The brass and the bullet are clearly distinct.
On television programs, the matching of one bullet with another that is allegedly fired from the same firearm is a cut-and-dried process. The bad guy goes to trial, is convicted and sent away to jail.
Know as ballistic forensics, the process is not so simple, in reality, and the results are becoming more controversial as the method is being challenged by scientists and in court. Clifford Spiegelman, a professor of statistics at Texas A&M, has written, "...forensic results are typically stated with uncertainty statements that cannot be supported."
Two Pieces
Two separate pieces of the cartridge may be used for a match--the bullet itself and the brass, what is left behind after the round is fired.
A bullet often malforms or even completely shatters when hitting an object. This can make it impossible to match with another bullet. A brass, if available, is much easier to work with.
Scratches
When a cartridge is loaded into a firearm, it begins to receive a series of scratches. As each step of the firing process forces the cartridge against more metal parts, even more scratches are added to the cartridge brass. These include scratches from the magazine, chamber and firing pin.
The bullet itself is scratched by the throat of the barrel and the lands and grooves inside a barrel.
Theory
The idea behind the matching of cartridges and bullets is that each gun leaves a unique pattern of scratches on those pieces. Each round fired from the same gun will have the same identical series of scratches, however small they may be. Viewed through a microscope these patterns will reveal which gun fired which cartridge.
Limits
To make a match effective, authorities need a piece of evidence directly attached to the crime scene. For example, a mostly whole bullet in a victim or an ejected brass that is left behind. Without evidence such as this, it becomes difficult to match a particular weapon to a crime.
Problems
The natural wear and tear on a firearm as rounds are fired through them will change the characteristics of the parts. This in turn changes the scratch pattern, making it difficult to determine if a gun previously fired a weapon.
Also, the act of normal maintenance on a firearm quite possibly will alter the scratch pattern.
Another challenge to the theory is that it is inherently flawed because it is only one person's subjective view of whether there is a match to begin with.
Tags: bullet itself, fired from, fired from same, from same, left behind