Typical results for a comparison of breech face impressions on cartridge case primers, using the NIST technique known as Congruent Matching Cells, or CMC. In pair A, almost all the cells from the first image match cells from the second image, indicating that the two cartridge cases were likely fired by the same gun. In pair B, some cells find similar cells, but they are randomly distributed, and therefore, not considered matching. Only the area of interest for each primer is shown. Portions of the primer surface that were not compared appear in white. The color scale indicates relative surface height in micrometers.