When lands and grooves grab and spin a projectile, or bullet, moving down the barrel of a firearm, they also create cuts in the projectile. These cuts leave behind distinctive markings that are at the heart of firearms comparisons. Lands, or ridges, refer to the high parts made when the factory makes cuts into the barrel of a firearm to make grooves, the low parts. These lands and grooves create markings known as striations on the projectile. Striations are straight and parallel to the longitudinal axis of the projectile. One can see that striations are more prevalent on soft lead bullets than they are on metal or jacketed bullets.
Silencers are gadgets that are designed to suppress or muffle the report of a gunshot. These can include a shirt wrapped around the barrel to a high-tech sound-absorbing screw-on attachment. Such attachments can also leave behind distinctive markings on projectiles, but these markings are not predictable. If a silencer does leave distinctive impressions on the projectile but is not available for examination by the time the forensic firearms examiner test-fires the gun, these impressions may hinder the scientist’s attempt to find a match between the bullet and the gun from which it was fired.
In addition to the presence of lands, grooves, striations, and twists, each rifled barrel has minute qualities that set it apart from all others. A rifled gun barrel is the result when the manufacturer carves spiral grooves into the parietal surface of the barrel in order to make projectiles spin as they are forced down the barrel. This spinning action allows the bullet to stabilize itself during flight. This is the same physics concept with footballs as the quarterback passes the spinning ball to the receiver waiting to catch the ball down the field. For this reason, gun barrels are said to be rifled in order to promote the spinning of a bullet.
For the purposes of forensics, a rifling tool cuts through each metal gun barrel a bit differently. As subsequent barrels are carved out, the cutting or carving equipment becomes worn with repeated use. This progressive wear and tear creates rifling patterns that differ from one barrel to another. Furthermore, repeated firing also wears down and damages the lands and grooves. This action makes each barrel unique as well as each bullet that passes through it.
In a nutshell, projectiles fired from the same firearm have the same striated patterns, but bullets fired from different ones do not. The microscopic striations found on a projectile are so unique that they are indicative that the bullet had to come from a particular gun excluding all others. This singles out one gun that sets it apart from others of the same make and model.
It is useful for the forensic firearms examiner to compare individualized striations. The first step to making this kind of comparison is to get an intact projectile fired from the suspect weapon. To do this, many firearms laboratories have a test-firing chamber. The forensic scientist then looks at the lab-fired projectile and compares it to the crime-scene bullet, using what is known as a comparison microscope. This microscope can juxtapose the images of the two projectiles to help make an accurate comparison.
For instance, projectiles found at the crime scene can be compared to ascertain whether they were fired from the same gun. If not, it can be concluded that more than one firearm was used. Similarly, separate bullets, each one retrieved from different crime scenes, can be compared to find out whether the same gun fired them. If there is a positive match, this can strongly indicate that the two crimes are connected. Most importantly, a projectile extracted from a shooting victim can be compared with a projectile that has been test-fired from a suspect weapon. A positive match would indicate that the weapon of suspect is the one used in the crime. This, in turn, can be the key to identifying the offender.
Two bullets do not have to be a 100% match in every detail to be considered solid evidence. Many times, they never do. The reason for this is that every projectile fired from a gun slightly changes the barrel and leaves behind soot and grit. These alterations inside the barrel and the deposited foreign materials change the impressed markings left on a subsequent projectile. Moreover, handling a soft bullet such as a lead bullet may add, remove, or change the existing markings. Even though two projectiles do not need to be exact copies, you have to find the same identical patterns on a minimum of three consecutive striations on each projectile for them to be a positive match with a gun.