SSS.8.1.3 - Unsuppressed Rifle Hazards - 10.3-in Barrel 5.56x45mm NATO MK18 Hazard Map Brief
/The Unsuppressed 10.3-in Barrel 5.56 MK18 Semiautomatic Rifle - Hazard Map
This Hazard Map Brief documents the free field blast overpressure hazards from an unsuppressed semiautomatic AR-15 rifle firing M193 55gr 5.56x45mm NATO ammunition from a 10.3-in barrel, as evaluated in Public Research Supplement 6.51.
The average Suppression Rating hazard contours at 1.6 m bare muzzle elevation from the instrumented blast test computed with the PEW-SOFT HD Blast Hazard Prediction Tool are shown in Figure 1. The Suppression Rating hazard scale is shown in Figure 2. Ground-truth anchored Suppression Ratings from the technical report are annotated 1.0 m left of the end cap (SR_muzzle) and 6 inches right of the shooter’s right ear (SR_ear).
Figure 3 and Figure 4 display free field blast overpressure [psi] and blast impulse [psi-ms] contours, respectively. Click or tap the figures to enlarge. Note that the figures also include decibel [dB] equivalents on the contour scales to assist the reader. Note that psi is a linear unit of pressure, whereas dB is a logarithmic unit. The same holds true for units of impulse. The dB units displayed are computed relative to a 20 micropascal reference pressure. For more information on linear and logarithmic pressure and impulse scaling, please reference SSS.2 - Methodology Introduction.
Computational parameters and blast overpressure hazard field observations are provided below.
8.1.3.1 PEW-SOFT HD Computational Model Parameters
PEW-SOFT HD code Version 2.2.4 Alpha was used.
Contour elevation is 1.6 m above grade. No reflecting surfaces, other than the ground, are present in the model.
Grid resolution is 0.076 m (3 in).
The average semiautomatic jet stability index sigma_sa is 0.0036.
fig 1. Unsuppressed 10.3-in Barrel 5.56x45mm NATO MK18 Free Field Hazard Map Produced by PEW-SOFT HD Blast Hazard Prediction Tool using PEW Science Test Data
Fig 2. PEW Science Suppression Rating Scale
fig 3. Unsuppressed 10.3-in Barrel 5.56x45mm NATO MK18 Free Field Blast Pressure Hazard Map Produced by PEW-SOFT HD Blast Hazard Prediction Tool using PEW Science Test Data
fig 4. Unsuppressed 10.3-in Barrel 5.56x45mm NATO MK18 Free Field Blast Impulse Hazard Map Produced by PEW-SOFT HD Blast Hazard Prediction Tool using PEW Science Test Data
8.1.3.2 Unsuppressed 10.3-in Barrel MK18 Hazard Map Observations:
As shown in Figure 1, an unsuppressed short barrel rifle firing the 5.56 NATO cartridge is exceedingly hazardous. The standard 10.3-in barrel MK18, when unsuppressed, produces a severe hazard field that fully envelops the weapon operator. The PEW Science Suppression Rating is intended to quantify blast load hazards in the regime produced by suppressed small arms. Outside this regime, blast loads produced by unsuppressed short barrel centerfire rifles induce Suppression Ratings at the extreme bottom end of the scale. Hearing damage to unprotected ears is likely to occur with limited exposure frequency to these events. Near the muzzle, immediate hearing damage is likely. In these cases, peak blast pressure and impulse contours are useful for characterization, as presented in Figure 3 and Figure 4.
When a small arm weapon system is suppressed, blast load waveshapes are very complex. This complexity is induced by long duration blowdown events from the silencer, as well as multiple pressure sources from the weapon systems that occur in similar amplitude regimes; events coalesce and become nested and varying frequency and phase components result. However, unsuppressed small arm weapon system combustion blast load waveshapes are relatively simple shockwaves. As such, the response of systems (like your ears) to their effects is more easily characterized by two primary parameters of the waves; their peak overpressure and primary positive phase duration.
Figure 3 displays how peak shock pressure from the unsuppressed AR-15 short barrel rifle varies spatially in the field.
Figure 4 displays how these short duration shocks create momentum transfer potential in the field. The blast load impulse from small arm system combustion is relatively low when compared to classical blast loads produced by higher energy explosions. There is less energy present in an AR-15 rifle cartridge than in an artillery round, for example.
Acute serious or life-threatening human injury is only likely at very high blast pressures in the free field, with longer durations. The human body is relatively resilient to blast overpressure hazards, in general. Blast loads by themselves are seldom immediately fatal; it is typically blast damage to nearby systems and components that induces blunt trauma and causes life-threatening injury in a blast event. Other common injury modes are induced by the translation of the human body due to impulsive structure or vehicle movement, followed by head or thorax impact with an object. Nonetheless, the below are reference injury thresholds from general explosion-induced blast overpressure loads in the free field, away from objects (Baker et. al, 1983). Below these thresholds, the injury mechanisms are expected not to occur. Likelihood of injury increases with pressures above the following thresholds, for impulses higher than shown in Figure 4:
The threshold for human eardrum rupture is approximately 5 psi = 35 kPa = 184.9 dB free field blast overpressure.
The threshold for human fatality from lung damage is approximately 10 psi = 70 kPa = 190.9 dB free field blast overpressure.
The above human injury thresholds are typically characteristic of larger energetic events than present in small arms. Again, the impulses associated with such loading are typically much higher than are generated in small arm system combustion. The blast impulse contours in Figure 4 highlight the relatively small amount of energy present in small arm weapon cartridges when compared to large caliber munitions, high explosives, and fuel-air weapons. For example, a human would most likely not experience lung collapse adjacent to the muzzle of an unsuppressed MK18; the blast load duration is too low and therefore so is the blast impulse. However, long term repeated exposure to blast overpressure loads displayed in Figure 3, even with accompanying relatively low amplitude blast impulses displayed in Figure 4, may induce traumatic brain injury (TBI). Postulated thresholds and frequencies of exposure to induce such injuries are subjects of current areas of research, word wide.
The above data is valid in the free field, away from all reflecting surfaces other than the ground, with the muzzle of the weapon located at 1.6 m elevation above grade. The indoor use of small arms, or their use near reflecting surfaces, exacerbates the risk of human injury due to blast impulse amplification. Silencers may reduce the degree to which that risk is exacerbated.
