SSS.6.167- HUXWRX FLOW 556 Ti and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle
/HUXWRX FLOW 556 Ti on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The FLOW 556 Ti is designed and manufactured by HUXWRX. It is a 5.56 mm centerfire rifle silencer, intended to suppress the 5.56x45mm NATO cartridge, with no barrel length restrictions. It has a 1.8-inch diameter and is 6.75 inches long. The silencer mounts to proprietary HUXWRX LH-threaded taper-mount muzzle devices. With the muzzle device installed, the total length of the system is 6.9 inches. The entirety of the silencer is DMLS (3D-printed) Grade 5 Titanium. The silencer weighs 10.4 ounces and the Flash Hider-QD 556 taper mount weighs 3.3 ounces, for a total system weight of 13.7 ounces, as tested. The FLOW 556 Ti can be obtained from Silencer Shop.
PEW Science is an independent private testing laboratory and also hosts the world’s only independent public 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 analysis production for this Sound Signature Review was funded in part by PEW Science Project PEW-HUX-088-001-24. Therefore, data pertaining to the FLOW 556 Ti in this Sound Signature Review is published with the express written permission of HUXWRX Safety Co. LLC.
! RESEARCH CAUTION !
It is important to note that higher distal gas velocity from HUXWRX 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 methods. 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 is published in Member Research Supplement 6.169 (Low Back Pressure 5.56 Rifle Silencer Head-to-Head Comparisons). Past Member Research Supplements 6.103 (5.56 NATO), 6.115 (7.62 NATO), and 6.124 (subsonic 300 BLK) are also available on the subject. All Member Research Supplements can be found here.
The testing and analysis presented in this Sound Signature Review are of the FLOW 556 Ti on the MK18 Automatic AR15 rifle 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.
Section 6.167.1 contains FLOW 556 Ti test results and analysis.
Section 6.167.2 contains overall Suppression Rating comparisons of the FLOW 556 Ti with with dedicated 223 and 30 caliber silencers on the current market, including the Off Grid Operator Ti, Surefire SOCOM556-MINI2, KAC QDC/CRS-PRT, KAC QDC/MCQ-PRT, Surefire SOCOM556-RC3, Otter Creek Labs Polonium-30, AAC M4-2000 Mod 08, PTR VENT 3, SilencerCo Velos LBP, BOSS Guillotine, CAT WB, PWS BDE 556, CAT ODB, Aero Precision Lahar-30L, Lahar-30, HUXWRX FLOW 762 Ti, Maxim Defense DSX, Thunder Beast Dominus, KAC 5.56 QDC, CGS SCI-SIX, Dead Air Nomad-30, YHM Turbo T2, Dead Air Sandman-S, HUXWRX FLOW 556k, Energetic Armament ARX, KAC QDSS-NT4, Rugged Razor556, Otter Creek Labs Polonium and Polonium-K, 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 6.167.3 contains an article summary and PEW Science laboratory staff opinions.
Summary: When paired with the 10.3-in barrel MK18 and fired with Federal XM193, the HUXWRX FLOW 556 Ti achieved a composite Suppression Rating™ of 41.3 in PEW Science testing.
When paired with the 14.5-in barrel M4A1 and fired with Federal XM193, the HUXWRX FLOW 556 Ti achieved a composite Suppression Rating™ of 41.1 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
6.167.1 HUXWRX FLOW 556 Ti Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the HUXWRX FLOW 556 Ti is shown in Table 1. The data acquired 1.0 m (39.4 in) left of the muzzle is available for viewing to all. This is a members-only review and includes pressure and impulse waveforms measured at the shooter’s ear. PEW Science thanks you for your support; further testing, research, and development of PEW-SOFT and the Silencer Sound Standard is made possible by members like you!
Table 1. HUXWRX FLOW 556 Ti Sound Metric Summary
6.167.1.1 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 public dataset and bolt-closing signatures. The waveforms are not averaged, decimated, or filtered. The data acquisition rate used in all PEW Science laboratory sound signature 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 laboratory testing; the recognized 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 556 Ti Ti 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 6-shot test are shown in Figure 2a. In Figure 2b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shots 2 and 5.
Fig 1a. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature
Fig 1b. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature
Figure 2a. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Figure 2b. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
The Flow-Through technology from HUXWRX has undergone a significant number of design iterations. The Standard research pedigree examines the performance of several of these designs, beginning with the HX-QD series from OSS. The reader is encouraged to examine the testing and analysis reports of the legacy OSS/HUXWRX models, which include both supersonic ammunition and subsonic ammunition combustion suppression. Section 7 of the Standard includes a Rankings Table that is filterable and sortable.
The test data herein were acquired with the HUXWRX FLOW 556 Ti on the PEW Science standard MK18 weapon system. This is the third HUXWRX FLOW model to be evaluated on this untuned AR15 short barrel rifle; previous evaluations were performed on the HUXWRX FLOW 556K (6.83) and the HUXWRX FLOW 762 Ti (6.114).
FLOW Model History: The FLOW 556K is a "k,” or “kurz” (short) model. It sacrifices gross bystander signature suppression for a smaller size and weight envelope, with very high ejection port blast hazard reduction on untuned weapon systems. The FLOW 762 Ti is a full-size silencer, with respect to most 5.56x45mm rifle silencer models. However, it is a .30 caliber model; the system is over-bored for the MK18 weapon system. Therefore, the FLOW 762 Ti sacrifices some bystander suppression for weapon system utility; it may be used with calibers larger than 5.56 mm. Nonetheless, as tested and analyzed by PEW Science, the latest generation of Flow-Through technology in the FLOW rifle silencers responds well to high overpressure input; over-bore is not as detrimental to performance as it is with many other silencer technologies, including the aforementioned previous generation HX-QD series.
FLOW Model Update: The FLOW 556 Ti removes the over-bore compromise from the FLOW 762 Ti. Externally, the design envelope of the two silencers is identical, with the FLOW 556 Ti weighing one ounce less than the 762 model. Internally, the design has been optimized for the 5.56x45mm cartridge. Therefore, the FLOW 556 Ti may be considered the full-size version of the FLOW 556k, but constructed from titanium instead of Inconel alloy. All three silencers use the same mounting system and all testing was performed with the same flash hider mount.
There are distinct features in the above measured test data that highlight both performance changes and parallels in the FLOW 556 Ti compared with that of the previous FLOW 556k and FLOW 762 Ti models. These include:
Lower amplitude initial jetting, albeit still with significant initial positive phase duration (Fig. 1b).
Limited rarefaction, like the FLOW 762 Ti, but with longer duration gas throttle peaking in late time (Fig. 1b).
Lower amplitude FRP divergence in both pressure space (Fig. 1) and impulse space (Fig. 2).
Similar heat-affected momentum propagation increase later in the shot string (Fig. 2).
In general, the combustion propagation from the FLOW 556 Ti is more controlled than the other two models (ref. Fig. 1b above, 6.83, and 6.114).
PEW Science Research Note 1: The HUXWRX Flow-Through technology is highly advanced. In report 6.114, it was determined that the longer effective contact time with the silencer internals still had high efficacy with over-bore. When the technology is adapted to be caliber-specific, as in the subject FLOW 556 Ti, gross performance increase is significant. The muzzle (bystander) Suppression Rating of the FLOW 556 Ti enters the 40-zone at 40.4. This is a very high level of performance on the untuned MK18 weapon system, only reached so far by the PTR VENT 3 (6.135). The technologies in the two silencers are radically different; the Purposely Induced Porosity (PIP) hybrid technology allows high early-time flow rate with significant momentum throttle to peak accumulation (ref. Fig. 1b, 6.135), whereas the Flow-Through HUXWRX technology facilitates high early-time flow with less downstream restriction. Flow-Through silencers rely on significantly lengthened overlapping flow path to maximize surface area contact, whereas PIP silencers rely upon porosity boundary layer pressure differential, forcing combustion gasses through a lattice structure. Both methods increase heat transfer efficiency and produce high gross suppression performance. However, there are performance drawbacks to both of the technologies; for example, it is likely that they both have larger than typical critical minimum size envelopes to achieve efficacy. Other drawbacks include potential debris buildup that may lower performance without strict cleaning schedules, due to the nature of their behavior. Finally, specifically with the HUXWRX Flow-Through technology, there is a pervasive performance characteristic that is extremely important to note (see Research Note 2).
PEW Science Research Note 2: By their nature, Flow-Through silencers increase wave particle velocity from their distal ends; this is a consequence of the primary goal of reducing gross system backpressure; minimizing both their alpha early-time flow parameter and their Omega (6.40) late-time gas momentum accumulation rate. By minimizing the overall flow restriction downstream of the weapon muzzle, both weapon kinematic influence and hazards to the operator (blast overpressure and toxic gasses) may also be minimized. However, two physical phenomena present, consistently, through the use of this technology:
Higher distal gas velocity 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 causing this impression has physical mechanism. This signature characteristic has been examined in-depth by PEW Science with 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). To accompany this current study, Member Research Supplement 6.169 has been authored, to include examination of the subject FLOW 556 Ti.
As a consequence of both the phenomenon in (1) and pressure field shape from the distal vent array, blast load reflections may be perceived as more severe to both bystanders and the operator when fielding Flow-Through silencers. Although the Suppression Rating characterizes personnel risk at the instrumented test locations (MIL-STD muzzle and 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.
PEW Science Research Note 3: As previously noted in the evaluation of the FLOW 762 Ti, full-size implementations of the latest HUXWRX Flow-Through technology do increase the computed Omega Metric. The rate of rise to maximum positive phase blast load impulse 1.0 m left of the silencer’s end cap in the free field does, in fact, drop. This is observable in the above test data (Fig. 2a) and calculable with the methodology outlined in Public Research Supplement 6.40. It is for this reason that users fielding the FLOW 556K will most likely experience a “louder” operator and bystander experience than users fielding the larger FLOW 762 Ti and FLOW 556 Ti; the full-size models exhibit more efficient performance and their holistic pressure field is less severe, despite the FLOW 556K having only a nominal differential in its operator (shooter’s ear) Suppression Rating. The free field acute operator hazard from ejection port blast on the untuned MK18 weapon system is significantly reduced with the FLOW 556k, but the severity of its muzzle signature, its first-round-pop (FRP), and the confounding factors described above in Research Note 2, will most likely result in personnel interpreting signatures from the FLOW 556 Ti as less severe, overall, especially when compared to the FLOW 556k. This is a postulation of likely user impressions contrasting the short and full-size FLOW silencer models. Further postulation for other technology comparisons follow:
PEW Science Research Note 4: The FLOW 556 Ti produces a significantly high Suppression Rating, indicating that the level of protection to the operator and bystanders, in the free field, on the untuned MK18 weapon system, is relatively high when compared to other silencers tested in the Standard. The dedicate bore diameter, the internal geometric optimization, and the lengthened flow paths in its large 1.8-in diameter and full-size length, all contribute to raising its Omega metric, maximizing heat transfer efficiency, and increasing sound suppression efficacy. Nonetheless, even with this high level of protection through purposeful design, it is likely that some personnel will still interpret signatures from the silencer as “boomy.” This likely impression is not limited to a bystander comparing the FLOW 556 Ti to traditional high backpressure silencers; on the contrary, it is likely that this impression may present in comparison of Flow-Through silencers with hybrid technologies as well; technologies in which particle velocity is further reduced but early-time flow rate remains high. This, specifically, is the subject of Member Research Supplement 6.169. The 5.56 silencers with the highest shooter’s ear Suppression Rating on this weapon system are put head-to-head in that report. The FLOW 762 Ti, although a 30 caliber silencer, is also included due to its relevancy to this report and the phenomenology under investigation.
Despite the above heavy performance caveats, the HUXWRX FLOW 556 Ti does exhibit extremely notable performance on this weapon system, and in the free field, represents one of the highest possible potentials for personnel hazard risk reduction on an untuned standard short barrel 5.56x45mm AR15 weapon system. The efficacy of the Flow-Through technology, as tested and analyzed progressively by the PEW Science laboratory, has increased with each design iteration. PEW Science postulates that the performance of the technology is reaching plateaus, based upon the size of the system required to maximize heat transfer efficiency. This is further discussed below.
PEW Science Research Note 5: The FLOW 556 Ti, like its sister 30 caliber model the FLOW 762 Ti, is constructed of Titanium. Titanium is a structurally resilient material, even at high temperature. However, some of its material properties become somewhat compromised under combined high temperature, pressure, and environments subject to aggressive gas jetting (such as the environment immediately outside a firearm muzzle):
Sparking, a visual signature component resulting from combustion of eroding metallic particulates from gas-impinged surfaces, is one often undesirable characteristic cited as an argument against the use of titanium in rifle silencers. This is particularly the case on systems which the temperature and pressure is most elevated (short barrel centerfire rifle cartridge systems). Nonetheless, the sparking phenomenon from some designs is greatly minimized. With high gross flow rate, elimination of sparking is more challenging. To further eliminate this visual signature contributor, material change may be warranted. Inconel (nickel alloy) or steel alloys are often used, but there is a weight penalty. It is likely that if a FLOW 556 Ti silencer was produced in an Inconel or steel version, the weight could approach double of the current model (approximately 20 ounces with no mount; approximately 24 ounces total). Such an increase is nontrivial for the same level of performance.
Structurally, the FLOW series has a benefit of reducing the total blast load duration to which the silencer internals are subjected. By venting expediently, blast load impulse accumulation inside the silencer is reduced, thus lowering structural demand. Even through significant system heating of the titanium body, it is anticipated that the FLOW 556 Ti will maintain overall integrity (localized erosion phenomena described in (1) notwithstanding). It may be possible that some components of the silencer could be changed by using a material with more robust high temperature strength. However, weight reduction through such an effort may not significantly reduce the postulated maximum 24 ounce mass above.
Without further increasing length and/or diameter, it is likely that the current performance-for-flow rate of the technology in the FLOW 556 Ti may plateau. It is important to note than increasing these gross parameters will most likely also increase the Omega metric. Whether or not that performance comprise is acceptable remains to be seen. It is postulated that this technology, like the hybrid PIP technology, may need to advance further for additional performance scaling. Furthermore, these postulated technological advancements are not limited to Flow-Through and PIP; as silencer technology continues to advance and DMLS (3D-printing) manufacturing technology continues to be more commonplace, blast load and impulse accumulation in early-time may advance further industry-wide. The balance of particle velocity, field shape, and gas momentum control remains one of the most significant challenges for advanced high flow rate technologies.
PEW Science Research Note 6: When comparing gross bystander suppression performance of the HUXWRX FLOW 556 Ti to that of other high performance silencers on this untuned MK18 weapon system in the free field, it is eclipsing the performance of the Maxim Defense DSX (6.110), Otter Creek Labs Polonium (6.75), Aero Precision Lahar-30L (6.119), and the Off Grid Operator Ti (6.166). This is a significant feat. The gross hazard reduction to the weapon operator, in the free field, of the FLOW 556 Ti is extremely high. On the untuned MK18 weapon system, it achieves an operator (shooter’s ear) Suppression Rating of 32.4. This is higher than the aforementioned FLOW 762 TI, PTR VENT 3, SilencerCo Velos LBP (6.134), and also higher than the the CAT/ODB/A1/718 (6.120). It is nominally higher than that of the aforementioned FLOW 556k. For the physical reasons discussed in Research Note 4, it is extremely important for the reader to understand the Suppression Ratings are computed with free field test data, and are therefore universal when translating the weapon and operator to different environments without reflections. However, when the environment becomes complex with many reflecting surfaces, performance translation of different systems is not yet possible; sound field shapes and particle velocities result in varied blast wave time-of-arrivals, as well as changes of blast load angles of incidence; these variations change amplitude and phase waveform components. As a result of these inevitable environmental complexities, user impressions will vary, as both the shooter and bystander. Nonetheless, hazard reduction efficacy in the free field, as characterized by the Suppression Rating, holds. The Suppression Rating comparisons are the current state-of-the-art, and the most “apples to apples” sound suppression performance comparisons that currently exist. Further research is ongoing.
PEW Science Research Note 7: The relative FRP severity from the FLOW 556 Ti on this weapon system is significantly improved over that of both the FLOW 762 Ti and the FLOW 556K. Member Research Supplement 6.169 further details this performance improvement.
PEW Science Research Note 8: 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 93 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 9: The closing time of the AR15 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 and the alpha parameter. 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.
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 10: Note that the muzzle Suppression Rating of the FLOW 556 Ti is 40.4 and the at-ear Suppression Rating is 32.4; different zones on the Suppression Rating Dose Chart. The lower back pressure of the FLOW 556 Ti contributes to a less severe ejection port blast signature. The back pressure is low enough to reduce the ejection port blast loads to the shooter more significantly than with many other silencers on the standard MK18 weapon system. Weapon tuning may influence hazard reduction efficacy, and is outside the scope of this study.
The signatures measured at the shooter’s ear are presented and analyzed below.
6.167.1.2 SOUND SIGNATURES AT SHOOTER’S EAR
Real sound pressure histories from the same 6-shot test acquired with PEW-SOFT at the shooter’s ear are shown below. Again, 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 primary sound signature pressure histories at the ear for all 6 shots are shown in Figure 3. The primary sound signature history is shown in Figure 3a. An annotated timescale is displayed in Figure 3b, for Shots 1 and 2. The real sound impulse (momentum transfer potential) histories at the ear from the same 6-shot test are shown in Figure 4. Again, full and shorter timescales are shown, this time of Shot 1, Shot 3, and Shot 5.
Figure 3a. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature
Figure 3b. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, Short Time Window
Figure 4a. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature
Figure 4b. HUXWRX FLOW 556 Ti 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature Peaks
As expected, the blast pressure and impulse measured at the shooter’s head indicate the FLOW 556 Ti is achieving a very high level of operator protection. The blowdown speed is significant and clear, definitive ejection port blast reduction indicators are present. As previously discussed in article 6.166, in systems in which ejection port blast is greatly diminished, the FRP generation at the shooter’s head, in impulse space, will present as “front-loaded.” This is evident in Figure 4b.
PEW Science Research Note 11: The amplitudes, durations, consistency, and balanced nature of the FLOW 556 Ti shooter’s ear signatures represent the current pinnacle of free field MK18 operator blast hazard reduction. Another notable attribute is how the heat-affected late-string blast momentum increase noted at the muzzle (ref. Fig 2b) is significantly less pronounced at the operator’s location (Fig. 4b). This differential is most likely attributable to both pressure field shape as well as the inevitable contribution of ejection port blast hazard. It is highly unlikely that a silencer can reduce ejection port blast hazard below that of an unsuppressed system. Therefore, there remains some level of ejection port blast hazard with an AR15 weapon system to which the operator will be subjected, regardless of silencer choice. However, caveats do exist:
PEW Science Research Note 12: It is likely that users may field the FLOW 556 Ti silencer on AR15 weapon systems that differ from the standard untuned MK18. For example, users may install a heavier recoil buffer, spring, or carrier assembly in their system(s). Users may also field different gas port orifice sizes, through the use of an adjustable gas block, different barrel gas port, or other means. Finally, users may change barrel length and gas system length, changing the duration of gas system impulse accumulation (dwell time) from the standard MK18 test case. All and any of these changes can influence the operator hazard reduction computed in this study. More succinctly, the operator (shooter’s ear) Suppression Rating of the system can and will change. This affects both the performance of the system equipped with the FLOW 556 Ti in absolute terms, as well as the relative comparison of systems equipped with other silencers. For example:
When a system is “untuned” the Suppression Rating comparisons hold. If a high backpressure silencer (e.g. Otter Creek Labs Polonium or YHM Turbo) is used, the ejection port blast hazard to the operator will be severe.
If the weapon system is “tuned” in order to reduce the propensity for high(er) amplitude ejection port blast hazard propagation by, for example, reducing the gas port orifice size, the shooter’s ear Suppression Rating of the tuned system with a high backpressure silencer will increase. This Suppression Rating increase (hazard reduction decrease), in conjunction with a relatively high muzzle (bystander) Suppression Rating from the high backpressure system, may increase overall suppression performance of the subject system to a greater relative degree than the same rifle tuned and used with the FLOW 556 Ti. If the rifle can achieve reliable function with a gas port orifice size reduction, unsuppressed, there is potential for a higher backpressure silencer to reduce ejection port blast hazard further than a lower backpressure silencer like the FLOW 556 Ti. An excellent case study illustrating this phenomenon is with the Maxim DSX SURG system test (6.111). Its shooter’s ear Suppression Rating is 32.5 on a tuned system; nominally the same as the FLOW 556 Ti shooter’s ear Suppression Rating on an untuned system. Would the FLOW 556 Ti achieve a higher shooter’s ear Suppression Rating with the SURG system? This is unknown, particularly because the reduced gas port orifice size of 0.053” vs. the standard MK18 0.070” may or may not produce reliable function without the added backpressure of the DSX silencer.
Operator hazard reduction is a function of all system parameters. It is also important to note that the above discusses only blast overpressure hazard reduction. Gas toxicity hazards, even if reduced by weapon tuning, will most likely not be reduced to the lower levels of FLOW 556 Ti use with a Maxim Defense DSX silencer, for example. This is particularly the case with rapid fire weapon operation, etc, in which the environment for expanding combustion gasses in the silencer becomes super-heated and downstream expansion is limited. This is sometimes colloquially referred to as the “gas stack” phenomenon.
PEW Science Research Note 13: The AR-15 weapon system, in general, is extremely hazardous the weapon operator, even when suppressed. Ejection port blast loads are severe. Tuning of the system by increasing buffer mass, decreasing gas port orifice area, or both, may reduce the amplitude and duration of ejection port blast loads from the weapon chamber. Reduction in severity from such weapon tuning is highly nonlinear and will vary based on silencer and weapon.
6.167.3 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the HUXWRX FLOW 556 Ti with that of several other silencers on the MK18 weapon system. 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 556 Ti and other rifle silencers, Using PEW-SOFT 5.56x45mm Supersonic Test Data and PEW Science Analysis
Figure 5 presents an overall summary of the postulated hazards to the operator and bystanders when fielding a variety of different silencers on the standard MK18 weapon system. The HUXWRX FLOW 556 Ti from this test program is shown in red. Hazards are expressed with the Suppression Rating; a holistic parameter that captures human inner ear damage risk potential from a measured impulsive complex overpressure signature during the entire time regime of weapon operation, including combustion, complete blowdown, and all mechanical operation, including the carrier group returning to battery, in the true free field. The parameter may be used with the dose chart at the beginning of this report. The PEW Science Suppression Rating is a damage risk criterion (DRC), a lower Suppression Rating indicates a higher personnel hazard in the free field - it is not a subjective quantity; it is an objective quantification of hearing damage risk potential.
The following gross conclusions can be made from the above:
The HUXWRX FLOW 556 Ti produces free field bystander hazard reduction on par with that of the PTR VENT 3 (6.135), which uses a completely different technology, as outlined in Research Note 1.
The free field operator (shooter) hazard reduction of the FLOW 556 Ti with the standard untuned MK18 weapon system is on par with that of the HUXWRX FLOW 556K (6.83), HUXWRX FLOW 762 Ti (6.114), and CAT/ODB/A1/718 (6.120). It is important to note that the mechanisms by which the operator hazard reduction on the standard untuned MK18 is achieved with the Flow-Through technology in the HUXWRX silencers and the SURGE BYPASS hybrid technology in the CAT ODB is different. The waveform parameters vary significantly as a result, and the Suppression Rating allows one to distill the overall severity into a single metric for free field hazard prediction. As discussed in Research Note 2 and published with in-depth analysis in Member Research Supplement 6.169, specific signature attributes and environmental factors can and will influence user perception and experience.
The HUXWRX FLOW 556 Ti system weighs less than 14 ounces total, as tested. The titanium versions of the CAT WB (6.129) and the aforementioned ODB are lighter, and the weight is also comparable with that of the PTR VENT 3. Caveats of weight reduction by using Titanium as a construction material are discussed in Research Note 5. Nonetheless, the FLOW 556 Ti objectively provides the highest holistic free field sound suppression performance of any silencer tested, and does so with significantly lower system weights than many other models.
Despite the shooter’s ear Suppression Rating of the HUXWRX FLOW 556k being only nominally lower than that of the full-size FLOW 556 Ti, the overall performance of the full-size silencer is significantly higher. This is discussed in Research Note 3.
The FLOW 556 Ti achieves shooter’s ear hazard reduction on the standard untuned MK18 weapon system that are typical of silencers on tuned AR15 systems (ref. Maxim Defense DSX SURGE testing in 6.111). This performance is notable, and also subject to caveats if other tuned system performance is to be compared, as discussed in Research Note 12 (member article only).
The efficacy of the Flow-Through technology in the HUXWRX FLOW 556 Ti is extremely high and appears to be the most efficient demonstrated by HUXWRX, to date. The performance for the length and weigh envelope is significant. Its larger diameter is critical for flow path length, and on most systems, does not interfere with operability.
PEW Science Research Note 14: Pure Flow-Through technology from HUXWRX does demonstrate some benefits over that of hybrid designs. One benefit is superior ejection port blast hazard reduction without the need for weapon tuning. And, with the introduction of the full-size 5.56-dedicated FLOW 556 Ti, the technology is shown to perform well for bystanders in the free field. Subject to aforementioned caveats, this performance is still extremely notable when compared to the performance of hybrid technologies implemented in designs like the Off Grid Operator Ti (6.166), Surefire SOCOM556-RC3 (6.151), SilencerCo Velos (6.134), CAT WB (6.129), FOR Systems Monarch 7.62 (6.155), and others. However, just as is the case with those technologies, Flow-Through technology can still not completely alleviate increased hazards on weapon systems in which the gas system delivers increased blast impulse in the unsuppressed state. A case study highlighting this issue is presented in the next publication in which the HUXWRX FLOW 556 Ti was evaluated on the standard M4A1 weapon system. The increased dwell time and gas port orifice size of the mid-length gas Daniel Defense 14.5-in barrel system create a more hazardous ejection port blast environment for the weapon operator.
As detailed in this report, signature at the operator’s head is a function of both muzzle and ejection port signatures from the AR-15 weapon system. Specific weapon system parameters will dictate modification efficacy.
Small arm weapon system suppression performance is a spectrum. The PEW Science Suppression Rating and the Silencer Sound Standard help quantify this spectrum for end users and industry, objectively.
6.167.3 Review Summary: HUXWRX FLOW 556 Ti 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 556 Ti achieved a composite Suppression Rating™ of 41.3 in PEW Science testing.
When paired with the 14.5-in barrel M4A1 and fired with Federal XM193, the HUXWRX FLOW 556 Ti achieved a composite Suppression Rating™ of 41.1 in PEW Science testing.
As with all weapon systems, the user is encouraged to examine both muzzle and ear Suppression Ratings.
PEW Science Laboratory Staff Opinion:
The HUXWRX FLOW 556 Ti is a lightweight full size 5.56 mm rifle silencer with significant back pressure reduction, without compromise of gross sound suppression performance. Optimized for the 5.56 mm cartridge, the FLOW 556 Ti improves upon the performance of the FLOW 762 Ti on AR15 weapon systems in both bystander and operator hazard reduction. At less than 14 ounces of system weight, its performance and maneuverability are significantly notable. Acute blast overpressure hazards from the ejection port of standard AR15 systems are significantly reduced through the use of this silencer. The silencer is constructed of 3D-printed Grade 5 Titanium. The Flow-Through technology implemented in the FLOW series from HUXWRX continues to represent one of the most advanced systems evaluated by PEW Science, to date.
Like the previous generation HX-QD 762 series, silencers in the HUXWRX FLOW series contain 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 FLOW 556 Ti removes the over-bore compromise from the FLOW 762 Ti. Externally, the design envelope of the two silencers is identical, with the FLOW 556 Ti weighing one ounce less than the 762 model. Internally, the design has been optimized for the 5.56x45mm cartridge. Therefore, the FLOW 556 Ti may be considered the full-size version of the FLOW 556k, but constructed from titanium instead of Inconel alloy. All three silencers use the same mounting system and all testing was performed with the same flash hider mount.
On an untuned standard MK18 weapon system, the FLOW 556 Ti operator hazard reduction, in the free field, is unmatched. Furthermore, due to its optimization for the 5.56x45mm cartridge, its bystander hazard reduction also has an extremely high level of efficacy. However, when moving to environments other than the free field, including those in which berms, tree lines, vehicles, and structures are present, silencers with HUXWRX Flow-Through technology may produce more severe user perception. A function of both wave particle velocity and field shape, this phenomenon is a pervasive performance attribute. Nonetheless, gas momentum rate throttling with silencers such as the FLOW 556 Ti and FLOW 762 Ti, when compared to shorter implementations in the FLOW 556K, does offer somewhat of a severity reduction. Low frequency inner ear response bias does remain, as reported and analyzed in detail in Member Research Supplement 6.169.
Free field performance balance of the FLOW 556 Ti, from a pure hazard reduction standpoint, is the highest measured by PEW Science, to date.
One consequence of high mass flow rate through a silencer is a potential increase in flash signature. However, the FLOW series silencers possesses flash-hiding features on their endcaps 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 yet evaluated the flash hiding performance of the FLOW 556 Ti. Qualitative measurements of flash using the FLOW 556K on short barrel 5.56x45mm weapons have indicated adequate flash suppression for lab personnel. Qualitative measurements of flash using the FLOW 762 Ti on short barrel 7.62x51mm weapons have also indicated adequate flash suppression for lab personnel, albeit with some sparking.
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. 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. The distal vents of the silencer produce thrust which actively tighten the silencer to the mount, during use. This feature is unique to HUXWRX rifle silencers; PEW Science confirms that this feature demonstrates efficacy in the field.
PEW Science has not evaluated the durability of the FLOW 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 556 Ti 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. HUXWRX reports this postulation to be correct, in accordance with their internal testing.
In this review, the HUXWRX FLOW 556 Ti 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.
