same firearm?+
No. CMC is a supporting metric within a broader comparison process; examiners still consider the overall pattern, known error rates for the method, and other case context. It strengthens the evidentiary basis for a conclusion but is applied alongside, not instead of, trained visual examination.
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