Firearms - The Basics

 A firearm examiner is someone who, through the use of a comparison microscope and various other methods and techniques, determines whether or not a fired bullet, discharged cartridge casing or discharged shot shell was fired in one particular firearm. 

 A comparison microscope is two microscopes joined by an optical bridge that allows for an examiner to compare evidence simultaneously, under magnification.  The field of view is separated by a hairline which allows an examiner to merge and compare unique imperfections present in the evidence in question. 

 The basis of firearm identification is the similarity in the reproduction of class and individual characteristics. 

Class characteristics, to name a few are caliber, number of lands and grooves, and direction of twist.  These characteristics are common to man various types of firearms. 

Spiral grooves are cut into a barrel of a firearm imparting spin on a bullet to give it stability in flight, so that it would not tumble end over end.  The cut or lower areas of the barrel are called "grooves" and the raised or higher areas are called "lands".  This spiral cutting is referred to as rifling. 

 Individual characteristics are characteristics that are exclusive to one particular firearm.  These characteristics are imparted on the firearm during its manufacturing process, during its use, abuse, and general wear.  Unique striae and imperfections constitute the individual characteristics which make no two firearms exactly alike.