Firearm evidence identification is the process of analyzing bullets or cartridge cases left at a crime scene to determine if they originated from a particular firearm. Statistical methods have long been developed and used to aid in such analyses. The Congruent Matching Cells (CMC) method is one such method developed at the National Institute of Standards and Technology (NIST) to quantify the similarity between two spent cartridge cases based on the markings left by the firearm barrel during the firing process. We introduce the first open-source implementation of the CMC method in the R package cmcR. The package will bolster forensic researchers' abilities to investigate, validate, and improve upon current statistical methodology in the field of forensic science.