Okay I found my particular notes on it and this is what is involved in firing a bullet.
When a bullet is fired, the propellent explodes and the bullet is forced through the barrel which is what will make the bullet spin keeping the trajectory stable. However, the barrel is smaller than the bullet itself and so as it passes through the barrel it produces distinctive groves on the bullet itself which is characteristic to particular guns. Fragments of the bullet such as the lead casing aroun it are also expelled along with some of the gunpowder residue. Gunpowder residue is important as it it establishes the following things:
- Whether the death was by suicide (in the case of death)
- Who fired the gun
- Matching the bullet fragments to a particular bullet
-Whether the several bullets fired are from the same gun or a box of bullets and
-Which bullet came from which gun
To do this you need to do a chemical analysis (usually a chemical assay) into the elements that make up the gunpowder residue and the casing fragments. Gunpowder is generally made of nitrates, an explosive of (again generally) a lead compound and a primer fuel of antimony sulphide.
A metal jacket around the bullet also helps identify which gun the bullet came from. These are usually compositions of copper and zinc with copper being the main component.
That's all the notes had. I'm not sure if you will understand it but this is generally what happens. Well at least the forensics behind it.
