Page 91 - JSOM Winter 2018

commonly fielded ammunition. Weather was generally partly TABLE 3 Study 2: Effects of Position
cloudy with gusts of wind up to 15 mph. Temperatures ranged Impulse,
from a high of 76°F (24°C) with a low 50°F (10°C) through- OP, mean mean (SD)
out measurements, with the majority of shots being taken at Position (SD), psi 95% CI (m × psi) No.
approximately 70°F (21°C). Average humidity throughout test Kneeling 5.23 (0.84) 3.93, 6.40 2.34 (0.57) 9
days was 37%, with no precipitation. Shots were taken at ap- Sitting 5.67 (0.48) 5.27, 6.40 2.32 (0.85) 4
proximately 1,867 ft above sea level. Students were measured Standing 4.51 (0.35) 3.97, 5.05 2.31 (0.82) 9
with B3 Gen6 blast gauges identical to those used in study 1. Prone 5.62 (0.98) 3.03, 6.84 2.27 (0.93) 13
Prone (IT) 6.09 (0.25) 5.94, 6.39 4.16 (0.30) 3
Notably, firing from inside or atop vehicles was associated with
higher OP measurements (Table 2). OP across all shots fired Prone (OT) 6.82 (0.38) 6.59, 7.26 2.95 (1.34) 3
was reported as a mean of 4.51 psi (standard deviation, 0.52; —, not measured; CI, confidence interval; IT, inside BearCat; OP,
overpressure; OT, atop BearCat; SD, standard deviation.
n = 89). Removing the rounds fired from the vehicles, the mean
OP dropped to 4.39 (0.56) psi (n = 76). Looking at the vehi-
cle rounds exclusively, notably higher OP was observed (mean, To isolate position and its relationship to OP, muzzle device
5.21 [0.27]; n = 13). Where possible, care should be taken to and barrel length were controlled for in the following anal-
minimize shooting from atop hard, metallic surfaces and from yses, and evaluated position was the only variable using the
within vehicles. Similar to data in study 1, impulse seemed ex- 29-in barrel with the 45° slanting muzzle brake (one of the
acerbated by firing from atop or within rigid structures. most common rifle configurations; Table 3). The seated posi-
tion remains the most problematic, but it became clear from
Evaluating the effects of position across all barrel lengths and our data that the closer the operator is to a reflective surface
muzzle devices, the seated position seemed to be the most (vehicle, hard surface, or on dirt), the OP on the shooter is
problematic (Table 3). Likely, this has to do with how the body more likely to exceed the current standard of 4 psi and likely
leans forward while in the seated firing position, which cants will exceed 5.6 psi. We suspect that the 45° muzzle brake on
the body further toward the muzzle while using the offhand the 29-in barrel is sufficiently far away from the operator that
for support. In addition, the seated shooting position creates a the shooter is in the cone of redirected blast resulting from the
somewhat concave form, which potentially may be collecting muzzle brake. Impulse was exacerbated by firing from within
and focusing blast. or atop rigid structures and vehicles.

As a general trend, moving the weapons platform closer to the To further investigate the effects of barrel length, barrel length
ground or resting it on a rigid structure seemed to increase the was isolated and investigated. Data are shown in Table 4.
OP experienced by the operator. Impulse did not seem to vary Once variation in muzzle device was controlled for by using
greatly between positions. only data from .50-caliber rifles with identical muzzle brakes,

TABLE 2 Study 2: Overpressure and Impulse by Shooting Position, Barrel Length, and Muzzle Device
Barrel Length Impulse, mean (SD)
Position (inches) Muzzle Device OP, mean (SD), psi 95% CI (m × psi) No.
24 None 4.26 (0.34) 4.00, 4.52 1.96 (0.31) 2
27 None 3.05 (0.15) 2.93, 3.16 4.14 (1.12) 2
Kneeling 28 45° slant 5.77 (0.15) 5.65, 5.88 2.69 (1.94) 2
None 3.36 (0.11) 3.30, 3.52 3.09 (1.20) 4
29
45° slant 5.23 (0.84) 3.93, 6.40 2.34 (0.57) 9
24 None 3.59 (0.31) 3.35, 3.83 3.27 (0.30) 2
Sitting
29 45° slant 5.67 (0.48) 5.27, 6.40 2.32 (0.85) 4
24 None 3.59 (0.31) 3.52, 3.83 2.25 (0.10) 2
26 45° slant 3.99 (0.12) 3.92, 4.14 3.43 (1.76) 3
26 Ported FH 3.02 (0.29) 2.71, 3.38 1.39 (1.09) 5
Prone
27 None 3.29 (0.14) 3.15, 3.41 1.57 (1.08) 5
28 45° slant 4.97 (—) — 1.45 (—) 2
29 45° slant 5.62 (0.98) 3.03, 6.84 2.27 (0.93) 13
24 None 3.23 (0.25) 2.93, 3.38 1.60 (0.41) 3
27 None 4.09 (1.25) 2.90, 5.59 2.94 (0.74) 4
Standing
29 None 3.40 (1.21) 2.61, 5.59 1.71 (0.18) 5
29 45° slant 4.51 (0.35) 3.94, 5.05 2.31 (0.82) 9
26 45° slant 3.38 (0.46) 3.02, 3.73 2.91 (0.21) 2
Prone (IT) 27 None 4.31 (0.13) 4.31, 4.54 3.70 (0.28) 3
29 45° slant 6.09 (0.25) 5.940, 6.390 4.16 (0.30) 3
27 None 4.64 (0.16) 4.521, 4.759 2.98 (2.16) 2
Prone (OT)
29 45° slant 6.82 (0.38) 6.585, 7.265 2.95 (1.34) 3
TOTAL — — 4.51 (0.52) — 2.61 (1.63) 89
—, not measured; CI, confidence interval; IT, inside BearCat; OP, overpressure; OT, atop BearCat; SD, standard deviation.

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