A TEST METHODOLOGY FOR THE COMPLETE CHARACTERIZATION OF THE
TASER® XREP MUNITION
Donald Sherman MSE and Cynthia Bir Ph.D.
Wayne State University Detroit MI 48201 USA
Objectives
A tripartite group of the Home Office Scientific Development Branch (United Kingdom),
the Canadian Police Research Center (Canada) and the National Institute of Justice (USA)
agreed to jointly fund an independent third party assessment of the mechanical, electrical and
physical characteristic of the TASER XREP munition. Wayne State University (WSU) in
Detroit, Michigan was selected as the independent test facility and work began in the February of
2008. Key test parameters were discussed among the funding agencies and a total of 10 tasks
were identified to measure the desired test parameters.
The goal of this research is to characterize and evaluate the XREP round. The
characterization will include physical, mechanical and electric properties and the evaluation will
be in terms of accuracy, precision and potential risk of injury. The parameters evaluated include
accuracy, precision, mass, length, electrical output (voltage, current and frequency spectrum)
velocity risk of penetration and the viscous criterion or VC.
Background
Many times in tactical situations there exists a need to deploy a restrained amount of
force. In these instances, less lethal weapons are a popular choice, with extended-range kinetic
energy rounds being the most common choice among law enforcement agencies. The term
extended-range kinetic energy round describes an entire class of less-than-lethal munitions.
These munitions, by definition, use kinetic energy as the means of transferring an incapacitating
force in the form of a ballistic impact.
Extended-range kinetic energy rounds are utilized in law enforcement activities as well as
in military “peace-keeping” missions. Regardless of the scope of their deployment, the rounds
always serve the same purpose; they persuade an unwilling party to comply without the use of
lethal force. The compliance is often a result of the pain caused by these munitions. The goal
is to inflict enough discomfort to solicit compliance without severe injury or fatality.
Historically, accuracy and precision have been problematic with extended range kinetic
energy munitions. Two of the same rounds, fired in the exact same manner, have resulted in
very different shot placements. This lack of precision makes it difficult for the end user to
determine how to aim for the best accuracy. In general terms, accuracy can be described as how
close a round is to a given point (i.e. the center of the target) whereas precision refers to how
closely two or more rounds impact with respect to each other. Therefore, a given round is most
useful when it has both good accuracy and good precision.
Another potential risk of injury results from the impact event itself. This assessment
relies on impact biomechanics to predict the risk of injury related to a given impact. The
tolerance of the human body to a given impact and the determination of the amount of energy or
force imparted by the round are key parameters to assess. Determining a risk of injury prior to
deployment in the field, allows the end user to make well-informed decisions.
One of the disadvantages of kinetic energy based munitions is their reliance on pain to achieve
an effect. The inclusion of other techniques of incapacitating a subject within the round will add
to the chances of a successful outcome following use. In the fall of 2007 Taser International
announced a pilot program for the eXtended Range Electronic Projectile or XREP munition
which offers this additional capability. The XREP is a 12 gauge round has two modes of
incapacitation, kinetic energy and neuro-muscular incapacitation (NMI). The round was initially
marketed as having “the same NMI bio-effect as the handheld X26 but with a range of 20
meters.”
Materials
Round Supply
At the time of testing the XREP round was in a pilot program and not commercially
available although the rounds tested were described as being representative of production rounds
by the manufacturers. All rounds were supplied directly to WSU from Taser. The rounds were
shipped in six batches. The batch numbers and quantities shipped are listed below in Table 1.
Batch #
Quantity
1
20 (plus 5 with no combustible charge)
2
40
3
55 (plus 15 with no combustible charge)
4
40
5
50
6
70
Total
275 (plus 20 with no combustible charge)
Table 1 – XREP Inventory List
Universal Receiver
A table mounted universal receiver (HS Precision Model UR01 Rapid City, SD) was used
to fire all rounds. The receiver was fitted with an 18 inch smooth bore cylindrical barrel and a
laser sight. The receiver was pneumatically driven and computer controlled to allow the testing
to be completely repeatable and user independent. The receiver’s laser sight was sighted in at
each firing distance using a 12 gauge bore sight.
3-rib Ballistic Impact Dummy
A biomechanical surrogate was recently developed and validated to determine the risk of
injury due to blunt ballistic impacts (Bir, 2000). The surrogate or 3-Rib Ballistic Impact Dummy
(3-RBID) was developed to provide a portable surrogate to evaluate non-lethal kinetic rounds in
terms of risk of injury. Three BioSID ribs are joined to a spine box with a polyurethane sheet
joining the ribs in the front. The impact surface measures 6.0 inches in height and 8.5 inches in
width. A urethane foam pad was placed in front of the polyurethane sheet to achieve biofidelity.
The 3-RBID was placed on a Teflon coated table to allow for a low friction interface
between the dummy and table. A non-contact RibEye system is used to measure the location of
each of the three ribs independently in three dimensions. The RibEye was attached to a data
collection system, which collected data at 20 kHz. Data collected from each impact was used to
obtain the magnitude and velocity of rib deflection.
High Speed Video
High-speed video (Redlake HG 100K Video Camera) was collected at 20,000 fps to
determine the exact location of impact and to determine the in-flight characteristics and impact
dynamics of the projectile.
Velocity
The velocity of each round was recorded with three Oehler skyscreens (Model 57). The
skyscreens were attached to an Oehler 35P chronograph, which provided a printout of each
measured velocity.
Mass
All masses were measured on an Ohaus scale (Model E0D120).
Methodology
Testing was performed in three phases. Phase one assessed the physical characteristics of
the munitions, including physical design, electric output, and durability. Phase two assessed the
in-flight characteristics, including the aerodynamics, the deployment of the cholla barbs, as well
as the accuracy and precision of the round. Phase three assessed the probability of injury from
the munition, including the risk of penetration and the blunt trauma effects.
Physical/ Electrical Characteristics Methodology
Task 1. Physical Design
The assessment of physical characteristics included a characterization of the physical
properties of the overall munition as well as the sub-munition pieces. Ten rounds were procured
for this task and the values for the physical dimensions were averaged. The properties which
were assessed included overall dimensions and mass in addition to the dimensions and mass of
the chassis and nose assembly separately. Additional dimensions were recorded for the torsion
spring fins, the frontal electrodes and the cholla barbs.
Physical characteristics were recorded using rounds that had been fired. First total mass
was measured and then the hand trap wire and tether were cut as close to the nose section as
possible and the nose and chassis sections were weighed independently. Next the munitions
were fitted together and total length was measured and then individual section lengths were
measured. Total diameter was recorded at the largest point, the rear faring while nose diameter
was recorded at the nose frame and chassis diameter was recorded at the middle of the chassis.
Fin area was calculated using the equation for the area of a trapezoid (1/2 x h x (a +b)) where h is
the height, a is the short leg and b is the long leg of the trapezoid.
An assessment of the force required to break the nose section free from the chassis was
also performed. The nose section of a live, electrically discharged munition was secured to the
crosshead of a uni-axial test machine (Instron) and the chassis was compressed about the
longitudinal axis at a constant velocity (0.5 mm/s). A digital output of the force/time was
recorded.
Task 2. Assessment of Electrical Output
The assessment of the electrical output of the munition included four separate tests to
measure the electrical output of the three most likely discharge modalities. These were;
discharge across the front barbs, discharge from a front barb to a rearward facing barb, discharge
from a front barb to a cholla barb and discharge from a front barb to the conductive hand trap
wire. A total of 20 rounds were to be used for this assessment, five for each of the four discharge
modalities.
Digital Oscilloscope leads (Tektronix Inc., model TDS 3012B, Beaverton OR) were
connected to both sides of a 1/100 voltage divider which was connected to the discharge points
of the munition in each scenario. The munition was then be activated by removing the insulator
placed between the battery and the chassis electronic housing. Determination of electrical output
for each scenario will include recording the duration, waveform, voltage, current, for each
individual round as characterized by oscilloscope recording. Oscilloscope data was recorded at
1.25 GS/sec.
Task 3. Durability
The durability of the XREP munition was assessed at two different temperatures and in
two different initial orientations. Twenty live rounds were tested at 23°C (room temperature)
and twenty live rounds were tested at -20°C. Ten rounds at each temperature were dropped from
an initial horizontal orientation and ten rounds were dropped from an initial vertical (barbs facing
downward) orientation. Testing began from an initial height of 1 meter. If the round did not
break then the height was increased by 50 centimeters and it was dropped again. This
incremental increase continued until the round broke or a height of 3 meters was reached. The
impact surface was a solid concrete surface. Breakage was considered a crack in the chassis,
separation of the nose assembly from the chassis or any permanent deformation which could
cause the round to malfunction. The aerodynamics and accuracy of the drop tested XREP rounds
was measured using the same methods described below in sections 4.2.1 and 4.2.3.
In-flight Characteristics Methodology
Task 4. Overall Aerodynamics
The aerodynamics of the XREP round was studied using two high speed video cameras
(Redlake Inc., model HG 100K, Tucson AZ) recording at 10,000 frames per second each.
Cameras were positioned at various distances from the barrel to record flight characteristics at a
range of deployment distances. Rounds were fired at the target from 4 different distances (5, 10,
15 and 20 meters). Ten rounds were fired at each distance. This allowed the overall
aerodynamics to be recorded at a range of distances from the barrel. The aerodynamics were
recorded at the muzzle, at impact as well as 5, 7.5, 10, 12, 15 and 20 meters. Data recorded
included attitude of the round (in degrees of pitch above or below horizontal), rotations per
second and velocity.
Task 5. Deployment of Cholla Electrodes
The deployment of the cholla electrodes at impact was recorded during the 40 impacts
used to assess the overall aerodynamics of the munition. Video of each impact was recorded
using a high speed video camera (Redlake Inc., model HG 100K, Tucson AZ) recording at
10,000 frames per second. Images show the orientation of the round at impact, decoupling of the
chassis from the nose section (via breakage of the fracture pins), and deployment of the 6 cholla
barbs from the chassis.
Eight (no additional rounds were available) rounds were used to determine cholla barb
placement and post impact discharge potential. An anthropomorphically correct, shielded test
surrogate served as the target to determine actual cholla barb placement. Shots were aimed at the
center of the thoracoabdominal mass. A resistive wire mesh vest was placed on to the surrogate
to measure electrical activity.
Task 6. Accuracy and Precision
The inherent accuracy of the round was assessed using a fixed position firing system.
The fixed position firing method will utilize a universal receiver for repeatable firing conditions.
A paper target containing a bull's eye and one inch grid marks was mounted down range at
distances of 5, 10, 15 and 20 and meters from the barrel. Ten rounds were fired at each distance.
The universal receiver was aimed using a laser sight prior to each impact. Rounds traveled
through 3 infrared light screens (Oehler Research Inc., Model 57, Austin TX) which will
determine the velocity.
After each impact the target paper was changed and the munitions. After each impact, X
and Y coordinate data were measured from the point of impact to the axis using digital calipers.
Task 7. Temperature Effects
Temperature effects were determined by repeating the tests for accuracy and precision as
well as recording the aerodynamics of the round at 50°C and -20°C. Rounds were placed in a
temperature controlled chamber (Espec model ESL-2CA) for 24 hours prior to testing.
Aerodynamics, accuracy and precision were measured as discussed previously in sections 4.2.1
and 4.2.3 respectively with the exception of the 20 meter distance which was ruled out after the
initial testing showed a significant vertical drop at this distance.
Probability of Injury
Task 8. Risk of Penetration
The skin penetration test protocol required the use of a combination of 20% ordnance
gelatin, 0.60 cm foam, and natural chamois. The Laceration Assessment Layer (LAL) which
consists of the foam and chamois was placed on the front face (towards the munition) of the
gelatin. The LAL layer was secured to the gelatin with adjustable elastic straps. The front face
of the surrogate was positioned at a 0-degree angle of incidence. The gelatin was cut to expose a
flat surface free from damage for each subsequent test.
Penetration effects were assessed at 2 distances; 2 and 5 meters. Ten fair hit impacts
were completed as part of each test distance. Test round velocities were determined
independently using three light screens (Oehler Research Inc., Model 57, Austin TX). After
completion of each test, the surrogate was visually inspected and evaluated for penetration. Test
round masses were recorded using digital balance (Ohaus, model E0D120, Pine Brook NJ).
Task 9. Risk of Blunt Trauma
The blunt trauma assessment was conducted to determine the probability of injury from
the impact of the XREP munition. This testing protocol required the use of the 3-RBID thoracic
surrogate to measure the viscous criteria (VC) for each impact. The 3-RBID was positioned on a
Teflon coated table to allow for a low friction interface between the surrogate and table. For the
purposes of this testing, a laser displacement (Robert A. Denton Inc. model RibEYE, Rochester
Hills MI) system was integrated into the surrogate. This system allowed for non-contact
deflection measurements to be made over a wide region of the sternum. Data were collected
using a TDAS (Diversified Technologies, Seal Beach CA) data acquisition system at 10,000 Hz.
Data collected from each impact were used to obtain the magnitude and velocity of rib
deflection.
Blunt trauma effects were assessed at 2 meters. Test round velocities were determined
independently using three light screens (Oehler Research Inc., Model 57, Austin TX). Ten
“successful” impacts were completed. A successful impact is one that hits no less than one inch
from any edge. For each impact that fell within the specifications, the injury parameter of VC
was determined. Based on previous research, it has been determined that a VC of 0.8 m/s will
result in a 50% chance of sustaining a thoracic skeletal injury at a level AIS > 2. This injury
level correlates to multiple rib fractures.
Results
Initial testing of the XREP revealed significant differences in the rounds from one batch
to the next. The first two batches were shipped electrically dead. Further batches revealed the
electrical activity of the round was intermittent and some rounds tested remained electrically
active more than 5 minutes after being activated. The rounds were also found to drop
considerably from the point of aim at the 20 meter mark. The drop was more than twice the drop
recorded for rounds tested at 15 meters.
Shortly after testing of the rounds was completed, TASER sent a letter to WSU indicating
that the rounds which were tested were not going to be released for commercial sales and that the
XREP platform was being re-designed. The second generation of the XREP munition was
intended to be available for testing in late 2008 but as of the date of the writing of this paper
(March 2009), they were still not available to be re-tested.
Conclusions
The testing methodology described above was established as a means to fully and
completely characterize the XREP munition. The methodology was developed with input from
the funding agencies, end-users and research personnel. It represents lessons learned through
years of research experience and is designed to provide end-users and law enforcement
administrators with accessible, easy to use results that will assist them in their decision on if and
when to use the XREP munition.
Appendix - Terminology
Accuracy - A measurement of how closely a measured value agrees with the true value. For the
current study, this represents how close the measured X and Y coordinates of the impact
are to the center of the target.
Precision - A measurement of how closely measured values agree with each other. For the
current study, this represents how close the various impacts for a given round are to each
other.
Circle of Precision - The smallest circle in which all ten impacts for a given round fit. The center
of the circle of precision is placed on the average X and Y coordinates.
Viscous Criterion (VC) – An injury criterion empirically derived to correlate impact to severity
of injury. The VC is calculated based on the amount of sternal deflection and the
velocity at which the deflection occurs. VC has been validated as a useful tool in
determining injury severity related to blunt ballistic impacts.
3-RBID - The 3-Rib Ballistic Impact Dummy is a biofidelic mechanical surrogate used for
evaluating injury risk of blunt ballistic impacts.
Penetration Assessment Layer (PAL) – The internal component of the penetration surrogate used
to assess the occurrence of penetration. The PAL is composed of 20% ballistic gelatin.
Laceration Assessment Layer (LAL) – The external covering of the PAL used to assess the
occurrence of laceration. The LAL is composed of an outer layer of natural (Sheep skin)
chamois and an inner layer of 0.60 cm closed cell foam.
