SSS.6.143 - Otter Creek Labs Polonium-30 and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle
/Otter Creek Labs Polonium-30 on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The Polonium-30 is designed and manufactured by Otter Creek Labs. It is a 30 caliber centerfire rifle silencer, intended to suppress many cartridges with projectiles appropriately sized to travel through the bore. It has a 1.625-inch diameter and is 5.8 inches in length, without a mount. The total length with the included direct thread adapter is 6.3 inches. The user may choose to install third-party adapters compatible with the 1.375”-24tpi system. The silencer is tubeless; the entirety of the welded assembly is constructed of H900 heat treated 17-4 stainless steel. As tested, the silencer weighs 13.9 ounces and the 1/2”-28tpi direct-thread mount weighs weighs 2.0 ounces, for a total system weight of 15.9 ounces. The Polonium-30 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 data production for this Sound Signature Review was funded in part by PEW Science Project PEW-OCL-077-001-23. Therefore, data pertaining to the Polonium-30 in this Sound Signature Review is published with the express written permission of Otter Creek Labs, LLC.
This Sound Signature Review contains single-test results using the Polonium-30 mounted with its direct-thread mount on the MK18 Automatic AR15 rifle, chambered in 5.56x45mm NATO with a 10.3-inch barrel. Federal XM193 55gr ammunition was used in the test. The standard PEW Science MK18 test host weapon system is described in Public Research Supplement 6.51.
Section 6.143.1 contains the Polonium-30 test results and analysis.
Section 6.143.2 contains Suppression Rating comparisons of the Polonium-30 with dedicated 223 and 30 caliber silencers on the current market, including the 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 6.143.3 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 Otter Creek Labs Polonium-30 mounted with its direct-thread mount achieved a composite Suppression Rating™ of 30.9 in PEW Science testing.
When paired with the Savage M10 20” .308 and fired with Federal XM80, the Otter Creek Labs Polonium-30 with its direct thread mount achieved a composite Suppression Rating™ of 37.1 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
6.143.1 Otter Creek Labs Polonium-30 Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the Polonium-30 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. Otter Creek Labs Polonium-30 Sound Metric Summary
6.143.1.1 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 Polonium-30 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. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature
Fig 1b. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature
Figure 2a. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Figure 2b. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
The Polonium-30 is, for all intents and purposes, structurally identical to the Polonium (6.75), other than the bore aperture. As a 30 caliber silencer, the Polonium-30 may also be used on 7.62x51mm weapons, as characterized in the bolt action evaluation in article 6.142.
In previous testing on the standard 5.56x45mm 10.3-in barrel MK18, the 6mm (243 caliber) Polonium exhibited significantly high pure muzzle suppression performance, with significant flow restriction (back pressure). On the same weapon system, the Polonium-30 examined in this article exhibits several performance differentials from its sister silencer, including:
More expedient blowdown and slower system kinematics (Fig. 1a).
More pronounced increase in coupled-jet amplitudes (Fig. 1b).
Increasingly severe primary jetting amplitude and coincident rarefaction (Fig. 1b).
Latent gas momentum stabilization with maxima shifting to later time as the shot-string progresses (Fig. 2b).
All four of the above performance differentials are a consequence of the silencer’s over-bore for this host platform, manifesting with less controlled combustion propagation, progressively more severe suppressed muzzle blast amplitude, with a higher gross flow rate (lower back pressure).
PEW Science Research Note 1: The Polonium-30 produces a more severe signature on the MK18 than does its sister silencer, the Polonium, and some of the performance attributes noted in the .308 bolt-action test of the Polonium-30 carry over to the 5.56x45mm combustion regime evaluation in this test. For example, the Polonium-30 does exhibit noticeable first-round-pop (FRP) on this platform that does manifest in a more severe hearing damage risk potential to bystanders during the first shot than the second shot. However, like in the 7.62mm bolt-action test, later shots in the string become more severe. In this respect, the Polonium-30 does exhibit FRP on this platform, but only relative in severity to the second shot. This measurable phenomenon, common between both tests of the silencer, is a consequence of the Polonium’s internal design losing efficiency as the bore approaches a critical diameter.
Nonetheless, despite the above performance characteristics, the over-bored Otter Creek Polonium-30 silencer is still able to control muzzle blast propagation relatively well for its size on the standard 5.56mm MK18 10.3-in short-barreled rifle, throughout the shot string. In fact, the Polonium-30 outperforms some other over-bored silencer on this platform, including the Q Trash Panda (6.61) in both pure suppression performance and consistency. Public testing of over-bored silencers on the 5.56x45mm MK18 weapon system by PEW Science has demonstrated typical inefficiencies, such as erratic flow rate. Such inefficiency characterized in measured impulse signatures from over-bored silencers may also be examined in test reports featuring products such as the Rugged Razor762 (6.58), the Energetic Armament ARX (6.82); the Rugged Razor556 (6.76), and the Dead Air Sandman-S (6.92). For the Otter Creek Labs Polonium-30 to eclipse the holistic performance of these silencers in both pure suppression and consistency, is notable.
PEW Science Research Note 2: The lower back pressure of the Polonium-30, compared with that of the Polonium, does pay dividends with regard to the severity of ejection port blast on the MK18 weapon system and hazard to the weapon operator. However, the more severe muzzle blast from the Polonium-30 limits the increase in the Suppression Rating at the shooter’s ear. Nonetheless, the hazard reduction with the Polonium-30 on this weapon system is more significant than the other aforementioned 30-caliber silencers. With regard to holistic performance on this weapon system, the Polonium-30 has a bystander signature that is relatively close in severity to that of the PWS BDE 556 in its short configuration (6.123), and somewhat less in severity than the bystander signature of the YHM Turbo T2 (6.98) and Aero Precision Lahar-30 (6.116).
Silencers that are in an overall lower performance class than the Polonium-30 on this platform include those such as Thunder Beast Dominus (6.105), as well as the aforementioned silencers in Research Note 1. Systems exhibiting significantly higher performance than the Polonium-30 on this platform include most dedicated 5.56mm silencers, including its sister silencer the Polonium (6.75), the Surefire SOCOM556-RC2 (6.52), CAT WB (6.129), and others. The reader is encouraged to utilize the PEW Science Rankings Table and Parametric Visualization tool to compare performance parameters. A summary chart is presented in Section 6.143.2 of this article.
PEW Science Research Note 3: 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 4: 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 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. There are other signature features measured at the shooter’s ear which may further assist with quantifying weapon kinematics.
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 5: Note that the muzzle Suppression Rating of the Otter Creek Labs Polonium-30 is 29.5 and the at-ear Suppression Rating is 23.9; the same zone on the Suppression Rating Dose Chart. The higher flow rate (lower back pressure) of the Polonium-30 from its larger bore aperture contributes to a less severe ejection port signature. However, its more severe muzzle signature does somewhat counteract that benefit, resulting in less of a hazard reduction to the shooter on the standard MK18 weapon system. This phenomenon also occurs with other silencers that exhibit reduced back pressure with simple over-bored geometry. The signatures measured at the shooter’s ear are presented below. The differences in gas dynamics at the shooter’s ear with the Polonium-30 and the standard Polonium are significant.
6.143.1.2 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, 2, and 3. 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 short timescales are shown.
Figure 3a. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature
Figure 3b. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, Short Time Window
Figure 4a. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature
Figure 4b. Otter Creek Labs Polonium-30 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature Peaks
When comparing the above signatures from the Polonium-30 on the MK18 to those from the standard Polonium on the same platform, the differences are significant. By only enlarging the bore aperture of the silencer, while maintaining baffle quantity and geometry, the Polonium design produces significantly different gas dynamics:
As noted at the muzzle previously in this article, the blowdown duration has significantly decreased. This is apparent in Figure 3a. Also apparent in the figure is the lack of significant latent rarefaction. The gas momentum has dropped significantly, from reduced ejection port blast.
The above phenomena is also observable in the muzzle-blast contribution to the shooter’s ear signature in pressure space (Fig. 3b) and impulse space (Figure 4). In addition to a lack of late-time positive phase impulse, the FRP momentum accumulation has a higher proportion of muzzle blast component than ejection port blast, unlike in the standard Polonium test. Note the front-loaded FRP in Figure 4b, indicative of a more balanced signature. This phenomenon has been highlighted previously in controlled test comparisons with back pressure differential (articles examining the Maxim Defense DSX system in 6.110 and DSX SURG in 6.111).
A nuanced difference in the at-ear signatures with the Polonium-30 from that of the Polonium is the gradually delayed momentum accumulation through the shot string (Fig. 4b). This is a consequence of the combination of over-bore dimension being over a critical diameter for the baffle geometry and the number of baffles in the system; the phenomenon is temperature-driven and also occurs in the muzzle signatures shown in this report, in addition to in the previously reported 7.62x51mm bolt-action test report. While measurable and consistent with the physics observed in the silencer’s performance on the other platform, the phenomenon is not of practical consequence to shooter risk on this platform.
PEW Science Research Note 6: In general, it is important to note that although the ejection port blast hazards are less significant with the Polonium-30 than with the Polonium on the standard MK18, the muzzle suppression is of lower performance and the net result at the shooter’s ear is still relatively hazardous. Therefore, a shooter may be able to lower their hearing damage risk potential with a Polonium on the MK18 by tuning the weapon more so than with a Polonium-30. Gas blowback phenomenon, however, may be more severe with the smaller-bored Polonium than the Polonium-30, regardless of tuning, with faster firing schedules, due to similar early time flow rate between the two silencers and the Polonium-30 exhibiting a higher gross flow rate.
It is important to note that not all silencers will possess a significant increase in shooter’s ear Suppression Rating from weapon tuning. 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.
6.143.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 6 presents a comparison of the PEW Science Suppression Rating of the Otter Creek Labs Polonium-30 to that of other rifle silences on the MK18 automatic AR15 rifle. The standard PEW Science MK18 test host weapon system is described in Public Research Supplement 6.51.
Figure 6. Suppression Rating Comparisons of the Otter Creek Labs Polonium-30 and other silencers, Using PEW-SOFT 5.56x45mm Supersonic Test Data and PEW Science Analysis
Figure 6 presents detailed PEW Science Suppression Ratings, computed at the muzzle and ear, for the listed silencers. The Suppression Rating is a holistic parameter that captures human inner ear damage risk potential from a measured impulsive 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.
It is very important to note that performance differentials outside the free field (e.g. near reflecting surfaces) may not scale linearly across all designs. This is due to blast load reflection factors varying with both amplitude and wave shape, along with other waveform components.
From the above data, it can be concluded that the Otter Creek Labs Polonium-30 design exhibits competitive performance with many other 30 caliber rifle silencers on the standard untuned MK18 weapon system. It’s shooter’s ear Suppression Rating is somewhat higher than that of the Polonium due to its higher flow rate from larger bore aperture. Note, however, the relatively lower bystander Suppression Rating. This is a direct consequence of the same enlarged aperture, and prevents the shooter’s ear Suppression Rating from increasing further, as well, due to the gross sound field.
With regard to holistic performance on this weapon system, the Polonium-30 has a bystander signature that is relatively close in severity to that of the PWS BDE 556 in its short configuration (6.123), and somewhat less in severity than the bystander signature of the YHM Turbo T2 (6.98) and Aero Precision Lahar-30 (6.116).
Silencers that are in an overall lower performance class than the Polonium-30 on this platform include those such as Thunder Beast Dominus (6.105), as well as the aforementioned silencers in Research Note 1. Systems exhibiting significantly higher performance than the Polonium-30 on this platform include most dedicated 5.56mm silencers, including its sister silencer the Polonium (6.75), the Surefire SOCOM556-RC2 (6.52), CAT WB (6.129), and others. The reader is encouraged to utilize the PEW Science Rankings Table and Parametric Visualization tool to compare performance parameters.
PEW Science Research Note 7: Due to the breadth of the Silencer Sound Standard testing and analysis pedigree, the reader is encouraged to examine the spectrum over which suppression performance on the MK18 has been demonstrated. There exist silencers evaluated on the 5.56x45mm short barrel rifle platform that have significantly lower performance. Because the PEW Science Suppression Rating is a damage risk criterion (DRC), a lower Suppression Rating indicates a higher personnel hazard in the free field. Therefore, such silencers are postulated to be more hazardous to the unprotected ear than silencers like the AAC M4-2000 Mod 08. To iterate, the Suppression Rating is a DRC - it is not a subjective quantity; it is an objective quantification of hearing damage risk potential.
Weapon tuning for use of the Otter Creek Labs Polonium-30 may be preferred or required by system operators desiring to optimize system performance and reduce hazard, but is postulated to be needed less than with a standard Polonium, for example. It is important to note that not all silencers will possess a significant increase in shooter’s ear Suppression Rating from weapon tuning. 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.
6.143.3 Review Summary: Otter Creek Labs Polonium-30 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 Otter Creek Labs Polonium-30 mounted with its direct-thread mount achieved a composite Suppression Rating™ of 30.9 in PEW Science testing.
When paired with the Savage M10 20” .308 and fired with Federal XM80, the Otter Creek Labs Polonium-30 with its direct thread mount achieved a composite Suppression Rating™ of 37.1 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 Otter Creek Labs Polonium-30 is a compact to mid-size 7.62mm rifle silencer. When compared to 5.56mm rifle silencers, it may be considered full size. The Polonium-30 possesses somewhat less suppression performance than its sister silencer, the Polonium. It exhibits performance that is competitive with several other 30 caliber silencers of similar size on the MK18 platform.
The rear of the Polonium is threaded for universal mount adaptation. The user may choose to install third-party adapters compatible with the 1.375”-24tpi system. This feature allows for significant adaptability of the silencer, should the included direct-thread adapter not be used.
The Polonium-30 outperforms some other over-bored silencer on this platform, including the Q Trash Panda in both pure suppression performance and consistency. Public testing of over-bored silencers on the 5.56x45mm MK18 weapon system by PEW Science has demonstrated typical inefficiencies, such as erratic flow rate. Such inefficiency characterized in measured impulse signatures from over-bored silencers may also be examined in test reports featuring products such as the Rugged Razor762, the Energetic Armament ARX; the Rugged Razor556, and the Dead Air Sandman-S. For the Otter Creek Labs Polonium-30 to eclipse the holistic performance of these silencers in both pure suppression and consistency, is notable.
With regard to holistic performance on this weapon system, the Polonium-30 has a bystander signature that is relatively close in severity to that of the PWS BDE 556 in its short configuration, and somewhat less in severity than the bystander signature of the YHM Turbo T2 and Aero Precision Lahar-30.
Silencers that are in an overall lower performance class than the Polonium-30 on this platform include those such as Thunder Beast Dominus, as well as the aforementioned silencers above. Systems exhibiting significantly higher performance than the Polonium-30 on this platform include most dedicated 5.56mm silencers, including its sister silencer the Polonium, the Surefire SOCOM556-RC2, CAT WB, and others. It’s shooter’s ear Suppression Rating is somewhat higher than that of the Polonium due to its higher flow rate from larger bore aperture. Note, however, the relatively lower bystander Suppression Rating. This is a direct consequence of the same enlarged aperture, and prevents the shooter’s ear Suppression Rating from increasing further, as well, due to the gross sound field.
Like its sister silencer, the Polonium, the Polonium-30 may fit the role of a “duty-use” silencer, in that it may be subjected to severe firing schedules. PEW Science postulates that the blast baffle geometry and construction of the Polonium-30, like the Polonium, is conducive to such use. However, PEW Science has not subjected the Polonium-30 to severe automatic firing schedules and encourages the user to contact the manufacturer for guidance prior to use in such applications.
In this review, the Otter Creek Labs Polonium-30 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.
