CrimePsych

In the field of forensic ballistics, studies have significantly shown that no two firearms are alike; that is, no two firearms when firing ammunition leave the exact same marks as another (on said ammunition), even if that other is an identical model from an identical manufacturer. This makes it very easy for experts in the field to differentiate between weapons in the same fashion that fingerprints differentiate between individual people.

How does this all work though i hear you ask?

Back in the 15th century in Vienna a gentleman by the name of Jaspard Zoller invented what is known as ‘rifling’ – this was added to weapon barrels of all sizes to enable the projectiles to spin along their long axis, gyroscopically stabilising the round and improving their stability and accuracy during flight.

In the past, as with muskets and other weapons that were loaded through the muzzle, there was no rifling within their barrels, assigning them to the category known as ‘smoothbore’. In addition to this, due to the fact that they were loaded with a single round at a time through the muzzle, the ammunition for smoothbore rifles and other weapons was very loose fitting. Subsequently, when firing these types of weapons the ball-round bounces down the barrel as opposed to travelling smoothly in a single direction, and as you can imagine, when leaving the barrel the trajectory of this type of ammunition is inaccurate and unpredictable to say the least.

While rifling was invented in the 15th century it did not become commonplace in weapons until the 19th century.

It is this rifling that gives firearms examiners the ability to compare a bullet removed from a victim to other rounds test fired in a forensics lab. Here’s an example…

A victim has been shot with a handgun and the round has been recovered from their body. Later, the police arrest a suspect and find a handgun in his car. Both the recovered round and the handgun are sent to a firearm examiner who test fires the weapon, usually into a water tank, so they can recover the round easily and without any damage to it.

If this was a smoothbore weapon it would be impossible to determine whether the rounds had been fired from the same weapon or not but as the weapon is modern and has rifling, we can. The rifling imprints the same unique marks on each round as it travels along the barrel and leaves the weapon. Studies have shown that weapons can fire thousands of rounds without ever having to worry about the rifling suffering abnormalities or damage that change the marks it leaves on rounds (although this has been called into speculation recently by a National Research Counsel report)

The examiner then users a special macroscope called a comparison macroscope to examine both rounds next to each other. That’s right, macroscope – a microscope generally magnifies to levels above 100x while a macroscope for use in ballistic analysis generally magnifies to a selection between 10x and 50x at the most.

This device is simply two macroscopes side by side connected via an optical bridge and then linked to a computer screen. By having a brace under each macroscope that the firearm expert can manipulate he or she is able to align the striations on the round as made by the rifling, in an attempt to find a match.

Our next article in this series will examine the actual methodology used by a forensic examiner when they conduct comparisons of this nature and we will also have a closer look at the different equipment used.