Abstract: The goal of this experiment was to identify an approximate maximum effective range for the string reconstruction method using three different nylon strings. A practice wall was used that had been created in the past for this type of reconstruction experimentation. This moveable section of wall has numerous existing bullet strikes. Some of these bullet strikes were reconstructed with string and by mathematical calculation to determine an approximate margin of vertical error for string reconstruction.

Introduction

How far away from the actual bullet strike can a string reconstruction be accurately performed? One of the largest limitations of the string method is the effect of gravity on the string. This is readily apparent to anyone who has attempted this type of shooting reconstruction. The nylon string that is typically used for this type of reconstruction sags lower and lower the farther you get from the impact point, causing the investigator to wonder just how accurately the string approximates the flight of the bullet. This article is an attempt to answer this question.

A typical way to perform a string reconstruction is to place a dowel rod or commercially designed probe through a set of holes that are left as a bullet passes through the drywall on both sides of a wall. The rod is then secured in place, protruding at least a foot or so on the side where the reconstruction is performed. The string is attached to the base of the rod near the wall so that the direction of the string can be compared to the protruding rod as the alignment is adjusted. If the bullet strike is lower on the wall, I prefer to attach the string so that it runs along the top of the rod. This way, a glance from above can confirm alignment of the horizontal aspect of the angle. At close ranges of less than ten to fifteen feet or so, the vertical alignment is performed by pinching the string between the thumb and index finger and aligning the string with the rod while sighting back down the string. Unfortunately, this alignment method does not work farther away because it is hard to see the small diameter of the rod and to tell whether the string is aligned closely. The string can be aligned from farther distances by starting the adjustment process with the string held up too high and then slowly lowering it until the string just meets the tip of the rod. This is accomplished by either attaching the string to a tripod to allow for fine adjustment as you walk back and forth to do the comparison or by using an assistant to watch and give instructions while you adjust the termination point of the string. The obvious question is, How far is too far?

The method of reconstruction by trigonometric calculation can provide a baseline to compare string reconstruction data if performed simultaneously. The calculation method uses the horizontal and vertical aspects of the impact angle and basic trigonometric formulas to project the flight path away from the strike in a linear fashion. Although a bullet does not travel in a straight line, this method can provide a very close approximation of the flight path.

Method

A practice wall made of typical 2" x 4" framing and drywall, with a number of pre-existing, full-penetration, 9 mm bullet strikes, was used. It has a pair of wooden legs that can be swiveled to allow for the adjustment of the vertical angle of the wall.

This wall was placed on a level concrete floor and a wooden dowel rod was placed through one of the bullet holes. The rod was secured in place and then a nylon string was attached for the string reconstruction. The string was stretched tightly to a distance of ten feet and was then attached to a camera tripod at a height that an average person might hold a handgun (Figure 1). The angle of the wall was then adjusted to get the rod close to the angle of the string. The fine adjustments of the string were then made using the controls on the tripod. The height of the string at ten feet was then recorded. The angle of the rod was calculated with a digital level and angle finder. The height of the trajectory at ten feet was then calculated and the results were recorded. This experiment was repeated out to fifty feet in ten-foot increments (Figure 2), and these results were recorded as well (Tables 1-3). This experiment was conducted with three types of nylon string similar to that found in commercial shooting reconstruction kits.

Results and Discussion

The reconstructions performed at 10' and 20' were on average about 1/2" and 2" off, respectively, from the calculated trajectory. This error is acceptable when performing shooting reconstructions because the goal is to provide an approximation of the location of the barrel of the weapon and not to try to find its precise position when it was fired. However, at 30', 40', and 50', the average error was about 6 1/2", 8 1/4", and 12 1/2", respectively. The data immediately shows that the error is always on the high side for longer distance reconstructions. This is because of the method of alignment of lowering the string until it matches the angle of the rod. This only aligns the portion of the string close to the impact point that has the uphill, for lack of a better term, portion of the sag. This means that the string curves up and away from the true trajectory, which can give the impression that the shooter was closer to the impact point than where the shooter truly was. The margin of error triples between 20' and 30' In light of this data, string reconstructions of longer than about 20' are strongly discouraged because of the large margin of vertical error. An argument could be made that the usefulness beyond twenty feet exists for demonstration of directionality. However, presenting this type of evidence to the layperson with instructions to disregard the vertical appearance of the string while observing the direction of the trajectory could lead to confusion for the jury. A string reconstruction beyond 20', even for the limited purpose of showing the direction of the trajectory, is particularly unwise if a reconstruction by calculation (or any other accurate reconstruction method) is to be performed, because of the inevitable conflicting results.

For further information, please contact:


Captain David Rose
The Ohio State University Police Division
901 Woody Hayes Drive
Columbus, Ohio 43210
(614)292-6367
rose.81@osu.edu

Figure 1
Close-range reconstruction.

Figure 2
Long-range reconstruction.

Table 1
100% nylon string #18 twist.

Table 2
100% nylon string #18 braid.

Table 3
100% nylon string #15 twist.