Why is primer residue on a shooter's hands studied in CSI?

Why is primer residue on a shooter's hands studied in CSI?

Richard Saferstein (Former Chief Forensic Scientist) gives expert video advice on: How does the study of firearms help in CSI?; How does a gun work?; How are firearms and ammunition collected in CSI? and more...

When it comes to some weapon, when they're fired, a primer residue may be deposited on the hands of the firer. This primer residue consists mainly of trace elements. With the presence of these trace elements, a forensic scientist may be able to ascertain whether an individual fired a weapon or was in close proximity to a fired weapon.