Trayvon Martin, George Zimmerman, and 'Intermediate Range'
Friday, May 18, 2012
So much has been said in recent days about the February shooting death of teenager Trayvon Martin by neighborhood watch volunteer George Zimmerman. Now facing second-degree murder charges, Zimmerman contends that he shot Martin during a physical altercation in which Martin was on top of him, beating his head against the ground.
Among 183 pages of documents released by the prosecutor yesterday is the autopsy report, which details the single gunshot wound to Martin’s chest. Originally leaked to a reporter with NBC, the report describes the shot as having been fired from ‘intermediate range’. This term has become the topic of discussion in news reports and analysis around the country, but what exactly does ‘intermediate range’ really mean?
Whenever a firearm is discharged, the combustion process forcefully expels various residues including soot and unburned/partially burned particles of gun powder. This process is far less efficient than the process of accelerating the bullet out of the muzzle, and the particles are terribly resistant movement through the air, so they travel only a rather short distance from the firearm before dissipating. If these residues strike something while they are still traveling away from the firearm—such as Martin’s skin or clothing—then it is known that he was at least within that short distance that the particles will travel.
We must add another term to our vocabulary: stippling. Stippling, which, when present on skin is also termed ‘powder tattooing’, refers to the pattern created by the expelled particles of gun powder. When these particles strike skin, they cause small punctate abrasions in a pattern that is controlled both by distance and angle. Gunshot residues leave the muzzle in a generally conical shape, which means that as the distance between the muzzle and the skin increases, the size of the stippling pattern likewise increases. If the muzzle is pointing nearly parallel to the body, the pattern will be fairly round; as the angle decreases, the pattern becomes more elliptical.
Gunpowder particles travel further than soot. If soot is present around a wound, the distance from the muzzle to the wound must be quite short—inches, really. If only stippling is present, the distance is greater. If nothing is present, we know only that the shot was fired from some minimum distance—any closer and stippling would have been present.
When the firearm is touching or is within a few inches of the skin, the hot gases from the muzzle will cause charring of the skin and other indicators that are not noted in the autopsy report, so it is clear that the firearm was not in contact with Trayvon Martin’s body.
So, again, what is ‘intermediate range’? To understand that term, you first have to understand that these terms are not standardized across the entire forensic science world. Most likely the medical examiner in this case was following the definition given by DiMaio (1999) who explains that an “intermediate-range gunshot wound is one in which the muzzle of the weapon is held away from the body at the time of discharge yet is sufficiently close so that powder grains expelled from the muzzle along with the bullet produce ‘powder tattooing’ of the skin” (p. 71). DiMaio explains that powder tattooing (stippling) begins at a distance of about four inches.
So how far out does intermediate range go? Well, that question is not easily answered without specific testing using the actual firearm and the actual ammunition (if any remains) or very good exemplars of both. Test firing is typically carried out at increasing distances until no more stippling is observed on the target material. Firing at pieces of white cloth, it is apparent that different ammunition can have remarkably different results; some 9 x 19 mm ammunition will deposit powder particles out to only around two feet, while other ammunition can leave traces out to six feet. The generally accepted average distance, though, is about three feet, which means beyond that no stippling will be present. This is what is known as ‘distant range’.
DiMaio, V. 1999. Gunshot wounds:Practical aspects of firearms, ballistics, and forensic techniques, 2nd ed. Boca Raton, FL: CRC Press.