“Forensic firearms identification includes well-established uses such as determining caliber and other general characteristics, but examiners are also frequently called on to testify whether a particular bullet was fired from a particular gun. A gun’s firing pin and the grooves on the inside of a gun barrel leave marks on cartridge casings when a bullet is fired, so a firearm examiner compares crime scene bullets to samples fired from the suspect gun and looks for matching patterns under a microscope.
According to the Association of Firearm and Tool Mark Examiners (AFTE), which sets standards for the field, a positive identification can occur when there is “sufficient agreement” between two or more sets of marks or patterns. The AFTE argues—as one of its members did as a witness for the state of Maryland in Abruquah’s appeal—that its methods are scientifically sound, widely accepted, and have low error rates in testing.
However, over the past decade many forensic methods, especially “pattern-matching” disciplines like bite mark and tool mark analysis, have been challenged by critics who argue that they rely on subjective interpretations that are nonetheless presented as scientific conclusions in courtrooms.”
https://reason.com/2023/06/22/maryland-supreme-court-limits-testimony-on-bullet-matching-evidence/