Criminal cases in Arizona often involve the use or alleged use of a gun. It is helpful to understand how the use of a particular gun can be connected to a specific crime and/or excluded from a specific crime.
There are a variety of ways to identify or exclude a firearm as having been involved in a particular crime – and these methods can be firearm specific or involve more general forensic techniques.
First the general. A gun can be tied to a particular suspect through the collection of fingerprints, DNA, or gunshot residue. If the gun is found at or near the scene, it is important to know whose fingerprints or DNA is present on the gun. Law enforcement will surely check and test the gun for both. Likewise, if a suspect is identified, the police (and defense) should ask whether gunshot residue was found on the suspect's hands and/or clothing. General evidence can also be collected (or excluded) from a gun that is in a suspect's possession. Is there, or should there be, any blood spatter on the gun? Can that blood spatter be tied to the victim through DNA testing? What about the absence of any spatter?
There are also specific firearms-related items of evidence that should be collected and compared by both law enforcement and the defense. Here are some examples:
Caliber – Is the caliber of the gun found in the suspect's possession the same or different as that used in the offense? The caliber of the gun used in the crime might be determined by an intact projectile (bullet) found in the victim or at the scene, or through a spent casing recovered at the scene.
Revolver or Semi-Automatic weapon – were spent casings recovered at the scene? If so, it's likely the firearm used was a semi-automatic weapon that ejected the spent casing after each round was fired.
Projectile and spent casings can also be used to exclude specific weapons. Is the weapon found on the suspect consistent or inconsistent with the evidence at the scene? There may be rifling marks on the projectile at the scene that can be compared in the laboratory to rifling marks left when testing a suspect gun. Likewise, spent casings at the scene might be compared to spent casings test-fired in the suspect gun. When analyzing spent casings, forensic technicians generally look for marks on the casing left by the firing pin and, in the case of a semi-automatic firearm, by the ejector mechanism.
On a case-by-case basis, there are many other possible firearms and ballistics types of evidence that may come into play. But in every firearms case, the defense (and the police) should examine each and every factor above to be sure that a particular suspect firearm can't be excluded from use in the crime.