"In mass-produced guns, different makes and models have standardizing characteristics. Since the 18th century, guns have been made with internal helical grooves cut into the barrels that are similar to the threads of a screw. They form "lands," or metal ridges between the grooves. The lands grip the bullet and give it accuracy, range and spin. The interior part of a gun barrel is the bore, and the caliber of a bullet is determined by the bore's diameter, expressed in hundredths of an inch or in millimeters. As already noted above, when a bullet travels through a gun barrel, the bullet's metal gets worn in a unique pattern by the harder metal of the barrel. Any bullet fired from a specific gun will show the same marks, unless there's been some intentional alteration between firings." -- "Ballistics: The Science of Guns" by Katherine Ramsland To READ MORE, please see: http://www.crimelibrary.com/criminal_mind/forensics/ballistics/3.html?sect=21

Adding information to the previous answer: -When a bullet is found and analized (based on the information given about the manufacturer standards) you can determined what kind (make and model) of weapon fired that particular bullet. -BUT, the most important thing is that during the time of usage, a particular gun develops unique patterns in the gun tunnel where the bullet travel when fired. This unique patters are "engraved" on each bullet fired. So, when a suspect is found a his gun is analized, balistic's technicians will fire some rounds of ammo against a body of water or gel (any soft resistance substance) that will not smash the bullet, and will let them compare the "tunnel engravings" on these lab-fired bullets with the ones found on a crime scene. You can say that this gun tunnel marks are like finger-prints. Hope this additional information makes it clearer.

The estreya on the bullet would be the same as the other bullet shot from the same gun. They would need a test shot into a water simulator thingy.

Every weapon, even though mass produced, has its own individual characteristics. serial numbers are different, but so is the rifling, or the inside of each weapons barrel. no two are exactly alike. If a bullet is found at a crime scene, it is gathered as evidence, logged and retained, until a suspect weapon is located for a comparison. when a bullet is fired and leaves the barrel of a weapon, the rifling leaves certain identifiable marks on the bullet. if a suspect weapon, of the same caliber(.38, .45. 9mm,etc.)is located, this weapon will undergo certain tests. meaning, the suspect weapon will be fired into a safety, water-based container. the bullet will be retrieved. under a mircroscope, the rifling of the crime scene bullet and the rifling of the suspect weapon bullet, will be compared. everyday, crimes are solved using this method. hope this helps.

Besides the barrel pattern on the projectile, firearms often leave other distinguishing marks on other components of the cartridge (which in the case of semi-automatic weapons that eject the spent cartridge) that can often be found at the location of weapon discharge. The firing pin leaves an indentation in the primer case of the cartrige that can be used to help match up (or rule out) a particular firearm. The ejection mechanism often leaves unique marks on the cartridge case. Anomolies in the weapon operation (a propensity to scrape or nick the cartridge on the ejection port during ejection, for example) can also be a clue. Unburnt powder embedded in the base of the projectile (if any) and in the cartridge case can be analyzed and compared with powder in cranies of the weapon. Finger-prints on the cartridge case can tie a cartridge case to a particular person. Cartridges often have mfg identifiers and other distinguishing markings stamped in the metal of the base of the cartrige case that can be matched to other cartridges in the weapon or possession of an individual. The act of discharging the cartridge causes intense pressure inside the cartridge case causing it to expand to the inside dimensions of the weapon chamber - any anomolies (like pitting, scratches, etc.) in the chamber often will be imprinted to the cartridge case. For example, a worn weapon may not have a tight-locking bolt, causing a bit of head-space expansion and case stretching when fired. In the case of a revolver, the rotating cylinder may have a bit of slop causing a slight misalignment of the round with the barrel's forcing cone, causing asymetrical deformation - in severe cases even shaving - of the bullet. So, even if two weapons are virtually identical in manufacture, over use and time the weapons will pick up characteristics unique to the particular weapon, from wear, improper care, replaced parts, etc. most of which leave telltales on the ammunition components used in the weapon. Hope this helps..