Otter Creek Labs Infinity 556K with 3 Different End Caps on the 5.56 MK18 AR-15 - Hazard Maps

This Silencer Hazard Map Brief documents the free field blast overpressure hazards from the Otter Creek Labs Infinity 556K rifle silencer with 3 different end caps fielded on the 10.3-in barrel MK18 AR-15 semiautomatic rifle firing M193 55gr 5.56 NATO ammunition.

All three cases are evaluated in technical Report 6.233 and the results tabulated in record numbers 7.1.270, 7.1.271, and 7.1.272 in the Rankings, respectively.

The average Suppression Rating hazard contours at 1.6 m silencer end cap elevation from the instrumented blast tests of the Infinity 556K silencer computed with the PEW-SOFT HD Blast Hazard Prediction Tool are shown in Figures 1, 2, and 3. The Suppression Rating hazard scale is shown in Figure 4. 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). Computational parameters and blast overpressure hazard field observations are provided below.

8.1.30.1 PEW-SOFT HD Computational Model Parameters

PEW-SOFT HD code Version 2.2.7 Alpha was used.
Contour elevations are 1.6 m above grade. No reflecting surfaces, other than the ground, are present in the models.
Grid resolutions are 0.076 m (3 in).
The average semiautomatic jet stability index sigma_sa of the silencer on the MK18 is 0.30 with the solid end cap, 0.23 with the partially vented end cap, and 0.15 with the fully vented end cap.

8.1.30.2 Otter Creek Labs Infinity 556K Suppression Hazard Map Observations:

The Otter Creek Labs Infinity 556K silencer is a hybrid design with high early time flow rate that controls combustion propagation well when using all three of its different end caps.

As distal venting is increased to include annular flow (with the partially vented end cap) and to include additional primary bore flow and additional annular flow (with the fully vented end cap), the forward hazard field changes shape. These changes occur directly proportionally to the blast momentum transfer out of the end of the silencer.

Somewhat counterintuitively, the partially vented end cap configuration of the silencer presents a somewhat reduced forward hazard field while still producing a slightly broader jet than the solid end cap configuration. The hazard changes are driven by frequency components from the additional but lower pressure annular vent flow and those tone changes are less severe for personnel. This phenomenon is analyzed in detail in the full Member Version of Report 6.233.

The operator hazards from blast overpressure while using the standard untuned MK18 with the Infinity 556K are very similar, regardless of end cap. However, gas blowback to the operator will be reduced with the partially vented and fully vented end caps, when compared to that of the solid end cap.