"Why we cannot rely on firearm forensics: A New Scientist investigation reveals that finding gunshot residues on a suspect does not mean they fired a gun."

One interesting passage from the article:

Even worse, it is possible to pick up a so-called "unique" particle from an entirely different source. Industrial tools and fireworks are both capable of producing particles with a similar composition to GSR. And several studies have suggested that car mechanics are particularly at risk of being falsely accused, because some brake linings contain heavy metals and can form GSR-like particles at the temperatures reached during braking.