SSS.6.83 - HUXWRX FLOW 556k and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle (Free Version)
/HUXWRX FLOW 556k on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The FLOW 556k is designed and manufactured by HUXWRX (formerly OSS). It is a 223 caliber centerfire rifle silencer, intended to suppress the 5.56x45mm NATO cartridge from barrels of any length. It has a 1.6-inch diameter and is 5.5 inches long. The silencer mounts to proprietary OSS LH-threaded taper-mount muzzle devices. With the muzzle device installed, the total length of the system is 5.6 inches. The entirety of the silencer is DMLS (3D-printed) 17-4 heat treated stainless steel. The silencer weighs 12.9 ounces and the Flash Hider-QD 556 taper mount weighs 3.3 ounces, for a total system weight of 16.2 ounces, as tested. The FLOW 556k can be obtained from Silencer Shop.
PEW Science is an independent private testing laboratory and also the world’s only publicly funded suppressed small arms research cooperative. Testing, data analysis, and reporting is generated with funding provided by PEW Science members. Any test data that is generated with any portion of private funding contains this disclosure. The testing and data production for this Sound Signature Review was funded in part by PEW Science Project PEW-HUX-030-001-22. Therefore, data pertaining to the FLOW 556k in this Sound Signature Review is published with the express written permission of OSS Suppressors, LLC.
This Sound Signature Review contains single-test results using the HUXWRX FLOW 556k with the flash hider mount on the MK18 Automatic AR15 rifle, chambered in 5.56x45mm NATO with a 10.3-inch barrel. Federal XM193 55gr ammunition was used in the test. The standard PEW Science MK18 test host weapon system is described in Public Research Supplement 6.51.
Please note that the FLOW 556k is a different silencer than the HUXWRX (OSS) HX-QD 556k (Review 6.64) in both construction and performance.
Section 6.83.1 contains the FLOW 556k test results and analysis.
Section 6.83.2 contains Suppression Rating comparisons of the FLOW 556K with dedicated 223 and 30 caliber silencers on the current market, including the Energetic Armament ARX, KAC QDSS-NT4, Rugged Razor556, Otter Creek Labs Polonium, Surefire SOCOM556-RC2, HUXWRX HX-QD 556 and HX-QD 556k, Q Trash Panda, CGS Helios QD, SilencerCo Saker 556, Rugged Razor762, and others.
Section 6.83.3 contains the review summary and PEW Science subjective opinions.
Summary: When paired with the 10.3-in barrel MK18 and fired with Federal XM193, the HUXWRX FLOW 556k mounted with the Flash Hider-QD 556 mount achieved a composite Suppression Rating™ of 33.1 in PEW Science testing. As with all weapon systems, the user is encouraged to examine both muzzle and ear Suppression Ratings.
Relative Suppression Rating Performance is Summarized in SSS.7 - PEW Science Rankings
6.83.1 HUXWRX FLOW 556k Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the FLOW 556k tested with the Flash Hider-QD 556 mount is shown in Table 1. The data acquired 1.0 m (39.4 in) left of the muzzle is available for viewing to all. The data acquired 0.15 m (6 in) right of the shooter’s ear is only available to membership supporters of PEW Science and the Silencer Sound Standard. 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.
Table 1. HUXWRX FLOW 556k Sound Metric Summary
6.83.1.1 SOUND SIGNATURES AT THE MUZZLE
Real sound pressure histories from a 6-shot test acquired with PEW-SOFT™ are shown below. Six cartridges were loaded into the magazine, the fire control group positioned to single-shot, and the weapon was fired until the magazine was empty and the bolt locked back on the follower of the empty magazine. Only five shots are considered in the analysis. The signatures of Shot 6 are displayed in the data presentation but are not included in the analysis to maintain consistency with the overall PEW Science dataset and bolt-closing signatures. The waveforms are not averaged, decimated, or filtered. The data acquisition rate used in all PEW Science testing is 1.0 MS/s (1 MHz). The peaks, shape, and time phasing (when the peaks occur in relation to absolute time and to each other) of these raw waveforms are the most accurate of any firearm silencer testing publicly available. PEW-SOFT data is acquired by PEW Science independent testing; the industry leader in silencer sound research. For more information, please consult the Silencer Sound Standard.
The primary sound signature pressure histories for all 6 shots with the FLOW 556k are shown in Figure 1a. The sound signatures of Shot 1 and Shot 2 are shown in Figure 1b, in early time. The real sound impulse (momentum transfer potential) histories from the same 5-shot test are shown in Figure 2a. In Figure 2b, a shorter timescale is shown comparing the impulse of Shots 1 through 4.
Fig 1a. HUXWRX FLOW 556k 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature
Fig 1b. HUXWRX FLOW 556k 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature
Figure 2a. HUXWRX FLOW 556k 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Figure 2b. HUXWRX FLOW 556k 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
As in the test of the previous generation HUXWRX (OSS) silencers, the gas jetting from the FLOW 556k is significantly faster than observed from high back pressure silencers like the Otter Creek Labs Polonium (Review 6.75) and the SilencerCo Saker 556 (Review 6.53). It also flows significantly faster than the Surefire SOCOM556-RC2 (Review 6.52). The flow rate of the FLOW 556k is also faster than significantly over-bored silencers like the Energetic Armament ARX (Review 6.82), the Rugged Razor762 (Review 6.58), and the Rugged Razor556 (Review 6.76).
Like the HX-QD 556 (Review 6.54) and HX-QD 556k (Review 6.64), the FLOW 556k produces distinctive waveform features consistent with low back pressure silencer behavior. These features are highlighted in Figure 1b, to include a coupled bullet exit event with endcap jetting, followed by a long duration initial positive pressure phase. These initial pressure pulses from the FLOW 556k are extremely similar to those observed in the HX-QD 556k tests, with three significant differences. The FLOW 556k:
Produces initial jetting with a slightly shorter primary positive phase.
Induces a slight jet formation delay after its initial coupled jet.
Produces lower amplitude jetting, overall.
PEW Science Research Note 1: The practical consequence of the above signature differences are higher performance sound suppression with a slight increase in flow restriction (back pressure). It is important to note that the back pressure increase is not significant. However, what is significant, is the increase in sound suppression performance. The FLOW 556k outperforms the HX-QD 556k at both the muzzle and ear on the MK18 by a significant margin. Furthermore, the FLOW 556k outperforms the full-size HX-QD 556 on the MK18 at the shooter’s ear while performing well enough at the muzzle to eclipse the Composite Suppression Rating of the HX-QD 556. The increase in sound suppression performance by the FLOW, compared with previous generation OSS HX-QD silencers, is nontrivial.
PEW Science Research Note 2: The impulse waveforms measured at the muzzle from the FLOW 556k (Figure 2) are unique. They share similarities with those measured from the HX-QD 556k, but again, with key differences. The aforementioned jet formation delay and shorter initial positive phase is evident in impulse space, just as it was in pressure space (Figure 1b). This data indicates that the FLOW is able to maintain a high flow rate (low back pressure; low PEW Science Omega Metric) while still throttling impulse generation. PEW Science postulates that more complex flow paths and increased surface roughness in the FLOW silencer, both enabled by the use of additive manufacturing, are responsible for significant performance dividends. Therefore, the HUXWRX FLOW 556k represents a significant practical advancement in small arm sound signature suppression technology. The FLOW technology is considered by PEW Science to be next generation.
PEW Science Research Note 3: The FLOW 556k exhibits noticeable first round pop (FRP) to bystanders, in that the severity of hearing damage risk to bystanders is postulated to be higher during the first shot with the FLOW 556k than with subsequent shots, in accordance with PEW Science inner ear modeling. The ancillary combustion during Shot 1 is observed in both pressure space (Figure 1b) and impulse space (Figure 2a). Interestingly, although the FLOW 556k possesses a larger impulse differential after Shot 1 than does the HX-QD 556k, it possess similar FRP to the HX-QD 556k.
PEW Science Research Note 4: Several rifle silencers are marketed as possessing increased flow rate or “low back pressure.” The method by which flow rate is increased in a silencer’s design is paramount to overall performance. The Silencer Sound Standard illustrates numerous examples of high flow rate achieved through primary axial over-bore resulting in significant performance detriment in suppressing the MK18 automatic short barrel rifle:
The Rugged Razor762 (6.58); significant over-bore makes this 30 caliber silencer inefficient for suppressing the 5.56x45mm cartridge.
The Energetic Armament ARX (6.82); similar to the Razor762 (and similar to the size of the FLOW 556k), this silencer possesses not only a high flow rate, but very simple baffle geometry, resulting in inefficient performance.
The Rugged Razor556 (6.76); it possesses similar over-bore to its sister silencer, the Razor762, but gains short barrel 5.56x45mm suppression performance from increased interstitial expansion volume (the removal of a baffle). Despite this increased efficiency, its primary flow rate increasing mechanism is still axial over-bore, and its overall sound suppression performance reflects this.
The above silencers are noted here due to the manufacturers advertising their suitability for use on 5.56 weapon systems. For contrast, the Surefire SOCOM762-RC2 (6.73) is also over-bored, and exhibits similar performance on the MK18. However, Surefire produces the SOCOM556-RC2 (6.52) for use with the 5.56x45mm cartridge.
The Silencer Sound Standard also illustrates some examples of primary axial over-bore resulting in performance compromise, but with favorable complimentary design features resulting in more balanced performance:
The Q Trash Panda (6.61); significantly over-bored for the 5.56x45mm weapon platform, this silencer is able to increase performance on the MK18 through significant expansion volume. Although the internal geometry of the Trash Panda, and its mounting system, do increase flow restriction (back pressure), the internal porting and over-bore somewhat balance its performance.
The CGS Helios QD (6.66); also significantly over-bored, this silencer possesses coaxial flow paths that may be accessed early in its core geometry. However, its primary expansion (blast) chamber is large, and can prevent efficient jet propagation to the anulus on the MK18. Despite this particular inefficiency, its internal design features, like in the case of the Trash Panda, result in balanced performance. It should be noted that when the blast chamber was bypassed during internal PEW Science R&D, its MK18 suppression performance increased.
All of the other MK18 datasets published by PEW Science, to date, use silencers that do not exhibit significant primary axial over-bore. The rest of the silencers can therefore be considered “dedicated to 5.56x45mm use.” Significant sound suppression performance dividends are often noted when silencers use dedicated bores. These performance increases are also possible with supplementary venting, as shown in this review of the HUXWRX FLOW 556k, all aforementioned HUXWRX (OSS) HX-QD reviews, and the review of the KGM R556 (6.60). The difference between primary axial over-bore and complex additional flow path venting, in silencer performance, is significant.
PEW Science Research Note 5: As in all semiautomatic AR15 weapon testing, a second pressure pulse originates from the ejection-port signature of the weapon and it occurs early enough in time such that its waves coalesce with that of the muzzle signature. However, in late time (at approximately 87 ms in Figure 1a) the mechanical noise of the bolt closing is observed. The pressure signature of Shot 6 does not display this event due to the bolt remaining open after the sixth and final round is fired from the magazine.
PEW Science Research Note 6: The closing time of the MK18 bolt is directly related to the flow restriction of a silencer for a given weapon system. PEW Science has determined bolt closing time variation from the unsuppressed state to be a reliable indicator of silencer back pressure, with strong correlation with the PEW Science Back Pressure Metric, Omega. However, PEW Science has also determined that the indicator is unreliable upon upper receiver fouling. Sound signatures are not influenced by this fouling, as these kinematics occur in late time, after gas venting to atmosphere. Momentum transfer, weapon condition (upper receiver fouling), and other factors, can significantly influence bolt closing time. PEW Science urges the reader to exercise extreme caution if using the published bolt closing time to make determinations regarding silencer flow restriction (back pressure) or weapon system kinematics. This type of calculation may provide erroneous results, as the weapon condition at the time of each test is not published data. The time-scale duration showing bolt closing time is only published by PEW Science such that the signature data pedigree may be verified.
The shape, timing, and magnitudes of the early-time pressure pulses and overall shape of the impulse waveforms measured at the muzzle, from shot-to-shot, are relatively consistent. The consistency of the waveform amplitudes highlight the silencer’s overall sound performance consistency at the muzzle after the FRP, as well as the relative consistency of the tested automatic rifle firearm configuration. Note the gradual impulse increase that occurs after Shot 3 (Figure 2). PEW Science postulates that as the internal temperature of the FLOW 556k stabilizes, this portion of the impulse history climbs; this is a consequence of the method of suppression employed by the silencer. The hearing damage risk induced by the sound signature during the shot string remains below that of FRP.
As typically indicated, first-round sound signatures always differ from subsequent shots, as the atmosphere within the silencer changes. The FRP phenomenon cannot always be shown by viewing only the peak sound pressure. This is one of the reasons why The Silencer Sound Standard requires examining multiple sound signature metrics. Ammunition consistency can play a role in the determination of FRP, however, the close examination of measured pressure and impulse waveforms typically excludes ammunition from the possible factors influencing true FRP, due to the relative consistency of most high quality factory ammunition.
PEW Science Research Note 7: Note that the muzzle Suppression Rating of the HUXWRX FLOW556 with the flash hider mount is 25.8 and the at-ear Suppression Rating is 31.8; which are different zones on the Suppression Rating Dose Chart. Unlike many rifle silencers, the high flow rate of the HUXWRX 556k, coupled with its above-average muzzle suppression for its size, is extremely beneficial to the sound signature at the shooter’s ear on the standard MK18 weapon system. The back pressure of the FLOW 556k is low, and the muzzle signature is suppressed adequately enough, that the weapon operator is subjected to less significant hearing damage risk. The HUXWRX FLOW556k possesses the highest Suppression Rating at the shooter’s ear on the MK18 weapon system than any silencer tested by PEW Science, to date. For context, inner ear response consequences of such low back pressure and competitive muzzle suppression on the MK18 weapon system are examined in the section below.
6.83.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the HUXWRX FLOW 556k with the Flash Hider-QD 556 mount to that of other rifle silences on the MK18 automatic AR15 rifle. The standard PEW Science MK18 test host weapon system is described in Public Research Supplement 6.51.
Figure 5. Suppression Rating Comparisons of the HUXWRX FLOW 556k and other silencers, Using PEW-SOFT 5.56x45mm Supersonic Test Data and PEW Science Analysis
From the above data, it can be concluded that the aforementioned low back pressure (low flow restriction; low PEW Science Back Pressure Metric, Omega) of the HUXWRX FLOW 556k results in significant sound signature reduction at the shooter’s ear on the MK18 weapon system, despite its truncated length. Notably, the muzzle (bystander) Suppression Rating of the FLOW 556k is higher than that of the CGS Helios QD and approaches that of the KGM R556. This significant increase in muzzle signature suppression performance, coupled with its extremely low back pressure, enable the HUXWRX FLOW 556k to achieve the highest at-ear (shooter) Suppression Rating on the MK18 measured by PEW Science, to date. The Composite Suppression Rating of the FLOW 556k on the MK18, at 33.1, is not only unprecedented for the size of the silencer, but it is the third highest Composite Suppression Rating measured on the MK18 by PEW Science, bested only by the Surefire SOCOM556-RC2 and the Otter Creek Labs Polonium. For a compact (k) silencer like the FLOW 556k to achieve this type of suppression performance on the MK18 weapon system, is nontrivial. As previously stated, the FLOW technology is considered by PEW Science to be next generation.
PEW Science Research Note 9: The HUXWRX FLOW 556k is the only silencer tested by PEW Science on the standard MK18, to date, to achieve an at-ear (shooter) Suppression Rating in the 30-zone on the Suppression Rating Dose Chart. The reduction in hearing damage risk by the FLOW 556k on the MK18, to the weapon operator, is unprecedented in PEW Science testing and analysis. The combination of bystander and shooter signature suppression, as reflected in the Composite Suppression Rating, is also unprecedented for its size. For reference, the FLOW 556k is almost an inch shorter than the Surefire SOCOM556-RC2 and Otter Creek Labs Polonium.
For further context, the FLOW 556k is slightly shorter than the Energetic Armament ARX and exhibits drastically different performance. Not only does the FLOW 556k have even lower back pressure than the ARX, its sound suppression performance results in significantly less hearing damage risk to both the weapon operator and bystanders. As the two silencers are very similar in size, the comparison of the Energetic Armament ARX and the HUXWRX FLOW 556k illustrates a significant technological performance gap that is considered by PEW Science to be generational. Along with its significant over-bore, the baffle design of the ARX is objectively primitive; the advanced flow path geometry of the FLOW 556k, as facilitated by additive manufacturing, places it in a higher performance class. The comparison of these two silencers presents a significant dichotomy in performance-based silencer design.
PEW Science Research Note 10: It is worth noting that only the full-size HUXWRX (OSS) HX-QD 556 approaches the at-ear (shooter) Suppression Rating of the FLOW 556k on this weapon system. The next closest shooter’s-ear competitor on the MK18 is the HUXWRX (OSS) HX-QD 556k. As the FLOW technology represents a design advancement by HUXWRX, PEW Science postulates that it may be reflective of the current “state of the art” in balancing flow restriction (back pressure) and sound signature suppression. Further study of the FLOW technology and other complex gas path technologies, are a continued subject of PEW Science research.
As previously discussed above in Research Note 4, several rifle silencers are marketed as possessing increased flow rate or “low back pressure.” The method by which flow rate is increased, and back pressure reduced, is paramount to overall performance. The Silencer Sound Standard illustrates numerous examples of high flow rate achieved through primary axial over-bore resulting in significant performance detriment in suppressing the MK18 automatic short barrel rifle. The Standard also illustrates some examples of primary axial over-bore resulting in performance compromise, but with favorable complimentary design features resulting in more balanced performance. Significant sound suppression performance dividends are often noted when silencers use dedicated bores, and these performance increases are also possible with supplementary venting. The difference between primary axial over-bore and complex additional flow path venting, in silencer performance, is significant.
The signature to which the shooter’s ear is subjected is a function of both ejection port and muzzle signature. When the silencer’s endcap is in closer proximity to the shooter, the severity is increased. The coalescing of the ejection port overpressure with the primary muzzle blast exacerbates the severity of the signature at the shooter’s head position. It is not ejection port signature, alone, that dictates the signature measured at the shooter’s head position.
6.83.3 Review Summary: HUXWRX FLOW 556k on the MK18 5.56x45mm AR15 with 10.3-in Barrel
When paired with the 10.3-in barrel MK18 and fired with Federal XM193, the HUXWRX FLOW 556k mounted with the Flash Hider QD-556 mount achieved a composite Suppression Rating™ of 33.1 in PEW Science testing. As with all weapon systems, the user is encouraged to examine both muzzle and ear Suppression Ratings.
PEW Science Subjective Opinion:
The HUXWRX FLOW 556k is a relatively compact 5.56mm machine gun rated rifle silencer that exhibits class-leading back pressure reduction and sound signature suppression performance for its size. Its sound suppression performance at the shooter’s ear on the standard MK18 is unprecedented. The silencer is constructed of 3D-printed 17-4 heat treated stainless steel and represents one of the most advanced systems evaluated by PEW Science, to date.
Like the previous generation HX-QD 556 series, the FLOW 556k contains a series of ported helical baffle components within the silencer core. Functionally, the core uses geometric features to induce turbulent flow, while early and continuously routing combustion gasses into annular space for down-stream venting to atmosphere. The distal end of the silencer contains significant exit flow area around its outer circumference. The sound suppression efficiency of this design is directly proportional to cartridge pressure, flow velocity, and duration. The FLOW technology, taking advantage of additive manufacturing (3D-printing), is more advanced than that of the previous generation HX-QD series and the measured performance illustrates this advancement.
The sound suppression performance of the FLOW 556k on the MK18 is nontrivial, especially for the size of the silencer. For example, the FLOW 556k is slightly shorter than the Energetic Armament ARX and exhibits drastically different performance. Not only does the FLOW 556k have even lower back pressure than the ARX, its sound suppression performance results in significantly less hearing damage risk to both the weapon operator and bystanders. As the two silencers are very similar in size, the comparison of the Energetic Armament ARX and the HUXWRX FLOW 556k illustrates a significant technological performance gap that is considered by PEW Science to be generational.
The aforementioned method by which the FLOW 556k shapes gas flow and allows it to exit the silencer is significantly different than many silencers on the current market. As shown in previous PEW Science reviews of the HX-QD 762, the flow characteristics of the silencer significantly reduce efficiency in the subsonic flow regime when compared to the supersonic flow regime. However, as the FLOW 556k is a 223 caliber rifle silencer, its use with subsonic cartridges is extremely uncommon. An improvement to the HX-QD technology before it, the FLOW technology now represents the best balance of Suppression Rating and back pressure reduction in the supersonic flow regime measured by PEW Science.
One consequence of high mass flow rate through a silencer is a potential increase in flash signature. However, the FLOW 556k possesses flash-hiding features on its endcap and increased internal surface roughness through additive manufacturing, compared with the previous generation HX-QD series. PEW Science postulates that these design factors influence flash suppression performance, but has not evaluated the flash hiding performance of this silencer.
The left-hand (LH) threaded taper mounts from HUXWRX are simple to operate. They may be installed on the weapon system with an adjustable wrench; the mount bodies, themselves, serve as wrench-flats. Newer iterations of the mounts may include additional wrench flat features. As the silencer is LH threaded to the mount, the mount is easily removed from the silencer, should the mount be detached from the weapon while still in the silencer. One can then tighten the entire assembly to the conventionally right-hand (RH) threaded barrel muzzle, and continued RH tightening will subsequently loosen the silencer from the mount. Although proper mount installation torque mitigates such a solution from being absolutely necessary, this mechanical feature is welcome for practicality.
PEW Science has not evaluated the durability of the FLOW 556k silencer system(s) on semiautomatic or automatic host weapons. Note that the nature of low flow restriction (low back pressure) silencers may positively influence durability, as the silencers are designed to vent more quickly than traditional designs, relative to the time regime of combustion. As the FLOW 556k is 3D-printed in its entirety, PEW Science postulates that it may exhibit increased durability when compared to the previous HX-QD generation. These phenomena are subjects of further research.
The FLOW 556k possesses extremely low back pressure. Therefore, short barrel gas-operated rifle tuning parameters may not differ from those used when firing the weapon system unsuppressed. Individual weapon system parameters dictate that some weapon systems are more sensitive to flow restriction than others.
In this review, the FLOW 556k performance metrics depend upon suppressing a supersonic centerfire rifle cartridge on a short barrel gas-operated rifle, which is an incredibly difficult task. PEW Science encourages the reader to remain vigilant with regard to all supersonic centerfire rifle suppression claims. The gas volume and combustion products created by the firing of the supersonic 5.56x45mm cartridge are significant; the measured pressure and impulse magnitudes, and their durations, illustrate this fact. Silencer performance on automatic (reciprocating) rifles depends on many factors. Weapon configuration may significantly influence total suppressed small arm system performance.
The hearing damage potential of supersonic centerfire rifle use is significant. PEW Science encourages the reader to consider the Suppression Rating when deciding on an appropriate silencer and host weapon combination for their desired use.
