31/08/2026
How a tiny cartridge case can identify the exact gun used in a crime.
Most people think ballistics is all about bullets. In reality, half the story is written on the cartridge case left behind at the scene.
Here's how it actually works in an FSL.
When a fi***rm is seized during investigation, it is never straightaway declared "the weapon of offence" just because it was recovered from the accused. It has to be scientifically linked to the crime cartridge or bullet. That link is established through comparison microscopy.
The seized fi***rm is test fired in a controlled manner inside the lab, usually into a water tank or cotton wool recovery box, so that the test fired cartridge case and bullet are recovered undamaged.
The test fired cartridge is then examined side by side with the crime cartridge under a comparison microscope. This instrument has two separate stages joined by an optical bridge, allowing the expert to view both specimens in a single field, split right down the middle. One half shows the crime exhibit, the other half shows the test fired exhibit, and they can be rotated and aligned in real time.
What is actually being compared:
Class characteristics first. Caliber, type of breech face, firing pin shape, number and direction of rifling marks. These narrow down the make and type of weapon.
Then individual characteristics. Every fi***rm, even two pieces from the same manufacturing batch, develops unique microscopic tool marks on its breech face, firing pin, extractor and ejector during machining, and these get further individualised with use. These marks transfer onto every cartridge case fired from that weapon, almost like a signature.
When the striation pattern and impression marks on the crime cartridge match the test fired cartridge to the point of individuality, the expert forms an opinion that both were fired from the same fi***rm.
This opinion, backed by photomicrographs and a reasoned report, becomes expert evidence and plays a crucial corroborative role in connecting the accused, the weapon and the crime scene before the court.
Small piece of brass. Big story to tell, if examined right.