SSS.6.95 - Otter Creek Labs Polonium-K and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle (Free Version)
/Otter Creek Labs Polonium-K on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The Polonium-K 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 4.8 inches in length, without a mount. The total length with the included direct thread adapter is 5.3 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 11.5 ounces and the direct-thread mount weighs weighs 2.1 ounces, for a total system weight of 13.6 ounces. The Polonium-K 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-032-001-22. Therefore, data pertaining to the Polonium-K 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 Otter Creek Labs Polonium-K 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 6.95.1 contains the Polonium-K test results and analysis.
Section 6.95.2 contains Suppression Rating comparisons of the Polonium-K with dedicated 223 and 30 caliber silencers on the current market, including the Dead Air Sandman-S, HUXWRX FLOW 556k, Energetic Armament ARX, KAC QDSS-NT4, Rugged Razor556, Otter Creek Labs Polonium, 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 6.95.3 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 Polonium-K mounted with the direct-thread mount achieved a composite Suppression Rating™ of 32.5 in PEW Science testing. As with all weapon systems, the user is encouraged to examine both muzzle and ear Suppression Ratings.
The full-size version of this silencer was tested on the MK18 in previous Sound Signature Review 6.75, in which it achieved a Suppression Rating of 37.7.
Relative Suppression Rating Performance is Summarized in SSS.7 - PEW Science Rankings
6.95.1 Otter Creek Labs Polonium-K Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the Polonium-K 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. 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. Otter Creek Labs Polonium-K Sound Metric Summary
6.95.1.1 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-K 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 Shots 1 through 6.
Fig 1a. Otter Creek Labs Polonium-K 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature
Fig 1b. Otter Creek Labs Polonium-K 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature
Figure 2a. Otter Creek Labs Polonium-K 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Figure 2b. Otter Creek Labs Polonium-K 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
The Otter Creek Labs Polonium-K is a compact (short, or kurz) version of the Polonium (Review 6.75). As a result, the Polonium-K shares some similar physical features with the full-size Polonium, including its baffle type. The principal physical difference between the Polonium and Polonium-K is a reduction in the number of baffles; the Polonium-K possesses only four.
With its similar physical characteristics and reduced number of baffles, the Polonium-K’s signature is influenced in the following ways on the MK18 host weapon, compared with that of the full-size Polonium:
The FRP, in both pressure space (Figure 1) and impulse space (Figure 2) is apparent, but its relative severity to subsequent shots, to bystanders, is relatively moderate. This was also the case with the full-size Polonium.
The flow rate of the Polonium-K is increased, compared with that of the full-size Polonium. This is evident in the peak amplitude and timing of the first major jet in Figure 1b, along with the rise-time to maximum initial positive phase impulse, dictating its Omega Metric (Figure 2a).
The consistency of the Polonium-K is not as great as with the full-size version, later in the shot string. Note the abnormal ancillary impulse accumulation in Shot 5 and Shot 6 (Figure 2a and 2b). While secondary combustion with the short 10.3-inch barrel MK18 host weapon is typical, the full-size Polonium was able to suppress the phenomenon through a full 6-round shot string. The Polonium-K, with only four baffles, is unable to fully suppress secondary combustion from unburnt powder as it starts to burn. As the shot string continues, this phenomenon becomes persistent. This is a performance limitation of some silencers that possess few baffles and simple geometry on the MK18; despite using a dedicated bore for the weapon,
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, until the aforementioned late-time irregularities resulting from primary axial over-bore.
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-K with the direct-thread mount is 32.5 and the shooter’s-ear Suppression Rating is 20.2; a lower zone on the Suppression Rating Dose Chart. The somewhat lower flow rate (higher back pressure) of the Polonium-K contributes to a more severe ejection port signature, increasing the overall severity of the signature on the standard MK18 weapon system. This phenomenon also occurred with the full-size Polonium on the MK18, albeit more severely. The Polonium-K’s muzzle signature is less severe than that of many silencer’s, to the shooter. This phenomenon allows the Polonium-K to be slightly less damaging to the shooter’s ear than some other silencers on this host weapon, even when possessing slightly higher back pressure. Despite this advantage, caution should be exercised by the weapon operator.
Holistic suppression performance comparisons on the MK18 weapon system are examined in the section below.
6.95.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the Otter Creek Labs Polonium-K 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-K 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 Otter Creek Labs Polonium-K has significantly higher sound suppression performance to bystanders than many of the silencers shown. For example, the Polonium-K is a full Suppression Rating category quieter to bystanders than the OSS HX-QD 556k (Review 6.64), the Rugged Razor556 (Review 6.76), the Dead Air Sandman-S (Review 6.92), and the Surefire SOCOM762-RC2 with the 3-Prong Mount (Review 6.73) on the standard MK18 host weapon.
PEW Science Research Note 4: The Polonium-K possesses a muzzle Suppression Rating over 15 points higher than that of the Rugged Razor762 (Review 6.58) and the Energetic Armament ARX (Review 6.82) on this weapon system. That extreme performance differential is largely due to the over-bore of those two silencers and their primitive design compared to other over-bored silencers for the 5.56x45mm combustion regime.
Interestingly, the Polonium-K possesses a muzzle (bystander) Suppression Rating that is less than half a category lower than the full-size Polonium (Review 6.75) on this weapon system. This highlights the diminishing returns of scaling its baffle quantity in number, to form the larger silencer, for its short barrel suppression performance.
PEW Science Research Note 5: At the shooter’s ear, the Polonium-K performs similarly to the full-size Polonium on the MK18, largely due to its similar muzzle suppression capability. Although its flow restriction (back pressure) is reduced compared with that of the full-size Polonium, the Polonium-K is still unable to prevent excess system pressure from venting to atmosphere during combustion blowdown through the barrel. This is the primary mechanism by which “blowback” manifests on a semiautomatic AR-15 suppressed weapon system, to the weapon operator. The reader may note that the Suppression Rating at the shooter’s ear with the Polonium-K on the MK18 is marginally lower than that with the Dead Air Sandman-S. This is an example of the superior muzzle signature of a silencer (the Polonium-K) making up for its higher backpressure. The Dead Air Sandman-S is simply too loud on the MK18, at the muzzle, to further protect the shooter’s ears, despite its lower flow restriction (lower back pressure) than the Polonium-K.
PEW Science Research Note 6: An extremely interesting comparison is the performance of the aforementioned Rugged Razor556 with that of the Otter Creek Labs Polonium-K. The Razor556 is an inch longer than the Polonium-K, and is a Suppression Rating category louder at the muzzle. This performance dichotomy may not be intuitive to some readers. PEW Science encourages the reader to examine the respective Sound Signature Reviews for a full explanation of the silencers’ behavior on the MK18, the technologies used in the silencers, and the relative performance of silencers on the same host weapons, as summarized in the PEW Science Rankings Section of the Silencer Sound Standard.
PEW Science Research Note 7: Notably, the Otter Creek Labs Polonium-K exhibits very close overall performance to the SilencerCo Saker 556 (Review 6.53) on the MK18, and the Polonium-K is shorter and lighter than the Saker 556. This comparison may be of interest to some readers due to some geometric similarities of the blast baffles in the two silencers.
PEW Science Research Note 8: Another noteworthy comparison is the performance of the Polonium-K to the HUXWRX FLOW 556k (Review 6.83). Both the Polonium-K and the FLOW 556k are compact 5.56 rifle silencers; however, their technologies are radically different. The Polonium-K is significantly outperformed by the FLOW 556k at the shooter’s ear, on the MK18. To bystanders, the Polonium-K outperforms the FLOW 556k, though the performance gap is not as great as the reverse relationship at the shooter’s ear. The Polonium-K represents extremely high conventional supersonic suppression performance in a compact envelope, at the cost of flow restriction (back pressure). The FLOW 556K represents a radical approach to keeping the flow rate as high as possible while still suppressing signature. PEW Science considers the dichotomy of the two silencers, and their designs, to be an excellent case study for the current silencer market.
It is important to note that 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.
Bystanders may perceive the Otter Creek Labs Polonium-K to be quieter than many silencers shown in Figure 5 when fired on the MK18 host weapon. Personnel firing the weapon may experience hearing damage risk on par with that from using the Dead Air Sandman-S.
6.95.3 Review Summary: Otter Creek Labs Polonium-K 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-K mounted with the direct-thread mount achieved a composite Suppression Rating™ of 32.5 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-K is a compact 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 other compact designs. Users should note that the Polonium-K is capable of shooting projectiles as large as 6mm (.243) and is also available in a full-sized version that is an inch longer.
The sound signature suppression performance of the Polonium-K 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-K is less severe than that of the full-size Polonium. 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-K 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-K 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-K is conducive to such use. However, PEW Science has not subjected the Polonium-K 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-K 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.
