SSS.6.60 - KGM R556 and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle
/KGM R556 on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The R556 is manufactured by KGM Technologies. It is a 223 caliber centerfire rifle silencer, intended to suppress the 5.56x45mm NATO cartridge from barrels of any length. It has a 1.5-inch diameter and is 6.625 inches in length with the mount installed. The user may also choose from a proprietary single-port muzzle brake or 3-prong flash hider, both of which secure to the silencer with a retention latch and a coarse thread aft of a sealing taper. The outer tube and mount are stainless steel and the internal baffle assembly is constructed of Inconel steel alloy. The welded endcap of the R556 possesses 10 circumferential radial vent orifices. This vent array comprises a non-adjustable version of the so-called KGM Adjustable Port End Cap (APEC). An adjustable version of this array is present in the sister silencer; the 30 caliber KGM R30. The silencer weighs 18.6 ounces with the 3-prong flash hider. The R556 can be obtained through your local dealer or distributor by contacting KGM Technologies.
Test Specimen Notes: The R556 evaluated in this Sound Signature Review was equipped with a tapered 3-prong flash hider mount without retention latch. Production models of the R556 include a serviceable latch for secondary retention. Gas dynamics and sound signature are postulated to be similar between the two configurations.
Additionally, the test specimen was subjected to significant firing schedules on short barrel machine guns by KGM prior to PEW Science testing, and possessed visible erosion on the leading edges of the silencer baffle mouths. The internal wear is considered to be normal by KGM and to be within serviceability tolerance. Baffle wear influence on the Suppression Rating of the R556 on the MK18 has not been evaluated, but is postulated to not be significant.
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-KGM-015-001-21. Therefore, data pertaining to the R556 in this Sound Signature Review is published with the express written permission of KGM Technologies, LLC.
This Sound Signature Review contains single-test results using the KGM R556 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 6.60.1 contains contains the R556 test results and analysis.
Section 6.60.2 contains Suppression Rating comparisons of the R556 with the Rugged Razor, and also with dedicated 5.56 mm bore rifle silencers, including the OSS HX-QD 556, SilencerCo Saker 556, and Surefire SOCOM556-RC2 on the MK18.
Section 6.60.3 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 KGM R556 mounted with the 3-Prong flash hider mount achieved a composite Suppression Rating™ of 28.6 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
6.60.1 KGM R556 Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the R556 tested with the 3-prong flash hider 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. KGM R556 Sound Metric Summary
6.60.1.1 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 R556 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 Shot 2 and Shot 3.
Fig 1a. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature, 28-AUG-2021
Fig 1b. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature, 28-AUG-2021
Figure 2a. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature, 28-AUG-2021
Figure 2b. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature, 28-AUG-2021
The R556 is a 223 caliber rifle silencer with a non-adjustable circumferential radial vent array. Therefore, the flow rate is higher than that from a typical 223 rifle silencer when firing the 5.56x45 NATO cartridge. The venting of combustion gasses out of the ten circumferential orifices in the array results in a relatively high mass flow rate (low back pressure; low PEW Science Omega Metric). The PEW Science Omega Metric for the 5.56x45mm cartridge is the subject of future publication. Venting of combustion gasses from the radial orifice array also produces jetting thrust, potentially influencing recoil kinematics to a degree that may be perceptible to the weapon operator. For reference, similar radial vent arrays are present on some silencer accessories such as the Dead Air E-Brake, and integrated into some rifle silencers, such as those manufactured by Silencer Tech, LLC and Mad Minute, LLC. Kinematic influence of silencer orifice vent arrays has not been evaluated by PEW Science.
The R556 possesses coaxial elements in its baffle stack, as well as a vented blast chamber. The design shares many similarities and features with Surefire, original SWR Omega, CGS Hyperion, and Dead Air Nomad technologies. The use of such gas paths combined with a circumferential distal axial vent array is becoming increasingly common in the U.S. market, though the combination of a circumferential radial array is less common.
The high flow rate of the R556 (a 223 caliber silencer with five baffles and a vent array) on the 5.56x45mm platform is highlighted by some similarities between the waveforms measured with the R556 and those measured with the OSS HX-QD 556 in Review 6.54 (a silencer with a circumferential axial vent array). The flow rate of the R556 is not as high as that of the HX-QD 556, but it does exhibit significant initial muzzle blast and bullet exit waveform coupling (Figure 1b), a relatively fast rate of rise to maximum peak positive phase impulse (Figure 2a), and a significant early-time positive pressure phase prior to rarefaction (Figure 1b).
PEW Science Research Note 1: Although the R556 exhibits some similarities in signature to the HX-QD 556 due to high flow rate, the mechanism by which the R556 achieves its high flow rate is different; the coaxial baffle chambers, along with a radial vent array, result in different gas dynamics than the more complex internal geometry of the OSS silencer and its axial vent array. Like many rifle silencers, the KGM R556 relies on the trapping of gas to suppress sound signature. The difference in measured impulse behavior is highlighted by the length of the positive phase impulse duration from the R556 (Figure 2a). Note that negative impulse onset of the R556 is delayed longer than that from the HX-QD 556, which possesses a positive phase duration of 3.5 ms (ref. SSS6.54.1.1, Fig 2a). Despite the fast rise time, the R556 is postulated to require increased venting time relative to the OSS design. Positive phase impulse duration is the subject of ongoing PEW Science internal research.
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. As is the case with the OSS HX-QD 556, the gas jetting from the KGM R556 is significantly faster than observed from the SOCOM556-RC2 and the Saker 556. Distinctive waveform features consistent with low back pressure silencer behavior are discussed above. The KGM R556 has somewhat reduced sound signature suppression performance compared to these dedicated 5.56mm silencers, measured 1.0 m left of the weapon muzzle. However, the performance deficit measured at the shooter’s-ear on the MK18 host weapon platform is reversed. The R556 possesses a less severe signature at the shooter’s ear on the MK18 than that of the SOCOM556-RC2 or Saker 556. This is an example of flow restriction (back pressure) influence on system performance. Unlike the Rugged Razor in Review 6.58, the KGM R556 possesses enough muzzle signature suppression, even with its vent array, that the at-ear signature is not overwhelmed. This is despite having lower flow restriction (back pressure) than the 30 caliber Rugged Razor.
The KGM R556 has significantly lower 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 KGM R556 is not significant enough to be noticeable to bystanders, in accordance with PEW Science inner ear response analysis.
PEW Science Research Note 2: 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 80 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 3: 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.
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 KGM R556 is 26.7 and the at-ear Suppression Rating is 22.9; the same zone on the Suppression Rating Dose Chart. The KGM R556 produces a significant overall signature on this host weapon system, but its high flow rate does assist with reducing ejection port signature, thus helping to reduce combined signature at the shooter’s ear. Comparisons with other silencers on the MK18 weapon system are provided below.
6.60.1.2 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. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, 28-AUG-2021
Figure 3b. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, Short Time Window, 28-AUG-2021
Figure 4a. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature, 28-AUG-2021
Figure 4b. KGM R556 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature Peaks, 28-AUG-2021
Unlike in the measurements at the muzzle, the FRP from the R556 at the shooter’s ear is significant enough to be noticeable to the shooter; albeit not severely. 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). Though the shape of the impulse waveforms measured at the shooter’s ear share some similarities with those measured with the OSS HX-QD 556, there are positive phase duration features that indicate a slightly lower flow rate than the OSS. PEW Science postulates that this occurs due to traditional coaxial baffle component geometries trapping more gas than the flow channels in the OSS design. Despite the vent array, the KGM R556 is not able to achieve the same flow rate. The flow rate is, however, faster than that of the 30 caliber Rugged Razor, but the at-ear signature is not as severe with the R556, despite the Razor having a lower average peak impulse. Duration components of the impulse signature significantly influence inner ear response; the decay from peak is more significant with the R556 than the Razor and much more consistent at the ear (ref. SSS6.58.1.2, Fig 4a).
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 review of the OSS HX-QD 762 with 7.62x51mm ammunition from a 20-in bolt action rifle (6.41) and in the review of the OSS HX-QD 556 in (6.54). In this current article examining the R556, there is a somewhat less pronounced decoupling of the ground reflection event from primary muzzle blast. This is due to the slightly higher back pressure of the KGM R556 when compared to that of the OSS HX-QD 556 on the MK18.
Again, the rate of rise to maximum peak positive phase impulse (Figure 4) is consistent with low back pressure (low PEW Science Omega Metric) for the platform. Inner ear response consequences of such low back pressure on the MK18 weapon system are examined in the section below. As discussed in the preceding section, the KGM R556 is a high flow rate 5.56x45mm machine gun silencer. Although its flow rate is measurably lower than that of the OSS HX-QD 556, it’s sound suppression performance is not significantly lower. This is highlighted by the somewhat lower at-ear Suppression Rating with the R556 than with the HX-QD 556; a consequence of the different mechanism used to increase flow rate (reduce back pressure) with the two silencers.
6.60.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the KGM R556 to that of the Rugged Razor, OSS HX-QD 556, 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 KGM R556 with the Rugged Razor, OSS HX-QD 556, Saker 556, and SOCOM556-RC2 , Using PEW-SOFT 5.56x45mm Supersonic Test Data
From the above data, it can be concluded that the KGM R556, with its 223 caliber bore and circumferential vent array, is a relatively efficient silencer for the MK18 host weapon platform. The at-ear Suppression Rating of the R556 is slightly higher than that from the Surefire SOCOM556-RC2 (6.52) and significantly higher than that from the SilencerCo Saker 556 (6.53), and the Rugged Razor (6.58). The R556 is quieter at the shooter’s ear than the Surefire and SilencerCo silencers due to flow rate, but quieter at the ear than the Rugged silencer due to both flow rate and muzzle suppression; this highlights the significant benefit of using a purpose-built silencer for a given cartridge; over-bored silencers for 5.56 may exhibit inherent suppression performance disadvantage. This is highlighted by the Razor’s severe muzzle signature that coalesces with the ejection port signature on this host weapon platform.
Despite the low PEW Science Back Pressure Metric, Omega of the KGM R556 with the 5.56x45mm cartridge, its at-ear Suppression Rating does not reach that of the OSS HX-QD 556 on the same platform. The performance delta between the two silencers at the muzzle is less significant. The two silencers achieve sound suppression and reduce flow restriction using differing mechanisms; again, the mechanism of the OSS is measured to be more efficient on this platform. In addition to the flow rate differences between the two silencers, the differences in the direction of venting (axially in the OSS versus radially in the KGM) is postulated to influence both the muzzle and ear signatures.
Bystanders may perceive the loudness of the KGM R556 to be on par with that of the OSS HX-QD 556 when fired on the MK18 host weapon. Personnel firing the weapon may experience hearing damage risk on par with that from using the Surefire SOCOM556-RC2.
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 KGM R556 does influence the at-ear signature of the shooter on this platform. This performance factor is significant on a short barrel automatic weapon system (the MK18).
6.60.3 Review Summary: KGM R556 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 KGM R556 mounted with the 3-prong flash hider mount achieved a composite Suppression Rating™ of 28.6 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 KGM R556 is a full-size 5.56 mm machine gun rated rifle silencer that possesses competitive sound signature suppression performance for a silencer with reduced back pressure. Users should note that full-size 5.56mm rifle silencers are often smaller than their full-size 7.62mm (.30) counterparts.
The use of the KGM taper retention mount is relatively straight forward; PEW Science has not evaluated the production version of the mount possessing a ratcheting tooth retention system. The suppression performance of the R556 is relatively competitive for a lower back pressure dedicated bore design on a short barrel 5.56x45mm weapon system. The size envelope and back pressure reduction of the silencer may be attractive to some users. For example, the R556 is identical in weight and almost in size to the 30 caliber Rugged Razor, yet exhibits more efficient performance on the MK18 host weapon platform; albeit without the cartridge versatility.
The practice of using of distal vent arrays to allow higher flow rate through silencers is relatively common, as referenced in the body of this review. The placement of the vents to produce radial thrust may influence weapon system kinematics. As intended, these systems may influence recoil dynamics, specifically. The kinematic influence of such systems has not been evaluated by PEW Science. The combination of radial external venting with internal coaxial elements is relatively uncommon. PEW Science postulates that the coaxial elements contribute significantly to the sound suppression performance of the R556.
The R556 is relatively durable. Although the test sample received by PEW Science did possess some erosion on the mouths of the baffles, KGM had subjected the silencer to extremely significant firing schedules prior to delivery to PEW Science. PEW Science postulates that many end users may not subject the silencer to such abuse. Nonetheless, the user is encouraged to contact the manufacturer to determine suitability for their application.
In this review, the KGM R556 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.
