Technically, ballistics is the science of projectile motion and conditions affecting that motion. But today, the term is used to describe the study of firearms and bullets.

Firearms were invented several centuries ago (around A.D. 1200) by the Arabs. Since then, significant advancements have been made to improve their aim and projectile speed and range. The major breakthrough came in the early 16th century, when it was discovered that a spiral groove etched into the gun barrel would cause the projectile to spin, making its flight more stable. This groove (also known as rifling) leaves distinctive striations on the bullet that are examined today in criminal investigations.

Variations in these striations allow experts to determine the type of gun the bullet was fired from. This arises from the method in which the barrel is manufactured. Firstly it is smooth bored, then reamed to specification diameter, before being rifled. Because the tools used wear down over time, no two barrels have identical rifling. Bullets fired from different guns will always have different striations. While each barrel imparts its distinctive striation pattern on every bullet fired from it.

Ballistics also includes examining the shells ejected from the gun for possible imperfections in the breech face, firing pin, extractor, or the ejector post -- all of which leave marks on the case head or shell casing. Another factor to be considered is the bullet impact. If the bullet has struck human bone or any hard object, it's shape changes radically. Often, firing tests performed by forensic scientists are used for comparison.

Contrary to popular belief, gun-powder residue on the hands is not a reliable test to see if a suspect has fired a weapon recently. Guns today use smokeless powder that leaves traces only detected through chemical testing. Even then, the test was designed to reveal the presence of nitrates, and many common household products now contain nitrates. Because of this, the practice has largely been abandoned.