SSS.6.54 - OSS HX-QD 556 and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle
/OSS HX-QD 556 on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The HX-QD 556 is designed and manufactured by 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 1.58-inch diameter and is 6.75 inches in length (from the tip of the flash-hider features on the endcap to the rear of the silencer). The silencer mounts to proprietary OSS LH-threaded taper-mount muzzle devices. The outer tube of the silencer is constructed of heat-treated 17-4 stainless steel, with the internal components being constructed of both 17-4 stainless steel and Grade 5 titanium. The silencer weighs 21.0 ounces with the Flash Hider-QD 556 taper mount, as tested. The HX-QD 556 can be obtained from Silencer Shop.
The HX-QD 556 Ti is also available, which reduces the system weight by 4.4 ounces while exhibiting similar sound suppression performance.
This Sound Signature Review contains single-test results using HX-QD 556 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.
Section 6.54.1 contains contains the HX-QD 556 test results and analysis.
Section 6.54.2 contains Suppression Rating comparisons of the HX-QD 556 with the SilencerCo Saker 556 and Surefire SOCOM556-RC2 rifle silencers on the MK18.
Section 6.54.3 contains the review summary and subjective PEW Science opinions.
Summary: When paired with the 10.3-in barrel MK18 and fired with Federal XM193, the OSS HX-QD 556 mounted with the Flash Hider-QD 556 mount achieved a composite Suppression Rating™ of 31.0 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
6.54.1 HX-QD 556 Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the HX-QD 556 tested with the direct-thread 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. 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. OSS HX-QD 556 Sound Metric Summary
6.54.1.1 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 HX-QD 556 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 4a. In Figure 4b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shot 2.
Fig 1a. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature, 28-AUG-2021
Fig 1b. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature, 28-AUG-2021
Figure 2a. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature, 28-AUG-2021
Figure 2b. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature, 28-AUG-2021
Significant differences in the the pressure and impulse waveforms shown in Figure 1 and Figure 2, respectively, are noted when compared to those from the Surefire SOCOM556-RC2 in Review 6.52 and the SilencerCo Saker 556 in Review 6.53. The gas jetting from the OSS HX-QD 556 is significantly faster than observed from the SOCOM556-RC2 and the Saker 556. Distinctive waveform features consistent with low back pressure silencer behavior are highlighted in Figure 1b, to include a coupled bullet exit event with endcap jetting, followed by a long duration initial positive pressure phase. This behavior is extremely similar to that exhibited by the OSS HX-QD 762 with 7.62x51mm ammunition from a 20-in bolt action rifle (Review 6.41).
In addition to low back pressure features in pressure-space, the HX-QD 556 also exhibits distinctive waveform features in impulse-space (Figure 2). Note that the rate of rise to maximum positive phase impulse is relatively fast. This behavior is consistent with a low PEW Science Omega Metric (low flow restriction; low back pressure). The OSS HX-QD 556 has significantly lower flow restriction (back pressure) than the Surefire SOCOM556-RC2 and the SilencerCo Saker 556. Omega metric data for the 5.56x45mm cartridge is the subject of ongoing PEW Science research. The measured first-round-pop (FRP) is visible in both the pressure and impulse regimes, in peak amplitude, timing, and wave shape, as is typical from a suppressed rifle. It should be noted that the FRP measured at the muzzle from the HX-QD 556 is significant enough that it is noticeable to bystanders, in accordance with PEW Science inner ear response analysis.
PEW Science Research Note 1: 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 91 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 2: 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.
PEW Science Research Note 3: The reader should note that even despite the aforementioned uncertainty of the bolt closing time indicator, the bolt closing time with the HX-QD 556 occurs significantly late in time when compared to that with the SOCOM556-RC2 and Saker 556. This is a consequence of the extremely significant back pressure reduction when using the HX-QD 556.
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.
Note that the muzzle Suppression Rating of the HX-QD 556 is 27.5 and the at-ear Suppression Rating is also 27.5; the same zone on the Suppression Rating Dose Chart. This behavior is highly atypical with the MK18 weapon system.
6.54.1.2 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 on the left. A zoomed-in timescale is displayed on the right, in a sound pressure region of interest for Shot 1 and Shot 2. The real sound impulse (momentum transfer potential) histories at the ear from the same 6-shot test are shown in Figure 4. Full and short timescales are shown.
Figure 3a. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, 28-AUG-2021
Figure 3b. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, Short Time Window, 28-AUG-2021
Figure 4a. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature, 28-AUG-2021
Figure 4b. OSS HX-QD 556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature Peaks, 28-AUG-2021
Similar to the measurements at the muzzle, the FRP from the HX-QD 556 at the shooter’s ear is significant enough to be noticeable to the shooter. Also similar to the measurements at the muzzle, there are waveform indicators measured at the ear consistent with low back pressure (low flow restriction; low PEW Science Omega Metric).
As observed in the gross muzzle pressure history (Figure 1), the so-called blowdown time measured at the ear is significantly shorter than observed with the Surefire SOCOM556-RC2 and the SilencerCo Saker 556. Although the signatures measured at the shooter’s ear in the figures above are composed of coalescing waves from the silencer endcap, its vents, and the ejection port, a significant waveform feature is decoupled with the OSS HX-QD 556; the ground reflection from the primary muzzle blast. This pressure decoupling is highlighted in Figure 3b.
When silencers with extremely low back pressure (low flow restriction; low PEW Science Omega Metric) are fired on a centerfire rifle, the blowdown occurs so quickly that late-time waveform features become visible in pressure space. This was highlighted in the aforementioned review of the OSS HX-QD 762 with 7.62x51mm ammunition from a 20-in bolt action rifle (6.41) in Figure 3a of that article. In this current article examining the HX-QD 556, there is an even more pronounced decoupling of the ground reflection event from primary muzzle blast. The blowdown is so fast on the MK18 weapon system with this silencer that the pressure history decays to an unprecedented 114.5 dB prior to ground reflection. This is due to the faster blow-down time with 5.56x45mm ammunition out of a 10.3-in barrel than that with 7.62x51mm ammunition out of a 20-in barrel. The MK18 sends less total gas volume through the silencer.
Finally, again, the rate of rise to maximum peak positive phase impulse (Figure 4) is consistent with low back pressure (low PEW Science Omega Metric). The characteristics indicating that the OSS HX-QD 556 (and all OSS HX-QD silencers) exhibit extremely low back pressure are definitive in PEW Science testing and analysis, and consistent across tested weapon systems.
Inner ear response consequences of such low back pressure on the MK18 weapon system are examined in the section below.
6.54.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the OSS HX-QD 556 to that of the SilencerCo Saker 556 and the Surefire SOCOM556-RC2 with two different mounts 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 OSS HX-QD 556 with the Saker 556 and the SOCOM556-RC2 , Using PEW-SOFT 5.56x45mm Supersonic Test Data
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 OSS HX-QD 556 results in significant sound signature reduction at the shooter’s ear on the MK18 weapon system, when compared to that from the Surefire SOCOM556-RC2 and the SilencerCo Saker 556. On the MK18, the at-ear Suppression Rating with the HX-QD 556 is half a category higher than with the 3-prong-equipped SOCOM556-RC2. The MK18 with the HX-QD 556 exhibits an ear signature a full category less severe than with the WARCOMP-equipped SOCOM556-RC2 or with the Saker 556. This is a direct consequence of the low back pressure (low flow restriction) generated by the HX-QD 556.
A notable performance metric of the HX-QD 556 on this weapon system is that the muzzle Suppression Rating is, for all intents and purposes, identical to that of the at-ear Suppression Rating. Bystanders may perceive the OSS HX-QD 556 to be louder than the SOCOM556-RC2 or Saker 556. However, the personnel firing the weapon will experience lower hearing damage risk.
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. The muzzle signature of the OSS HX-QD 556 is quiet enough such that the at-ear signature is still quieter than those from the other silencers. This performance balance is notable on a short barrel automatic weapon system (the MK18).
6.54.3 Review Summary: HX-QD 556 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 OSS HX-QD 556 mounted with the Flash Hider-QD 556 mount achieved a composite Suppression Rating™ of 31.0 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 OSS HX-QD 556 is a full-size 5.56mm machine gun rated rifle silencer that exhibits class-leading back pressure reduction with a notable balance of sound signature suppression performance that is competitive with many silencers on the market. The HX-QD 556 is also offered in a titanium version which reduces the weight of the system by 4.4 ounces. Users should note that full-size 5.56mm rifle silencers are often smaller than their full-size 7.62mm (.30) counterparts.
The HX-QD 556 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 aforementioned method by which the HX-QD 556 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 HX-QD 556 is a 223 caliber rifle silencer, its use with subsonic cartridges is extremely uncommon. The OSS HX-QD series 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. The HX-QD 556 possesses flash-hiding features on its endcap. PEW Science has not evaluated the flash-hiding performance of this silencer.
The left-hand (LH) threaded taper mounts from OSS 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.
The silencer is also offered in a titanium version, as stated above. The Ti version may be attractive to some users due to the weight reduction. Durability of the steel silencer is expected to be higher than that of the Ti silencer. PEW Science has not evaluated the durability of the HX-QD 556 silencer system(s) on semiautomatic or automatic host weapons. Note that the nature of low flow restriction (low back pressure) silencers may influence durability. These phenomena are subjects of further research.
The HX-QD 556 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 HX-QD 556 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.
