Okay, I think I found my answer:
I found this at
Blogs, Ballistics in Forensics - Reading Between the Ridges in a Gun Barrel - Amazines.com Article Search Engine
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.
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