SSS.6.75 - Otter Creek Labs Polonium and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle
/Otter Creek Labs Polonium on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The Polonium is designed and manufactured by Otter Creek Labs. It is a 243 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 13.5 ounces and the direct-thread mount weighs weighs 2.1 ounces, for a total system weight of 15.6 ounces. The Polonium can be obtained from Otter Creek Labs Dealers.
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-OCL-025-001-22. Therefore, data pertaining to the Polonium 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 with the 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 6.75.1 contains the Polonium test results and analysis.
Section 6.75.2 contains contains Suppression Rating comparisons of the Polonium with dedicated 223 and 30 caliber silencers on the current market, including the Surefire SOCOM556-RC2, HUXWRX HX-QD 556, Q Trash Panda, CGS Helios QD, SilencerCo Saker 556, Rugged Razor762, and others.
Section 6.75.3 contains the review summary and PEW Science subjective opinions.
Summary: When paired with the 10.3-in barrel MK18 and fired with Federal XM193, the Otter Creek Labs Polonium mounted with the direct-thread mount achieved a composite Suppression Rating™ of 37.7 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
6.75.1 Polonium Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the Polonium 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. Otter Creek Labs Polonium Sound Metric Summary
6.75.1.1 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 Polonium 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 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 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature
Fig 1b. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature
Figure 2a. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Figure 2b. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Immediate similarities in the the pressure and impulse waveforms shown in Figure 1 and Figure 2, respectively, are noted when compared to those from the SilencerCo Saker 556 in Review 6.53. In addition to the significantly delayed gas jetting after the bullet exit event (Figure 1b), the initial rise to maximum peak positive phase impulse possesses a gradual slope (Figure 2a). Both of these waveform similarities indicate that the Polonium possesses a higher PEW Science Back Pressure Metric, Omega, than silencers such as the Surefire SOCOM556-RC2 (Review 6.52), the Q Trash Panda (Review 6.61), the Rugged Razor762 (Review 6.58), and the CGS Helios QD (Review 6.66), on this platform. Omega metric data for the 5.56x45mm cartridge is the subject of ongoing PEW Science research.
The measured first-round-pop (FRP) from the Polonium is visible in both the pressure regime (Figure 1b) and the impulse regime (Figure 2b), 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 Polonium is postulated to be somewhat noticeable to bystanders, in accordance with PEW Science inner ear modeling. The gross relative influence on overall hearing response severity to bystanders of the Polonium FRP on this platform, to subsequent shots, is moderate.
Although the Polonium and Saker 556 share some gross waveform characteristics in their muzzle signatures, the Otter Creek Labs Polonium is quieter than the SilencerCo Saker 556 on the MK18 weapon platform, to both bystanders and to the shooter.
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 75 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.
The shape, timing, and magnitudes of the early-time pressure pulses and overall shape of the impulse waveforms measured at the muzzle, from shot-to-shot, are relatively consistent. The consistency of the waveform amplitudes highlight the silencer’s overall sound performance consistency at the muzzle after the FRP, as well as the relative consistency of the tested automatic rifle firearm configuration.
As typically indicated, first-round sound signatures always differ from subsequent shots, as the atmosphere within the silencer changes. The FRP phenomenon cannot always be shown by viewing only the peak sound pressure. This is one of the reasons why The Silencer Sound Standard requires examining multiple sound signature metrics. Ammunition consistency can play a role in the determination of FRP, however, the close examination of measured pressure and impulse waveforms typically excludes ammunition from the possible factors influencing true FRP, due to the relative consistency of most high quality factory ammunition.
PEW Science Research Note 3: Note that the muzzle Suppression Rating of the Otter Creek Labs Polonium with the direct thread mount is 37.6 and the at-ear Suppression Rating is 19.7; a lower zone on the Suppression Rating Dose Chart. The lower flow rate (higher back pressure) of the Polonium contributes to a more severe ejection port signature, increasing the overall severity of the signature to the shooter on the standard MK18 weapon system. This phenomenon also occurs with the SilencerCo Saker 556 on the MK18, albeit slightly more severely. The Polonium’s muzzle signature is less severe than the Saker’s, to the shooter. This phenomenon allows the Polonium to be slightly less damaging to the shooter’s ear than the Saker 556 on this host weapon, even when possessing slightly higher back pressure. Despite this advantage, as with the use of the Saker 556 on the standard MK18, caution should be exercised by the weapon operator. The signatures measured at the shooter’s ear are presented below.
6.75.1.2 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. A zoomed-in timescale is displayed in Figure 3b, in the region of peak sound pressure for Shot 1, Shot 2, and Shot 3. The real sound impulse (momentum transfer potential) histories at the ear from the same 5-shot test are shown in Figure 4. Again, full and short timescales are shown.
Figure 3a. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature
Figure 3b. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Pressure Signature, Short Time Window
Figure 4a. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature
Figure 4b. Otter Creek Labs Polonium 5.56x45mm NATO MK18 Automatic Rifle Ear Sound Impulse Signature Peaks
Unlike in the measurements at the muzzle, the FRP from the Polonium at the shooter’s ear is not significant. What is significant is the positive phase signature after primary muzzle blast (Figure 3b, ~35 ms). This increase in gas momentum may also be observed in the abnormally high late-time impulse waveforms beginning at approximately 35 ms in Figure 4a. This gas jetting, consisting of coalescing pressure waves from the late-time muzzle blast and primary ejection port pressure waves, is so intense that significant negative phase develops after its decay (50 ms, Figure 3b). These at-ear signature features are extreme and indicate the presence of significant gas flow momentum near the shooter’s head. Extremely similar phenomena was noted in the SilencerCo Saker 556 at-ear waveforms on this platform. This behavior is typical on the MK18 with silencers not possessing significant over-bore or other backpressure reducing features such as baffle assembly porting.
PEW Science Research Note 4: Shorter than the 20-in barrel bolt-action rifle host weapon, the 10.3-in barrel automatic rifle places the blast source in closer proximity to the shooter’s ear. In addition to the silencer endcap being closer, the ejection port allows combustion products at higher than atmospheric pressure to vent relatively suddenly in the time regime of so-called chamber blow-down. This venting time, and thus the vented pressure amplitude and wave shape, varies depending upon the pressure history in the chamber and barrel, which are in turn dependent upon the flow restriction (back pressure) of the silencer. Both the close proximity of the muzzle to the shooter’s ear, and the additive ejection port pressure coalescing with the muzzle signature wave front(s), result in a more severe signature at the shooter’s ear than 1.0 m left of the weapon muzzle. Therefore, the at-ear Suppression Rating is lower than the muzzle Suppression Rating with this particular silencer on the MK18 host weapon. Like the Saker 556, the Polonium produces significant ejection port signature at the shooter’s head position with this weapon system. This is a direct result of its relatively low flow rate (relatively high back pressure).
6.75.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 13 presents a comparison of the PEW Science Suppression Rating of the Otter Creek Labs Polonium with the direct-thread mount 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 5. Suppression Rating Comparisons of the Otter Creek Labs Polonium and other silencers, Using PEW-SOFT 5.56x45mm Supersonic Test Data and PEW Science Analysis
From the above data, it can be concluded that the Polonium suppressor has significantly higher sound suppression performance to bystanders than most of the other silencers shown. For example, the Polonium is over a Suppression Rating category quieter to bystanders than the Q Trash Panda, on the MK18. The Polonium is over two Suppression Rating categories quieter to bystanders than the Rugged Razor762 on the MK18, which represents an extreme performance differential. Unlike those aforementioned 30 caliber silencers, the Polonium is a 243 caliber silencer and not as significantly over-bored for the 5.56x45mm cartridge.
PEW Science Research Note 5: At the shooter’s ear, it is clear that the higher flow restriction (lower flow rate; higher back pressure) of the Otter Creek Labs Polonium adversely influences the sound signature. The standard MK18 test host possesses a standard carbine buffer spring and H2 buffer mass; the inertial and mechanical resistance in the reciprocating system, when using the Polonium, is inadequate to resist motion long enough for internal system pressure to drop below thresholds seen when using silencers such as the Trash Panda, Helios QD, and Razor762. The back pressure reducing features in those silencers (over-bore) allows for a significantly higher flow rate than possessed by silencers like the Polonium and Saker 556. Typically, other methods of back pressure reduction are required when over-bore is not used (e.g. porting of the baffle stack in the Surefire SOCOM556-RC2, or complex path geometry in the HUXWRX HX-QD series).
PEW Science Research Note 6: As previously discussed in Note 3, The Polonium’s muzzle signature is less severe than the SilencerCo Saker’s, to the shooter; that is, the influence of the signature emitted from the silencer’s end cap, to the shooter, is less severe. This phenomenon allows the Polonium to be slightly less damaging to the shooter’s ear than the Saker 556 on this host weapon, even when possessing slightly higher back pressure.
In a related note, despite the Polonium having significantly higher back pressure than the Rugged Razor762, it is slightly quieter to the shooter’s ear on this weapon system. Again, the signature to which the shooter’s ear is subjected is a function of both ejection port and muzzle signature. When the silencer’s endcap is in closer proximity to the shooter, the severity is increased. 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 gross signature suppression performance of the Otter Creek Labs Polonium and the Surefire SOCOM556-RC2 is relatively similar, though the methods of signature suppression used in both silencers are radically different. The lower flow restriction (lower back pressure) of the SOCOM556-RC2, than the Polonium, produces a less severe ejection port signature on the MK18, and the SOCOM556-RC2 accomplishes this without significant detriment to muzzle signature suppression. PEW Science postulates that sound signature suppression optimization for the short-barrel automatic 5.56x45mm AR15 platform, in this manner, is one of four primary mechanisms:
Silencer over-bore.
Baffle stack venting arrays.
Annular venting.
Continuous venting (lengthened flow path geometry).
The above four mechanisms of optimizing silencer sound suppression, and flow rate, are continued subjects of PEW Science internal research.
6.73.3 Review Summary: Otter Creek Labs Polonium 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 mounted with the direct-thread mount achieved a composite Suppression Rating™ of 37.7 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 Otter Creek Labs Polonium is a full-size 5.56mm machine gun rated rifle silencer that possesses extremely competitive sound signature suppression performance with many silencers on the market, while possessing higher back pressure compared to some designs. Users should note that the Polonium is capable of shooting projectiles as large as 6mm (.243).
The sound signature suppression performance of the Polonium on the MK18 weapon system is significant; the suppression performance to bystanders (muzzle Suppression Rating) is extreme for its size. It should also be noted that the back pressure (flow restriction) of the Polonium is significant, and on-par with that of the SilencerCo Saker 556. Users fielding such silencers on the AR15 weapon system may be advised to take flow restriction into consideration. Methods by which users may mitigate some of the adverse effects of high silencer flow restriction on the MK18 include increased buffer mass, spring force, and/or adjustable gas port orifice size. So-called “tuning” of the AR15 weapon system, for both suppressed function and optimized signature, is outside the scope of this article.
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. It should be noted that the silencer may accept 6mm projectiles through its bore, further widening potential use cases.
The Polonium is intended to be 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 is conducive to such use. However, PEW Science has not subjected the Polonium 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 Polonium 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.
