Why is gun powder residue studied in CSI?

Why is gun powder residue 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 a bullet penetrates a target, if the weapon was close to the target (generally within three feet), residual material from the gun powder may be deposited upon the target. By studying the pattern of the deposition of that material around the bullet hole, a forensic scientist may be able to ascertain the distance the weapon was held from the target.