Page 90 - JSOM Winter 2018

follows: The two courses are presented as study 1 and study TABLE 1 Study 1: Overpressure and Impulse by Shooting Position
2 and are described in detail with accompanying results from and Barrel Length
both trainings. General conclusions are drawn about operator Position and Impulse,
exposure to OP under various conditions, and the final section Barrel Length OP, mean mean (SD)
describes implications and recommendations resulting from (inches) (SD), psi 95% CI (m × psi) No.
this work. Kneeling
20 5.04 (—) — 1.74 (—) 1
29 3.48 (1.10) 1.43, 4.89 1.83 (1.42) 8
Study 1
Prone
General Description 20 5.81 (0.99) 4.52, 9.22 2.97 (1.18) 34
To begin assessing shooter OP exposure, evaluations of shoot- 29 4.69 (1.26) 2.96, 8.02 2.24 (0.91) 113
ing position and barrel length were conducted to compare Standing
shooter OP exposures while firing.
20 8.20 (0.46) a 7.84, 8.55 3.27 (1.94) 2
29 4.07 (1.21) 2.56, 6.14 2.18 (1.09) 31
In the first course (3 days), OP was characterized on a total
of 451 rounds fired by 20 shooters from various .50-caliber Prone (BearCat)
rifles with 20- and 29-inch barrels while participating in a .50 20 3.90 (1.69) 2.65, 5.91 1.31 (0.76) 3
Cal/Hard Target Weapons training course taught by Tacflow 29 3.72 (0.13) 3.55, 3.81 2.69 (2.38) 6
Academy (www.tacflow.com). These measurements yielded Standing (BearCat)
224 usable data points for analysis. Shooters engaged targets 20 — — — —
from the seated/kneeling, prone, and standing positions while 29 4.20 (1.38) 3.00, 5.85 2.18 (1.18) 4
outside, as well as inside and atop an armored vehicle (Lenco Tower
BearCat); from the top of a fire training tower; and through 20 4.27 (0.34) 4.01, 4.76 3.08 (0.49) 4
a window in an enclosed room. Ammunition varied between
shooters based on target and department supply, with M33 29 — — — —
Ball (660 grain) being a commonly fielded ammunition. Window
20 5.15 (0.90) 4.48, 6.58 3.83 (0.91) 7
Weather was overcast with occasional showers and gusts of 29 3.97 (1.51) 2.29, 7.76 4.23 (1.26) 11
wind up to 43 mph. Temperatures ranged from a high of 91°F Total for 20" 5.56 (0.93) 3.11, 9.22 2.60 (1.13) 51
(33°C) to a low of 54°F (12°C), with most shots being fired at Total for 29" 4.43 (1.22) 1.02, 8.42 2.25 (1.12) 173
approximately 77°F (25°C). Average humidity throughout the TOTAL 4.69 (1.56) 1.02, 9.21 2.33 (1.12) 224
test days was 77%. Shots were taken at approximately 597 ft —, not measured; CI, confidence interval; OP, overpressure; SD, stan-
above sea level. dard deviation.
a Suspected complication with shooter ammunition or sensor placement.
Students were given five Blackbox Biometrics, Generation 6
(B3 Gen6; www.b3inc.com) blast gauges affixed to the shooter Study 2
with gauges on the front of their hats or head, on the right
and left shoulders, and on both wrists along the distal radioul- General Description
nar joint (the position at which people generally wear a wrist Based on information collected during study 1, there was a
watch). The B3 Gen6 gauge does not register peak OP <2.5 greater focus on the variation in barrel length in this data
psi. Visual analysis of shooter positioning allowed us to de- collection, and special attention was paid to muzzle devices.
termine which gauge provided the best incident OP measure. Broadly, muzzle devices were broken down into the follow-
Incident OP was used because all known safety tables are tied ing categories: (1) no muzzle device (just a target crown on
to incident OP, because reflective OP can vary wildly from ex- the end of the barrel; (2) a 45° slant muzzle brake, similar to
perienced OP based on the orientation of the sensor. Reported the one fielded by Barrett M82A1 rifles; (3) ported flash sup-
values use a left-shoulder gauge, because research experience pressors with a series of holes drilled around them in a 270°
notes that for a predominately right-handed cohort, this ori- arc, with the bottom portion remaining undrilled to minimize
entation gives the most accurate incident OP measurement dust and debris disturbance while firing (similar to an A2-
(Table 1). style flash hider but with holes instead of slots); and (4) a sup-
pressor (generally, the AWC Turbodyne or Barrett .50- caliber
Conclusions: Study 1 suppressors).
Overall, .50-caliber rifles can produce excessive OP, and re-
search has overlooked the roles of rifle configuration, shooting Measurements were collected from of a total of 220 rounds
environment, or shooter position on the OP shooters receive fired by 12 shooters from various .50-caliber rifles with 24-,
when using these rifles. Notably, the most common firing po- 26-, 27-, 28-, and 29-inch barrels without a muzzle brake and
sition operators used with the weapon platform was prone, with a 45° muzzle brake participating in a .50 Cal/Hard Target
which consistently produced OP in excess of 4 psi. Of note, Weapons course instructed by Tacflow Academy over 4 days.
longer barrels resulted in decreased OP across shooting posi-
tions, but the effect was marginal and inconsistent (Table 1). Usable OP data were identified from a total of 89 rounds fired.
Impulse did not vary meaningfully in aggregate across barrel Shooters in this course engaged targets from seated/kneeling,
length, but shooting in confined or rigid spaces did seem to standing, and prone positions while outside, atop an armored
increase experienced impulse to operators. The inconsistent vehicle (Lenco BearCat), and from inside an armored vehicle
decrease in OP from barrel length is likely due to the muzzle (Lenco BearCat). Ammunition varied between shooters based
devices. on target and supply, with Hornady 750 grain A-MAX being a

88 | JSOM Volume 18, Edition 4 / Winter 2018