Bullet fingerprinting is:

Bullet fingerprinting- Primary markings- The rifled firearm leaves its signature on the caridge case and on the bullet. With all rifled firearms, the bullet is slightly larger than the barrel, and as it passes through the barrel, its sides are marked by the rifling of the barrel. Secondary markings- They are produced on the surface of the bullet by imperfection on the inner surface of the barrel. These irregularities are produced by sticking of paicles of the bullet to the bore when shots are fired and is known as metallic fouling. They also result accidentally during the manufacturing process, usually microscopic in nature and have random distribution. They are useful in identifying the specific gun which was fired.