SSS.6.52 - Surefire SOCOM556-RC2 and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle (Free Version)
/Surefire SOCOM556-RC2 on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The SOCOM556-RC2 is manufactured by Surefire. It is a 223 caliber centerfire rifle silencer, intended to suppress the 5.56x45mm NATO cartridge from barrels greater than or equal to 10 inches in length. It has a 1.5-inch diameter and is 6.4 inches long. The silencer mounts to the host firearm with a proprietary mounting system; the user may choose from various Surefire flash hider and muzzle brake mounts. The outer tube and end cap are constructed of heat treated stainless steel. The baffles are constructed of Inconel steel alloy. As tested, the silencer weighs 16.9 ounces and the 3-prong flash hider weighs 3.9 ounces, for a total system weight of 20.8 ounces. The SOCOM556-RC2 can be obtained from Silencer Shop.
This review contains results from two separate tests. PEW Science tested the SOCOM556-RC2 on the MK18 with the SOCOM 3-Prong flash hider mount and also with the WARCOMP closed-tine flash hider mount. The SOCOM 3-Prong flash hider mount is non-ported and contains so-called “labyrinth seals.” The presence of the seal rings, and more importantly the lack of ports, results in the 3-prong flash hider system exhibiting significantly different gas dynamics than does a ported WARCOMP-equipped SOCOM556-RC2 system. Similar phenomena were measured and analyzed in the PEW Science test of the SOCOM762-RC2 on a bolt-action rifle in Sound Signature Review 6.26.
Caution: PEW Science does not recommend the WARCOMP mount for prolonged suppressed use, for most operators. Personal protective equipment (PPE), including eye and ear protection, is recommended for the operation of suppressed small arms, in general.
Both sets of test results in this Sound Signature Review are of the SOCOM556-RC2 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 tests. The standard PEW Science MK18 test host weapon system is described in Public Research Supplement 6.51.
Section 6.52.1 contains test results using the SOCOM 3-Prong flash hider mount.
Section 6.52.2 contains test results using the WARCOMP closed-tine flash hider mount.
Section 6.52.3 contains an in-depth comparison of the sound signatures and gas dynamics of the two mount systems.
Section 6.52.4 contains Suppression Rating comparisons of the two systems.
Section 6.52.5 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 Surefire SOCOM556-RC2 mounted with the SOCOM 3-Prong flash hider achieved a composite Suppression Rating™ of 35.7 in PEW Science testing. When mounted with the WARCOMP flash hider on the same host weapon and fired with the same ammunition, the SOCOM556-RC2 achieved a composite Suppression Rating of 29.4. 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
6.52.1 SOCOM556-RC2 Sound Signature Test Results (SOCOM 3-Prong Flash Hider)
A summary of the principal Silencer Sound Standard performance metrics of the SOCOM556-RC2 tested with the 3-Prong Flash Hider 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. Surefire SOCOM556-RC2 (SOCOM 3-Prong Flash Hider) Sound Metric Summary
6.52.1.1 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 SOCOM556-RC2 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 4a. In Figure 4b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shot 2.
Fig 1a. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature, 28-AUG-2021
Fig 1b. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature, 28-AUG-2021
Figure 2a. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature, 28-AUG-2021
Figure 2b. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature, 28-AUG-2021
The pressure and impulse waveforms shown in Figure 1 and Figure 2, respectively, share many similarities with those fired from bolt-action weapons. The measured first-round-pop (FRP) is visible in both the pressure and impulse regimes, in peak amplitudes, timing, and wave shapes, as is typical from a suppressed rifle. It should be noted that the FRP measured at the muzzle from the SOCOM-556RC2 is not significant.
PEW Science Research Note 1: Other than the amplitude, the overall shape and trends of the impulse waveforms in Figure 2 are very similar to those measured from the Surefire SOCOM762-RC2 silencer in Review 6.26 fired on a bolt-action rifle. This is not always the case for silencer designs across different cartridges and barrel lengths. Variations across flow regimes, cartridge types, and barrel lengths, are subjects of ongoing PEW Science internal research.
PEW Science Research Note 2: While a second pressure pulse originates from the ejection-port signature of the weapon, 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 while the muzzle Suppression Rating with the 3-prong flash hider is 35.4, the at-ear Suppression Rating is in a lower zone on the Suppression Rating Dose Chart. Caution should be exercised by the weapon operator. More detailed information and analysis of the sound signatures measured at the shooter’s ear is available in the PEW Science member version of this review.
Data acquired with the WARCOMP-equipped weapon system follows.
6.52.2 SOCOM556-RC2 Sound Signature Test Results (WARCOMP Flash Hider)
A summary of the principal Silencer Sound Standard performance metrics of the SOCOM556-RC2 tested with the WARCOMP Flash
Hider is shown in Table 2. The data acquired 1.0 m (39.4 in) left of the muzzle is available for viewing to all. As stated in Section 6.52.1, 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 2. Surefire SOCOM556-RC2 (WARCOMP Flash Hider) Sound Metric Summary
6.52.2.1 SOUND SIGNATURES AT THE MUZZLE
Real sound pressure histories from the second 6-shot test with the SOCOM556-RC2 acquired with PEW-SOFT™ are shown below, this time with the closed-tine WARCOMP flash hider. 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 sixth shot signatures 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 SOCOM556-RC2 on the WARCOMP are shown in Figure 5a. The sound signatures of Shot 1 and Shot 2 are shown in a smaller time window in Figure 5b. The real sound impulse (momentum transfer potential) histories from the same 6-shot test are shown in Figure 6a. In Figure 6b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shot 2.
Fig 5a. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle WARCOMP Muzzle Sound Pressure Signature, 28-AUG-2021
Fig 5b. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle WARCOMP Muzzle Sound Impulse Signature,28-AUG-2021
Figure 6a. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle WARCOMP Muzzle Sound Impulse Signature, 28-AUG-2021
Figure 6b. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle WARCOMP Muzzle Sound Impulse Signature, 28-AUG-2021
As was shown in the test with the 3-Prong Flash Hider, the measured first-round-pop (FRP) with the WARCOMP mount is evident when comparing Shot 1 to Shot 2 (Figures 5b and 6b).
As was observed in bolt-action rifle testing with the Surefire SOCOM762-RC2 silencer, irregularities in the wave shapes occur with the WARCOMP, when compared to the test using the 3-Prong Flash Hider. Direct comparisons are provided in Section 6.52.3 of this review. Despite these irregularities, the overall late-time shapes of the impulse waveforms measured at the muzzle, from shot-to-shot, are consistent, highlighting the silencer’s overall sound performance consistency at the muzzle after the FRP, as well as the relative consistency of the tested firearm configuration. Note the highly irregular early-time gas jetting of the first shot in Figure 5b, beginning at a time of 29.9 ms. This is high pressure gas leakage from the silencer mount interface, originating from the WARCOMP mount ports, as measured 1.0 m left of the silencer end cap.
PEW Science Research Note 6: The muzzle sound signature of the SOCOM556-RC2 is more severe when using the WARCOMP mount; although the peak amplitudes are not significantly different, the wave shapes and their durations vary. There is a significant change in gas dynamics with the WARCOMP mount due to early-time gas venting from the mount interface. Again, Section 6.52.3 of this review contains direct comparison of the WARCOMP and 3-Prong Flash Hider.
The gas jetting out of the mount interface with the WARCOMP increases the severity of the sound signature to bystanders and lowers the muzzle Suppression Rating to 29.2. Note that the at-ear Suppression Rating of the SOCOM556-RC2 with the WARCOMP mount on the MK18 occupies an even lower zone on the Suppression Rating Dose Chart than it did with the 3-prong flash hider. Caution should be exercised by the weapon operator. This phenomenon is examined in further detail in the at-ear waveforms available in the PEW Science member version of this review. The at-ear waveform differences are extremely significant.
Comparisons of the 3-prong flash hider and WARCOMP equipped systems are provided below.
6.52.3 SOCOM 3-Prong Flash Hider and WARCOMP Flash Hider Mount Comparison
Below, direct comparisons of the measured pressure and impulse waveforms with the two different mounts used in the two tests of the SOCOM556-RC2 are shown. 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!
Figure 9a and Figure 9b show first-shot muzzle overpressure and impulse comparisons, respectively. These comparisons directly illustrate the changes in gas dynamics at the muzzle due to WARCOMP mount leakage at the silencer-mount interface.
Fig 9a. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signatures - Mount Comparison, 28-AUG-2021
Fig 9b. SOCOM556-RC2 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signatures - Mount Comparison, 28-AUG-2021
The 3-Prong Flash Hider is non-ported and equipped with “labyrinth seals;” these circumferential machined ridges on the mount help to prevent gasses from escaping the mount interface. PEW Science postulates that the porting is the most significant factor influencing the difference in muzzle blast pressure propagation from the mount interface.
The mount leakage phenomenon that occurs with the WARCOMP was observed in the previous PEW Science testing and analysis of the Surefire SOCOM762-RC2 on a bolt-action rifle. During the PEW Science testing of the SOCOM556-RC2 on the MK18, five primary differences are noted:
The silencer (SOCOM762-RC2 vs. SOCOM556-RC2)
The ammunition (7.62x51mm vs. 5.56x45mm)
The barrel length (20-in vs. 10.3-in)
The weapon operating mechanism (bolt-action rifle vs. automatic gas-operated rifle)
WARCOMP timing (the mount was simply installed on the bolt-action host weapon, whereas the mount was properly timed on the MK18)
Despite the above five differences, the testing and analysis results are consistent. The silencers possess similar locking collars, the ammunition propellant combustion occurs with sufficient pressure to induce blast waves when released to atmosphere suddenly, the secondary blast waves from unburnt propellant from the shorter barrel only contribute further to the issue, the leakage happens in early time so weapon type is inconsequential, and finally, blast waves from the mount interface expand spherically, in three dimensions. Timing of the mount, even if it did influence blast propagation amplitude, would only do so negligibly in the free field.
The WARCOMP performance differential on the MK18 is examined in further detail in the PEW Science member version of this review. The waveform differences at the shooter’s ear are extremely significant.
6.52.4 Suppression Rating Comparison (3-Prong Flash Hider vs. WARCOMP)
Figure 11 presents a comparison of the PEW Science Suppression Rating of the Surefire SOCOM556-RC2 on the MK18 automatic AR15 rifle with the 3-prong flash hider mount and the closed-tine WARCOMP mount.
Figure 11. Suppression Rating Comparisons with the Surefire SOCOM556-RC2 and different Mounts, Using PEW-SOFT 5.56x45mm Supersonic Test Data
From the above data, it can be concluded that the SOCOM556-RC2 suppressor has significantly higher sound suppression performance when used with the 3-prong flash hider mount, instead of the WARCOMP mount. The aforementioned gas leakage out of the mount interface from the WARCOMP ports is so severe that it not only lowers the muzzle Suppression Rating by half a category, but it drops the at-ear Suppression Rating severely. The extremely low at-ear Suppression Rating with the WARCOMP mount is a function of the relatively high blast pressure originating from the mount interface leak and its proximity to the shooter’s ear; the MK18 possesses a 10.3-in barrel length.
Note that the at-ear Suppression Rating is lower than the muzzle Suppression Rating with this particular silencer on the MK18 host weapon, regardless of mount.
6.52.5 Review Summary: SOCOM556-RC2 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 Surefire SOCOM556-RC2 mounted with the SOCOM 3-Prong flash hider achieved a composite Suppression Rating™ of 35.7 in PEW Science testing. When mounted with the WARCOMP flash hider on the same host weapon and fired with the same ammunition, the SOCOM556-RC2 achieved a composite Suppression Rating of 29.4. As with all weapon systems, the user is encouraged to examine both muzzle and ear Suppression Ratings.
PEW Science Subjective Opinion:
The Surefire SOCOM556-RC2 is a full-size 5.56mm machine gun rated rifle silencer that possesses competitive sound signature suppression performance with many silencers on the market, while possessing reduced back pressure compared to some designs. 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 Surefire locking collar is relatively straight forward. The pure sound suppression performance is also relatively high on a short barrel 5.56x45mm weapon system. The size envelope, durability, ease of mounting, and the suppression performance of the SOCOM556-RC2, may make it an attractive option for many users. While the back pressure reduction of the silencer is moderate, it is not a class-leader in this area. This performance factor may not be of practical concern for some users and is weapon-system dependent.
Two mounts (the SOCOM 3-Prong Flash Hider and WARCOMP) were used in the testing of the SOCOM556-RC2 silencer, in the same test session. Previously, PEW Science tested the SOCOM762-RC2, with both mounts. Despite each silencer being tested on different platforms (the 762-RC2 on bolt-action rifle and the 556-RC2 on the MK18), the testing and analysis results indicate that the use of the WARCOMP is unfavorable, regardless of platform.
Surefire advises that the WARCOMP mount is intended for operators that are using their weapon unsuppressed and may use their silencer to suppress their weapon system infrequently. The WARCOMP provides an enhancement to shooting dynamics when unsuppressed, and these benefits are part of the intended design. Note that PEW Science has not performed an in-depth evaluation of the WARCOMP without a silencer mounted. For users that may only seldomly suppress their weapon system, the WARCOMP mount may offer benefits that make it an attractive choice.
The SOCOM 3-Prong Flash Hider with “labyrinth seals” and no porting offers significantly different suppressed performance than the WARCOMP with all Surefire rifle silencers. The seals are marketed by Surefire to prevent aft gas leakage at the mount interface. PEW Science data indicates that this claim is true, but the absence of ports is postulated to be a significant factor.
PEW Science highly recommends the use of a non-ported “labyrinth seal” equipped mount with the SOCOM556-RC2 for the reasons stated in this review. It is possible that prolonged use of a ported WARCOMP with a silencer could result in higher sight or hearing risk to an operator if not properly equipped with the recommended PPE. Hearing and eye protection is recommended for the operation of suppressed small arms, in general. It is also possible that prolonged use of a ported WARCOMP mount may result in the need for more frequent mount maintenance intervals, as the silencer may seize or become “carbon-locked” to the mount more easily due to the combustion products venting through the WARCOMP ports. PEW Science highly recommends users of the Surefire mounting system clean their mounts regularly.
In this review, the SOCOM556-RC2 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.
