The article first describes the operation of the three main types of firearms most likely to be involved in a shooting crime: handguns, shotguns, and rifles. Barrel markings and the direction of their twist, along with any barrel imperfections (striations), are unique to each manufacturer and model of firearm. Barrel markings and striations impressed on bullets found at a crime scene can be analyzed and compared with bullets test-fired from a suspect weapon to determine whether it fired the crime-scene bullets. Residue patterns can provide information on how a firearm was used. The distance a person or an object is from a gun when it is fired will determine whether or not residue from the firing will be deposited and in what pattern. Types of residue patterns based on this distance are described. Residue from the gun's firing can also be found on the suspect. Methods of testing for such residue are also explained. Bullets recovered from a crime scene should only be collected after their exact location has been recorded. The bullets or casings should be marked in a manner that leaves the striations and/or firing marks untouched. Recovered weapons at the crime scene should be picked up by the trigger guard; however, if evidence such as blood or tissue is on the trigger guard, the weapon can be picked up by the grip. FBI recommendations for the packaging of evidence to be sent to the laboratory for testing are outlined.