SSS.6.169 - Research Supplement: Low Back Pressure 5.56 Silencer Performance - Flow-Through vs. SURGE BYPASS, PIP, and LBP. Head-to-Head Detailed Comparisons (Article Preview)

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HUXWRX FLOW Series vs. Combat Application Technologies CAT/WB/A1/718 vs. SilencerCo Velos LBP vs. PTR VENT 3 on the 5.56 MK18 with 10.3-inch Barrel

There are a variety of technologies implemented in modern rifle silencers. Principally, silencers reduce the severity of the signature produced by a small arm weapon system by modifying the propagation of combustion gasses such that they enter the ambient atmosphere at a lower temperature and pressure than they would otherwise, had a silencer not been installed on the weapon. These parametric differentials, along with the rate of gas propagation, dictate the resulting suppressed small arm signature.

Currently, there exist three different primary classes of rifle silencer technology used to suppress the 5.56x45mm NATO rifle cartridge:

  1. Conventional designs, in which shock and blast loads are significantly reflected in early time, rearward toward the muzzle orifice, with later time gas propagation significantly restricted to atmosphere. High-performance examples of such silencers include the Maxim Defense DSX (6.110), Otter Creek Labs Polonium (6.75), Aero Precision Lahar-30L (6.119), and others. 

  2. High Flow Rate designs, in which blast reflections in the proximal expansion chamber are redirected, reduced, or otherwise altered to reduce the influence on reciprocating weapon function, with later time gas propagation expediently venting to atmosphere. Examples of these silencers include the HUXWRX HX-QD 556 (6.54), HX-QD 556K (6.64), FLOW 556K (6.83), FLOW 762 Ti (6.114), and FLOW 556 Ti (6.167) which use various generations of HUXWRX Flow-Through technology.

  3. Hybrid designs, in which elements of both Conventional and High Flow Rate silencers are used, coupled with other technology variations and staged elements to provide parametrically varied performance attributes. Such attributes include the ability to reduce early time shock reflections while significantly altering the rate of proximal venting. These silencers may allow for minimal reciprocating weapon functional influence, while at the same time significantly suppressing signature to the operator and bystanders. Hybrid designs span a large range of the market in both brand and performance in different combustion regimes.  These silencers include the PTR VENT 3 (6.135), CAT/WB/A1/718 (6.129), SilencerCo Velos LBP (6.134), the Surefire RC2 family, the Surefire SOCOM556-RC3 (6.151), and others.  Technologies implemented in these types of silencers are varied, including PTR Purposely induced Porosity (PIP), elements of Surefire Total Signature Reduction, CAT SURGE BYPASS, and others.

The above three classes of silencers produce varied performance on different weapon systems and with different ammunition types. Of the three classes, the most significant overall performance potentials remain confined to Hybrid designs, in accordance with the PEW Science Silencer Sound Standard public research pedigree, to date. There exists a subcategory of this class of silencers, that is shown to demonstrate efficacy in multiple flow regimes. To date, the best example of such a subcategory includes silencers like the Combat Application Technologies CAT/ODB/A1/718 (6.122). These silencers not only combine performance parameters from all three classes, but have been shown to demonstrate so-called pressure agnostic behavior, in which input variation exemplified by both supersonic and subsonic cartridge combustion regimes both result in high signature suppression efficacy.

It is important to note that higher distal gas velocity from Flow-Through designs produces low frequency-biased inner ear response. This signature characteristic is interpreted by bystanders and operators as sounding “boomy,” more so than that of silencers generating high flow rates through different mechanism (including those using PTR PIP or CAT SURGE BYPASS).  While such descriptors of sound signatures are subjective in nature, the phenomenon responsible for this impression has physical mechanism.  This signature characteristic has been examined in-depth by PEW Science and analysis was previously published in Member Research Supplement 6.103 (supersonic 5.56 NATO), Supplement 6.115 (supersonic 7.62 NATO), and Supplement 6.124 (subsonic 300 BLK).  This current article is authored to include analysis of the recently examined HUXWRX FLOW 556 Ti.

As a consequence of both the phenomena above and pressure field shape from the distal vent array, blast load reflections may also be perceived as more severe to both bystanders and the operator when fielding HUXWRX Flow-Through silencers. Although the Suppression Rating characterizes personnel risk at the instrumented test locations (MIL-STD muzzle and MIL-STD shooter’s ear), testing and analysis is performed in the true free field away from any reflecting surfaces other than the ground, which is 1.6 m below the weapon system muzzle. Bystanders and operator personnel may be located near berms, vehicles, structures, tree-lines, and other obstacles that act as reflecting surfaces. In these environments, it is likely that Flow-Through silencers and other silencers exhibiting the measured traits of low frequency-biased signatures presented in the above Research Supplements, will present as “louder;” the already more noticeable late-time components of their signatures will be further exacerbated by the later-time wave reflection components of the total signature to which personnel are subjected.  All weapon systems have more severe signatures near reflecting surfaces.  However it is postulated that Flow-Through silencer signatures may be perceived as even more severe in these environments.  Research is ongoing.

This Research Supplement compares a select group of silencers, in the supersonic ammunition combustion regime tested on the PEW Science standard MK18 weapon system.  In general, increasing gross flow rate through a silencer can significantly reduce ejection port blast hazards to the weapon operator on standard untuned AR15 rifles.  However, certain designs may significantly outperform others in this regard.  All six (6) of the selected silencers analyzed in this study have relatively high shooter’s ear Suppression Ratings on the standard untuned MK18.  However, there are significant signature differences between the models.  Quantifying these performance differentials is essential for characterizing hazards to the system operator and bystanders.

6.169.1 Quantifying Sound Suppression Performance

The severity of a suppressed small arm weapon signature, to the human inner ear, is quantified by the PEW Science Suppression Rating (Figure 1). As the Suppression Rating is a Damage Risk Criterion (DRC), it only expresses the degree to which a human may experience hearing damage, and it does so in the true free field, away from reflecting surfaces other than the ground 1.6 meters below the weapon muzzle. Although complex signature characteristics are included in its computation, the end result is only a DRC. Suppression Rating Rankings can be found in Section 7 of the Standard.

The nature of human inner ear response, and human interpretation of such response, dictates further signature examination if one desires information outside the purposes of DRC use. For example, end users may be interested in “how a silencer sounds,” rather than in the hearing damage risk potential of using the silencer on their weapon. The two phenomena are not necessarily coupled for all users.

The Suppression Rating DRC allows the end user to group silencers with similar hearing damage risk potential on a variety of weapon systems. After such grouping, further analysis can provide insight into the aforementioned phenomena. To that end, six silencers evaluated by PEW Science are included in this members-only Research Supplement to examine relative First Round Pop (FRP) and overall sound signature suppression performance characteristics. The silencers in this group were selected for their high flow rate characteristics and relatively high operator (shooter’s ear) Suppression Ratings on the untuned MK18 5.56x45mm NATO MK18 weapon system. The six silencers behave differently, even despite some of them exhibiting similar performance quantified by the PEW Science Suppression Rating.  Their 5.56 MK18 suppression performance provides an excellent case study comparing current “low back pressure” rifle silencer technology (Figure 2).

The published Sound Signature Reviews of these six silencers, on the standard MK18, are linked below (chronologically, in order of PEW Science laboratory testing and analysis publication). Their technology class and subcategory or designation is noted:

Fig 1. PEW Science Suppression Rating Scale

*Note that the FLOW 762 Ti is included in this publication, despite being a 30 caliber silencer, due to the relevance of the current generation HUXWRX Flow-Through technology performance comparison with the dedicated bore FLOW 556 Ti analysis recently published.  The 30 caliber CAT/ODB/A1/718 SURGE BYPASS example is omitted for brevity.

Because the Suppression Rating is a DRC, it characterizes the postulated risk of hazard to bystanders, or the weapon operator, from a suppressed small arm weapon system. Despite achieving a similar Suppression Rating, some silencers may have certain signature characteristics that differ from others, and those differences are of interest to users, as noted above. Flow-Through, SURGE BYPASS, LBP, and PIP all respond differently to changes in muzzle blast pressure input.

Sound signature suppression is not the only performance attribute of interest to many users. Back pressure reduction is often paramount for reducing gas toxicity and weapon over-function. The higher flow rate models above do not represent an exhaustive list of silencers that may reduce back pressure; there exist other models such as the Surefire SOCOM556-RC3 (6.151). The RC3 is intended to exhibit higher gross flow rate, reduce over-function on reciprocating weapon systems, and reduce gas toxicity to the end user. The silencer uses a higher flow variant of Surefire Total Signature Reduction technology to reduce back pressure. This silencer is not included in this research supplement because it exhibits significantly lower operator hazard reduction performance than the six aforementioned models.

In addition to flow rate, flash reduction, and back pressure reduction, sound suppression performance of centerfire rifle silencers, particularly during the first shot, is of significant concern to many weapon system operators. Relatively high sound signature amplitude during the first shot from a suppressed weapon system is referred to as First Round Pop (FRP). Prior to the weapon system being fired, a sound suppressor is filled with air from the surrounding environment; this air occupies the internal silencer volume and supports ancillary combustion during the first shot. It is this ancillary combustion that may increase sound signature relative to subsequent shots. The FRP phenomenon is present and measurable in all suppressed systems unless the internal silencer atmosphere is purged such that ancillary combustion is not supported within the silencer.

Fig 2. Suppression Rating Comparisons Of Rifle Silencers Using PEW-SOFT 5.56×45mm NATO MK18 Test Data

Typically, silencers possessing a high flow rate also possess lower sound suppression performance with subsonic ammunition and potentially lower bystander suppression performance, in general. However, there are significant exceptions. Hybrid designs have the potential to exhibit high performance in multiple regimes.

Although changes to gross gas flow rate (flow restriction, or back pressure) strongly correlates to sound signature suppression, there are other flow dynamics and frequency components of silencer sound signatures that result in varying signature severity to the human inner ear for a given suppressed system. These gas dynamics can significantly influence some signature characteristics. Furthermore, certain personnel may have preexisting hearing damage or other hearing sensitivity characteristics that differ from the 95th-percentile inner ear response with which the PEW Science Suppression Rating correlates. The impact of these differences on the human perception of silencer sound suppression performance has been quantified by PEW Science.

This research supplement is intended to provide more information to PEW Science members with regard to specific sound signature characteristics of the tested configurations in the aforementioned analytical test reports and to help frame objective loudness comparisons between six low back pressure 5.56 rifle silencers that use various technologies to suppress AR15 weapon signatures. Both FRP and total sound signature suppression regimes are examined. This supplement is part of ongoing PEW Science small arm weapon system sound signature research. PEW Science thanks you for your support.

The full version of this article is only available to members. You can support PEW Science testing, research, and development with a membership, here. State-of-the-art firearm sound signature testing and research conducted by PEW Science is supported by readers like you. Thank you for your support!