SSS.6.102 - Otter Creek Labs Lithium and the Heckler and Koch SP5 9x19mm Subgun
/Otter Creek Labs Lithium on the HK SP5 9x19mm Subgun with 8.9-in barrel
The Lithium is designed and manufactured by Otter Creek Labs. It is a 9mm centerfire submachinegun silencer, intended to suppress many cartridges with projectiles appropriately sized to travel through the bore, up to and including .308 Winchester with specific barrel length and firing schedule restrictions. The Lithium has a 1.5-inch diameter and is 5.9 inches long. The silencer may be attached to a variety of weapon systems depending on the user’s choice of mount; the included 1/2-28tpi direct thread mount increases the system length to 6.1 inches. Other mounting options are possible due to the rear of the silencer body being threaded with the SilencerCo Bravo thread pattern. The entirety of the Lithium is constructed of 6Al-4V Grade 5 Titanium; it is a fully-welded tubeless silencer. The silencer weighs 5.7 ounces and the included direct thread mount weighs 1.2 ounces, for a total system weight of 6.9 ounces, as tested. The Lithium 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 Lithium 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 Lithium with the direct thread mount on the semiautomatic Hecker and Koch SP5 subgun, chambered in 9x19mm NATO with a 8.9-inch barrel. Speer Lawman 147gr ammunition was used in the test, in which its velocity was subsonic. The standard PEW Science HK SP5-A2 test host weapon system is described in Public Research Supplement 6.85.
Section 6.102.1 contains the Lithium test results and analysis.
Section 6.102.2 contains Suppression Rating comparisons of the Lithium with the HUXWRX CA$H 9K, Resilient Suppressors RS9, and GSL Phoenix fired on the Heckler and Koch SP5-A2.
Section 6.102.3 contains the review summary and PEW Science subjective opinions.
Summary: When paired with the 8.9-in barrel HK SP5-A2 with 80-deg locking piece and fired with Speer Lawman 147gr, the Otter Creek Labs Lithium mounted with the direct thread mount achieved a composite Suppression Rating™ of 55.4 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
6.102.1 Otter Creek Labs Lithium Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the Lithium 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 Lithium Sound Metric Summary
6.102.1.1 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 returned to battery with an empty chamber. The HK MP5 family of submachineguns do not possess a bolt-hold-open feature. 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 Otter Creek Labs Lithium 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, Shot 2, and Shot 3.
Fig 1a. Otter Creek Labs Lithium subsonic 9x19mm HK SP5-A2 Semiautomatic Subgun Muzzle Sound Pressure Signature
Fig 1b. Otter Creek Labs Lithium subsonic 9x19mm HK SP5-A2 Semiautomatic Subgun Sound Pressure Signature
Figure 2a. Otter Creek Labs Lithium subsonic 9x19mm HK SP5-A2 Semiautomatic Subgun Muzzle Sound Impulse Signature
Figure 2b. Otter Creek Labs Lithium 9x19mm HK SP5-A2 Semiautomatic Subgun Muzzle Sound Impulse Signature
The Otter Creek Labs Lithium is a lightweight and relatively compact pistol caliber silencer specifically intended for use on submachineguns (though it may also be used on pistols and rifles). PEW Science has not examined the behavior of the Lithium on semiautomatic handguns; an inertial decoupler assembly is most likely required for efficient operation with such systems and is beyond the scope of this article.
The behavior of the Lithium when suppressing the subsonic 9x19mm cartridge is characterized by relatively high performance signature suppression to bystanders for its size, with gas dynamics that are more restrictive than 9mm silencers possessing higher flow rates, such as the Resilient Suppressors RS9 (Review 6.90) and the HUXWRX CA$H 9K (Review 6.97). However, the flow rate of the Lithium is postulated to be somewhat higher than that of dedicated pistol silencers for the cartridge. More robust characterization of this behavior is the subject of future PEW Science research.
The first-round-pop (FRP) of the Lithium is significant to bystanders, in accordance with PEW Science inner ear modeling. The FRP divergence in pressure space occurs relatively early (Figure 1b) with corresponding impulse accumulation being pronounced during primary blowdown and decaying to stabilized flow after a time of approximately 32 ms (Figure 2). After FRP, the signature of the Lithium displays notable consistency.
PEW Science Research Note 1: As in most semiautomatic 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 85 ms in Figure 1a) the mechanical noise of the bolt closing is observed. The pressure signature of Shot 6 still displays this event due to the bolt not remaining open after the sixth and final round is fired from the magazine on the HK SP5-A2 weapon system. Nonetheless, the bolt is closing on an empty chamber.
PEW Science Research Note 2: The closing time of the HK SP5-A2 bolt is directly related to the flow restriction of a silencer used with the 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 with rifle cartridges, particularly on the MK18 weapon system. PEW Science Omega Metric research in the subsonic 9x19mm combustion regime with the HK SP5-A2 and SP5K-PDW weapon systems is underway. Note that the PEW Science HK SP5-A2 possesses an 80-deg locking piece in its bolt carrier group, which may result in lower bolt carrier group rearward velocity than systems possessing standard angle locking pieces. This lower rearward velocity may result in delayed forward return velocity, when compared with other systems.
It is also important to note that PEW Science has determined bolt closing time to be an unreliable indicator upon upper receiver fouling in some weapon systems, such as the MK18. Sound signatures are typically not influenced by this fouling, as these kinematics occur in late time, after gas venting to atmosphere. Note that the roller-delayed blowback action of the HK SP5-A2 is significantly different than the direct gas impingement operated locked bolt action of the MK18. These factors, in addition to the difference in combustion regime amplitudes of supersonic 5.56x45mm and subsonic 9x19mm, result in a different influence of mechanical weapon noise to the total signature to which bystanders and the shooter is subjected.
As always, it is important to note that 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 semiautomatic firearm configuration.
PEW Science Research Note 3: Note that the muzzle Suppression Rating of the Otter Creek Labs Lithium with the direct thread mount is 55.2 and the shooter’s-ear Suppression Rating is 38.3; which are the different zones on the Suppression Rating Dose Chart. This demonstrates a typical performance attribute for silencers fielded on submachineguns. The gross suppression of a silencer, as well as its flow rate, influences the holistic signature on the standard HK SP5-A2 weapon system. The signatures measured at the shooter’s ear are presented below.
6.102.1.2 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 6-shot test are shown in Figure 4. Again, full and short timescales are shown.
Figure 3a. Otter Creek Labs Lithium subsonic 9x19mm HK SP5-A2 Semiautomatic Subgun Ear Sound Pressure Signature
Figure 3b. Otter Creek Labs Lithium subsonic 9x19mm HK SP5-A2 Semiautomatic Subgun Ear Sound Pressure Signature, Short Time Window
Figure 4a. Otter Creek Labs Lithium 9x19mm HK SP5-A2 Semiautomatic Subgun Ear Sound Impulse Signature
Figure 4b. Otter Creek Labs Lithium 9x19mm HK SP5-A2 Semiautomatic Subgun Ear Sound Impulse Signature Peaks
The sound signatures measured at the shooter’s ear from a suppressed roller-delayed blowback submachine gun are complex. However, there are key waveform features that allow for strict interpretation of various events occurring during the gunshot, and their contribution to the overall signature.
PEW Science Research Note 4: There is a major performance factor displayed in the above data that differentiates the performance of the Lithium on the SP5 from that of the HUXWRX CASH 9K and the Resilient Suppressors RS9. The factor is best described as gas propagation delay. This delay is a direct illustration of the lower flow rate of the Lithium when compared to that of the aforementioned high flow rate silencers. Note the time at which FRP divergence begins in pressure space (Figure 3b) and impulse accumulation builds in impulse space (Figure 4b), during the first shot. Prior to a time of approximately 29 ms at the shooter’s ear, the weapon combustion signature differences among shots are minimal. Because the combustion gasses are retained within the silencer for this duration, the subsequent venting is delayed to later in time for greater coalescence with action unlocking, mechanical noise, and ejection port pressure pulse. This combined signature timing results in less pronounced FRP to the shooter; the ancillary combustion from FRP is nested with other signature components and becomes “a wash” to the shooter, with respect to magnified hearing damage risk during the first shot compared to subsequent shots. However, the lower flow rate of the Lithium highlighted by this phenomenon, combined with its closer end cap standoff, also contributes to greater gross impulse accumulation at the shooter’s head with all shots, subjecting the shooter to an overall greater hearing damage risk than silencers with higher flow rates.
The above phenomenon is nuanced and may not be immediately intuitive to the reader. Masking of FRP hearing effects to the shooter when they are present to bystanders, by pushing the timing of FRP propagation into the realm of other weapon dynamics, is serendipitous to the shooter during FRP but can be a holistic performance detriment, especially with a compact silencer that has an end cap pressure origin closer to the shooter’s head. For contextual comparison of this phenomenon for the extreme case, the reader is referred to the testing of the GSL Phoenix (Review 6.86). The Phoenix is a much larger silencer, and is quieter overall than the Lithium. However, the Phoenix has a flow rate that also results in muzzle signature coalescence with the ejection port signature that may be unfavorable to the shooter in the free field, when compared to silencers with higher flow rates. Maximizing the Suppression Rating to the weapon operator on the HK SP5 requires a combination of high flow rate and muzzle signature suppression. This balance has objectively been proven challenging to achieve.
PEW Science Research Peer Review Notes: Internal engineering reviews and external engineering peer reviews have been conducted of PEW Science data for the HK SP5 (MP5) system, focusing on weapon system kinematics. The roller movement and trunnion disengagement events are most likely the contributors at approximately 30.6 ms (Figure 4b) . Note that the bolt head is able to move prior to roller disengagement. Late-time event kinematics, such as cartridges being stripped from the magazine and the bolt returning to battery have also been verified (Figure 1a, 85 ms and Figure 3a, 83 ms).
Due to the relatively low pressure amplitude and duration of subsonic 9x19mm combustion compared with supersonic 5.56x45mm combustion, the influence of the above signature factors on the holistic system signature, especially to the shooter, is more significant.
PEW Science Research Note 8: Unlike the case at the muzzle and to bystanders, the FRP from the Otter Creek Labs Lithium at the shooter’s ear on this weapon system is significantly suppressed (see Research Note 4, above). Elevated relative hearing damage risk to the shooter from FRP with the Lithium on this host is greatly reduced, in accordance with PEW Science inner ear modeling.
6.102.2 Suppression Rating Comparison (Subsonic 9x19mm from the HK SP5-A2)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the Otter Creek Labs Lithium with the direct thread mount to that of the HUXWRX CA$H 9K, Resilient Suppressors RS9, and GSL Phoenix on the HK SP5-A2 system. The standard PEW Science HK SP5-A2 test host weapon system is described in Public Research Supplement 6.85.
Figure 5. Suppression Rating Comparisons of the Otter Creek Labs Lithium with the HUXWRX CA$H 9K, Resilient Suppressors RS9, and GSL Phoenix on the HK SP5-A2, Using PEW-SOFT 9x19mm Subsonic Test Data and PEW Science Analysis
The Otter Creek Labs Lithium is slightly quieter than the Resilient Suppressors RS9 (Review 6.90) on the HK SP5-A2, to bystanders, despite being shorter. However, the reduction in flow rate (increased back pressure) of the Lithium compared with high flow rate silencers like the RS9 and the HUXWRX CA$H 9K (Review 6.97) result in a Suppression Rating at the shooter’s ear that is more severe. Its flow rate, coupled with its muzzle (bystander) Suppression Rating and closer end cap standoff to the shooters head, results in the Lithium producing a more severe signature to the shooter than the GSL Phoenix (Review 6.86).
For its size, the Otter Creek Labs Lithium produces a relatively high Suppression Rating to bystanders, on the HK MP5 system. While its FRP to bystanders is significant, the experience to the shooter may differ, as discussed in the preceding section of this article.
The PEW Science Suppression Rating quantifies inner ear damage risk. Human perception of sound may, or may not, always correlate. More detailed information regarding this phenomenon is presented periodically in PEW Science Member Research Supplements.
The signature to which the shooter’s ear is subjected is a function of both ejection port and muzzle signature. When the silencer’s endcap is in closer proximity to the shooter, the severity is increased. The coalescing of the ejection port overpressure with the primary muzzle blast may exacerbate the severity of the signature at the shooter’s head position. It is not ejection port signature, alone, that dictates the signature measured at the shooter’s head position.
Although the published suppressed HK SP5 dataset is limited, two significant conclusions drawn in previous articles once again hold true in this data and analysis:
The roller-delayed blow back MP5 operating system is still a blow back system. The delay, even when maximized with the 80-deg locking piece, still does not completely eliminate gas sensitivity during suppressed use. Silencer back pressure still influences the signature at the shooter’s ear on this suppressed weapon system.
The Flow-Baffles in the HUXWRX CA$H 9K and the early-time venting in the Resilient Suppressors RS9 significantly reduce back pressure. The suppression performance of those two silencers to the shooter in the subsonic pistol cartridge regime is notable, and the MP5 is gas sensitive enough for their back pressure reduction to have efficacy, even with muzzle (bystander) suppression performance lower than that of the Otter Creek Labs Lithium.
Further study of the suppressed roller-delayed blow back system is a subject of future PEW Science research.
6.102.3 Review Summary: Otter Creek Labs Lithium on the HK SP5 9x19mm Subgun with 8.9-in barrel
When paired with the 8.9-in barrel HK SP5-A2 with 80-deg locking piece and fired with Speer Lawman 147gr, the Otter Creek Labs Lithium mounted with the direct thread mount achieved a composite Suppression Rating™ of 55.4 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 Lithium is a compact and lightweight 9mm silencer that exhibits high sound signature suppression performance for its size. At 6.9 ounces and 6.1 inches long in the direct thread configuration, the Lithium is lighter and smaller than many other 9mm submachinegun silencers on the current market, while also able to suppress centerfire rifle cartridges on full length barrels. The application versatility of the Lithium is notable.
The baffle aperture of the Lithium is enlarged, which along with ancillary porting, does somewhat increase early time flow rate. On the HK MP5 system, the silencer is measured to perform more similarly at the shooter’s ear to submachinegun silencers with high muzzle signature suppression performance, like the GSL Phoenix. While such a comparison does somewhat highlight its flow restriction, the gross suppression characteristics are notable due to the silencer’s relatively small size and light weight. The welded titanium tubeless design of the Lithium allows for this weight in the size envelope, as well as adequate strength for rifle cartridge suppression. The user is encouraged to contact the manufacturer for specific guidance on applicability of the Lithium on different host weapons. Barrel length and firing schedule restrictions should be observed.
The included direct thread mount is easy to install in the Lithium, and possesses hex-wrench features on both the mount exterior and interior; a welcome feature for convenience. The exterior wrench feature is recessed, which reduces overall system length. The mount may be removed and other mounts compatible with the so-called “SilencerCo Bravo” threading system may be installed. This feature allows the Lithium to be used with a variety of weapon systems.
In addition to fixed-barrel weapons, the Lithium may be used on semiautomatic handguns. The use of a silencer on such a weapon, for example, on a modified tilting-barrel Browning action, may often be accomplished through the use of an inertial decoupler assembly. PEW Science has not evaluated the Lithium in such a configuration. In general, PEW Science recommends the user contact both the silencer manufacturer and weapon manufacturer prior to creating and operating a suppressed weapon system from these components. Flow dynamics, system mass, and mechanics may significantly influence reliability, performance, and operational longevity of such systems.
In this review, the Otter Creek Labs Lithium performance metrics depend upon suppressing a subsonic centerfire pistol cartridge on a roller-delayed blowback submachinegun. While the sound signature of such cartridges can be suppressed to levels that may result in the desire of the shooter and bystanders to not wear hearing protection, PEW Science encourages the reader to remain vigilant with regard to all subsonic pistol cartridge suppression claims. The gas volume and combustion products created by firing a subsonic centerfire pistol cartridge such as 9x19mm are still significant; the measured pressure and impulse magnitudes, and their durations, illustrate this fact. Silencer performance on automatic (reciprocating) weapons depends on many factors. Weapon configuration may significantly influence total suppressed small arm system performance.
The hearing damage potential of subsonic submachinegun use is not insignificant. PEW Science encourages the reader to consider the Suppression Rating when deciding on an appropriate silencer and host weapon combination for their desired use.
