Primer GSR

A discussion of the collection, analysis, interpretation, and reporting of GSR requires an understanding of the formation of primer residue particles. Most residue originating from the barrel of a gun is burned, unburned, or partially burned propellant (gunpowder) and contains metal particulates, such as lead, copper, brass, or nickel from jacketing material. Firearms examiners use this type of GSR to determine the distance between the muzzle of a gun and a target. When forensic trace evidence examiners receive a request to look for GSR on the hands or clothing of a suspected shooter, they search for residue from the primer.

The firing pin of a gun hits the back of the cartridge, activating the shock-sensitive primer, which ignites the gunpowder, forcing the bullet down the barrel of the gun and on its path. The heat and pressure within the cartridge vaporize the metals from the primer. Vapors escape from any area of the weapon not gas tight, like the breach area and muzzle. The heat of this explosive reaction and subsequent cooling results in the condensation and formation of tiny metal-containing particles. These particles fall on anything in the vicinity of the fired weapon, including the hands of the shooter, and typically measure 1 to 10 microns (10-6 m) in size (for comparison, a typical human hair is approximately 100 microns in diameter). Finding and viewing primer GSR particles require a high-powered microscope, such as an SEM.

Gunshot residue particles can be removed easily from the surfaces they land on. Regular activities, such as putting hands in pockets, rubbing hands together, or handling items, can wipe them away.⁴ The washing of hands can remove most, if not all, particles. Rates of loss vary widely with the activity of the subject. Depending on conditions and activity, particles may be removed from a shooter's hands within 4 to 5 hours after a shooting event.⁵ They also can transfer from a surface or person to another individual; the amount depends on the number of GSR particles on the contaminated surface (e.g., a person's clothing or hands) and likely will be a small percentage of the total number of particles present. Tests show that people standing within 3 feet to the side of a shooter may have GSR on their hands, whereas those standing 10 or more feet in the same direction typically will not.⁶ This can vary with the type of gun and ammunition, number of shots fired, and the environment of the shooting. Gunshot primer residue also can travel downrange with each firing of a weapon.⁷ Long guns, like rifles and shotguns, tend to leave less GSR on shooters than handguns.⁸

GSR Samples

Investigators collect primer GSR with adhesive lifters, sometimes referred to in supply catalogs as dabs or stubs. Several companies sell them, usually as a kit with gloves, instructions, an information form, and tape to seal the kit when finished. The adhesive contains carbon, which colors it black and makes it able to conduct electrons in the SEM. Analysts also can use clear adhesive lifters; however, these require an extra step of carbon coating to prevent charging from the electron beam hitting the sample during analysis.

The adhesive is located on an aluminum stub fixed into the cap of a plastic container. Removing the cap exposes the tape, and the sample collection official can press the adhesive--without ever touching it--to the sampling surface. The submitting officer completes the information form, which provides collection-site data (e.g., condition of the subject's hands, known activity prior to collection, estimated time of shooting, and exact time of collection), as well as the type of gun and ammunition used in the event, if known.

Investigators should use one lifter per collection site. Some kits contain two (left hand and right hand), and others feature four (left back, left palm, right back, and right palm). One lifter can suffice when sampling an entire hand, front and back.⁹ Lifters from separate hands can be considered and analyzed as one subject's sample at the lab. Finding particles on the left hand versus the right hand or back versus palm holds no significance because analysts do not know the activity of the hands between the time of the shooting and the time of collection and because both hands likely are in the vicinity of the fired weapon. Investigators can press lifters to the face, hair, or clothing if they suspect that the hands have been cleaned between shooting and collection or covered at the time of the event.

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