Fingerprints recovered from guns are found on the ‘spent’ or fired shell casings. When a bullet is fired it is sent through the barrel of the gun at an amazing speed, as part of the firing mechanism the primer in the cartridge ignites at an extreme temperature of 2000 degree Celsius sending hot expanding gas down the barrel of the gun. These extreme conditions have been thought to make retrieval of fingerprints off bullets and cartridge casings exposed to these conditions impossible. With new technology coming out of the University of Leicester, London it is now possible to visualize fingerprints off of spent casings, even if those casings have been washed in hot water and soap.
Fingerprint residue contains natural salt secretions that corrode metals at the points where the salt contacts the metal. This microscopic fingerprint corrosion leaves a permanent mark on metal that cannot be washed away. The heat of the firing of the gun also does not alter these marks.
The new technique that has been established to visualize these types of prints relies on running electrical current over the metal object , such as a bullet casing or a gun, that has been coated with a fine toner like powder. When a charge is run down the metal object the fine conducting powder is attracted to these areas of corrosion revealing a powdered fingerprint.
This technology can also be used in fires to recover prints of metals exposed to very high temperatures.