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FIRED CARTRIDGE CASE EJECTION PATTERNS FROM

SEMI-AUTOMATIC FIREARMS

 

 

 

 

 

 

 

 

 

 

All firearm positions and movements studied were discerned from 30 years of investigation of officer-involved shootings by the lead investigator. Each position and movement had been performed by police officers in dynamic, rapidly unfolding life and death shooting situations.

 

A note must be added here to further clarify the results from some of the test positions so as to better interpret them. Some of the officers involved in this study were unable to correctly perform some of the test conditions (tests 7, 8, & 9) as asked by the researchers. The test conditions called for the shooters to fire while they were in motion not after they had completed the motion. The shooters who did not perform the test conditions correctly did not spin and shoot, instead they spun, stopped, and then shot. Preliminary testing indicates a dramatic change in spent cartridge casing placement when the firearm is fired while being moved rapidly. Therefore, the authors believe the data presented in these tests (7-9) significantly understates the distance and variability of the ejection pattern.

 

 

Results

 

The results of this study demonstrated how unpredictable spent cartridge casing ejection patterns are even when many variables are controlled. A total number of 7,670 bullets were fired from eight different firearms in the course of this study. Spent cartridge casing locations are illustrated through the use of scatter diagrams and pie charts. During the presentation of this information reference is made to quadrants one, two, three, and four. The quadrants are orientated such that quadrant one and two are in front of the zero point and would indicate spent cartridge casings being ejected in front of the study participant, while quadrants three and four would represent the spent cartridge casings being ejected behind the zero point and to the rear of the study participant. A spent cartridge casing found in quadrant one or four would indicate ejection to the right of the zero point, while spent cartridge casings found in quadrant two or three would indicate ejection to the left of the zero point. When a spent cartridge casing position is specified through angular reference a negative degree value indicates a cartridge casing behind the zero point, while a positive degree value indicates a cartridge casing in front of the zero point. Zero to 180 degrees begins to the right of the participant and follows an arch to the left. In both cases the zero reference is located at the zero stake in the center of the grid. See Figure 1 below for example.

 

Since all the data obtained from this study is too large to fit in this article the authors have chosen to present specific results in a broad to precise fashion. Significant variables such as firearm position, motion, grip, weapon type, and ammunition will be taken into consideration one-by-one. This one-by-one approach is to illustrate the impact of each individual variable on the spent cartridge casing patterns. First, all 7,670 spent cartridge casing locations will be presented without any other variables being considered. Next, in section two, the impact of firearm position and grip alone will be illustrated while firearm type, ammunition, and weapon motion will be left as an unknown. Then in section three weapon type results will be illustrated, followed by section four, which is firearm motion and finally section five, ammunition type. This is to help demonstrate how unpredictable spent cartridge casing locations are when several variables are not accounted for and are still unpredictable even when these variables are accounted for. The authors’ goal in this way of presenting the results is to illustrate how imprecise determining shooter’s location is even when several important variables are known and accounted for and not just when variables are unknown.

 

Further statistical analysis of the tests presented below is contained in a table located after each of the scatter plots. This information is presented in two sections within a single table so that the reader can see both the angles, in degrees, in which the spent cartridge casings flew after being ejected from the weapon and the distance, in inches (and cm), that they traveled from the shooter. First the mean (average), standard deviation, median (the middle number), and mode (most common number) are given for the angles (degrees) in which the spent cartridge casings flew from the center point where each participant fired his/her firearm. A negative number from 0 to 90 signals the spent cartridge casing flew behind and to the right of the participant; a negative number from 91 to 180 signals the spent cartridge casing flew behind and to the left of the participant. A positive number from 0 to 90 is to the front and right of the participant and 91 to 180 is to the front and left of the participant. Secondly, the mean, standard deviation, median, and mode of the spent cartridge casings for that specific test are given in inches (cm) for each individual quadrant, described above. This tells the reader how far in inches (cm) the spent cartridge casings landed from the center point in any direction. These tables are to help the reader understand more specific areas in which the spent cartridge casings landed for each of the individual tests illustrated below.

 

All Weapons and Tests

 

Figure 2 shows the percentage of the spent cartridge casings for all test positions and test firearms inclusive. 73.6% of the spent cartridge casings fell in the 90-degree section to the right and rear of the shooter (Quadrant 4). This confirms what experts cite as the location that spent cartridge casings should land in when ejected from the firearms used in this study. However, this still leaves 26.4% of the spent cartridge casings to be accounted for (Hueske, 2006; Haag, 2006). This means over 2,000 spent cartridge casings landed outside of the area most often cited by experts. It also does not consider that the test quadrant to the right and rear occupies 225 square feet and a specific cartridge casing in that quadrant could be almost anywhere in the 225 square feet.