Guns and bullets are used in the most serious crimes, and they also happen to leave the most damning evidence.
While it’s often obvious when initially walking into a crime scene whether a gun was used – even if the victim survived and is being treated by paramedics – a full examination of the scene is always required to look at all factors that impacted the event. When a gun is involved, however, crime scene investigators will look for specific clues such as bullet casings, bullet holes, spatter patterns, and perhaps even a dropped weapon – either at the scene or nearby, such as in a body of water or a dumpster.
After the scene is photographed, diagrammed and all evidence referenced, the shell casings, bullet fragments, weapons and other evidence can be gathered, preserved and catalogued for further study back at the lab. Bullets suspected to be drilled into soft plaster or furniture will not be pried out. The area around and containing the bullet will be cut out so that the bullet can be removed carefully at the lab, and thereby preserve the channel that the bullet made.
1. Striations on a fired bullet
Once a bullet is recovered, either from an impact point in a soft wall or pulled out of flesh by a medical examiner or a physician, that bullet will become a primary piece of evidence. Every gun barrel is rifled during manufacture, or finished inside the barrel with rotating grooves to impart spin to a bullet in order to improve accuracy during flight. The resulting spiralling grooves and lands (the flat parts between the grooves) leave mirrored markings on the bullet itself.
If you are able to fire another bullet from the same gun, an investigator can match the grooves under a special microscope which displays both bullets side by side, comparing the strata. Since there are several processes involved in rifling a barrel, each barrel is unique. Just like fingerprints, a bullet can be paired to a weapon with nearly perfect accuracy.
Further, even without a weapon, the striations can identify a type and model of firearm, so detectives will know what to look for.
2. Gunpowder residue leaves expected patterns
When a gun is fired, the bullet is not the only thing that comes out of the barrel. Burning powder particles also expectorate and will create a pattern on the object closest to the barrel. This pattern varies by how far the weapon is from the target. By knowing the patterns of a particular firearm, a consensus can be reached about how close or how far away the gun was. If there are no powder marks on the hand holding the gun or the forehead around the bullet hole, it is unlikely that the victim shot himself, so suicide can be ruled out no matter how convincing the pose looks to the untrained eye.
3. Trajectory, ricochet and bullet holes
Sticks carefully inserted in bullet holes can indicate direction of the bullet, and, if many holes are found in a scene, these sticks can help triangulate: where the shooter was standing; how tall he (or she) might have been; and many more details. Bullets that ricochet will collect trace evidence from where they bounce, and add further detail for the investigator.
4. Hidden fingerprints
A gunman will finger bullets as he loads them into a cylinder or magazine, and leave tiny quantities of salty sweat with each touch. When a bullet is fired away from its casing, tremendous heat is instantly transferred to that metal, vaporising the moisture and setting the salts from those prints. The salts become molten and a chemical reaction with the metal etches the fingerprints permanently into the casing.
5. Firing pin impressions
Firing pins leave individual marks on the primer (the explosive cap that ignites the gunpowder when struck by the pin on the tip of the hammer) at the very bottom of each bullet. The alignment, size of impression, and age of the firearm also contribute to individualised marks as the hard metal (the pin) make impressions on the soft metal (the primer).
6. Bullet damage to tissue
Wounds can provide an incredible amount of information about sequence of hits, distance, velocity, bullet type, caliber, and more – whether a bullet fragment is recovered or not. Bevelling around an entrance wound holds clues, fracture lines in dense bone such as skulls can identify distance, velocity and direction of fire. Grazing bullets leave skin tags – little flags of ragged skin that are pulled up and torn as a projectile passes by at high speed. All of these indications help validate the opinions of a medical examiner.
Every one of these indicators can become a comprehensive study in itself. While we only have room for a highlight from each factor, there have been: books written about each subject; experts have spent decades studying only trajectory, for example; and hundreds of thousands of tests repeated over and over again verify the science and validate the findings.
Your Turn: Do you think a bullet fired from a gun can be matched to that gun a decade later, after it has fired thousands of additional rounds? Sound off in the comments below.