As discussed in the previous lesson, firearms have individual characteristics that make them unique. These markings are some of the primary objects of study in forensic ballistics.
Given a fired bullet to determine the caliber, type, and make of firearm from which it was fired
Given a fired shell to determine the caliber, type, and make of firearm from which it was fired
Given a fired bullet and a suspected firearm to determine whether the fired bullet was fired from the suspected firearm
Given a fired shell and a suspected firearm to determine whether the fired bullet was fired from the suspected firearm
Given two or more fired bullets to determine whether they were fired from the same firearm
Given two or more fired shells/cartridge cases to determine whether they were fired from the same firearm
No two barrels are microscopically identical as the surface of their bores all possess individual characteristic markings.
When a bullet is fired from a rifled barrel, it becomes engraved by the riflings and this engraving on a bullet fired from one barrel will be different from that on a similar bullet fire from another barrel. And, conversely, the engraving on bullets from the same barrel will be the same.
Every barrel leaves its "thumbmark" on every bullet which is fired through it, just as every breech face leaves its "thumbmark" on the base of the fired cartridge case.
The first thing to do in the examination of bullets is to conduct a visual examination of the bullets to familiarize with all the markings appearing on them.
Conduct an examination of the bore of the firearm.
Determine the conspicuous characteristics appearing on the bullet or any markings appearing therein.
Markings appearing on test bullet #1, but does not appear on succeeding test bullets should be disregarded. Such markings are called "accidental markings" and come from foreign substances.
If a bullet is undersized or the bore of the firearm is badly worn out, there will be a cylindrical passage of the expanding gas and it will appear as a dark or black mark in the picture.