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