The great forensic scientist Edmond Locard is known as the French Sherlock Holmes. Locard’s exchange principle is that every contact leaves a trace
A gun fired leaves residues that are revealed in the hair, on skin, and most markedly on the thumb and forefinger of the shooter. Unfortunately for the criminal investigator, and fortunately for the perp, the residues can be transferred. Not just to another person using the gun, but by the act of shaking hands, or other physical contact.