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’.
References
DiMaio, V. 1999.
Gunshot wounds:Practical aspects of firearms, ballistics, and
forensic techniques, 2nd ed. Boca Raton, FL: CRC Press.
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