What Firearms Examiners Can Do
Higashi spent 18 ye ars as a firearms examiner with the Los Angeles County Sheriff's Office before he joined the Colorado Bureau of Investigation five years ago. In his more than 20-year career he has worked thousands of cases. Higashi would tell you that he is a tool mark examiner. The firearm is the tool, leaving its mark on the fired bullet and the spent cartridge casing.
When a law enforcement agency submits evidence, Higashi will examine the firearm itself, any projectile fired from a gun, and any recovered fired cartridge casing. He'll then examine the expended ammunition to determine the caliber of the fired object.
Firearms examiners like Higashi can provide a list of possible firearms that may have fired the projectile. This is based on the barrel's twist rate and whether the barrels' lands and grooves twist to the left or to the right. Of course, this only works with projectiles fired from rifled barrels.
The fact that the lab can supply a list of manufacturers whose barrels match the twist rate and direction of the fired projectile is significant. In cases where the gun isn't immediately recovered, investigators can limit their search to only certain firearm brands.
Firearms examiners can determine whether the submitted firearm is the source of the expended ammunition. They can also determine how many guns were responsible for firing all of the expended ammunition and whether recovered shell casings were fired from the same gun. The laboratory's tests are non-destructive, so the marks on the fired bullets and cartridge cases will remain for an indefinite period of time. This is significant in cold cases, or cases that might not be prosecuted for many years.
Most firearms examiners can "function test" firearms to determine whether the firearm functions properly or has been modified. They can also test fire weapons to determine expended cartridge casing trajectories to aid with scene reconstruction. Investigators can speak with their local firearms examiners to request any special testing or analysis that may be available to assist with a current investigation.
Note that firearms examiners can't say who fired the gun or when the gun was fired. With the advent of trace DNA, hopefully most agencies are taking the proper steps to preserve any trace biological evidence on the firearm at the time it's collected.
What You Can Do
As is true in so many cases, the first responding patrol officer has the ability to ensure that any firearm evidence is identified, preserved, and (depending on your agency) collected properly. The best rule of thumb is always: first, do no harm.
As previously stated, DNA can be obtained from the trigger, grip frame, or any other part of the firearm that's handled. This DNA may consist of minute amounts of epithelial or skin cells. That's why it's so important to protect the gun from the elements as well as excessive handling.
Higashi prefers to receive firearms that don't have anything inserted directly into the gun's barrel. It's important to submit the gun to the laboratory in a safe condition, but nothing needs to be directly inserted in the barrel to ensure that the weapon is functionally safe.
Because cartridge casings can retain fingerprints as well as the tool marks from the gun, it's best to package these carefully so the casing can't roll around. Also package each shell casing separately.
Fired projectiles are particularly delicate. The striations of the barrel are preserved in the lead or copper jacket of the fired bullet. Fired bullets must be packaged in a manner that won't damage these impressions. I prefer small cardboard boxes with soft cotton or paper lining the box. As with cartridge casings, package each fired projectile separately, in its own packaging. This is especially important in preserving the markings that can be entered and matched in the NIBIN database.