SSS.6.196 - Otter Creek Labs Infinity and the M4A1 Mid-Gas 5.56x45mm 14.5-in Barrel Automatic AR15 Rifle (Free Version)

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Otter Creek Labs Infinity on the M4A1 Mid-Gas 5.56x45mm AR15 with 14.5-in Barrel

The Infinity is a 30 caliber centerfire rifle silencer designed by Otter Creek Labs.  The Infinity is intended to suppress many cartridges with projectiles appropriately sized to travel through the bore, and may be used with severe firing schedules. It has a 1.73-inch diameter and is 6.6 inches long without a mount.  The silencer does not include any mounts; the proximal end of the Infinity is threaded 1.375-24 tpi to accommodate “HUB” compatible mounting systems.  The silencer body is 3D-printed from Inconel 718 alloy and the retail package includes multiple heat-treated stainless steel end caps that change system behavior.  The user may choose to install different end caps to achieve varying flow states appropriate for the combustion dynamics of the chosen cartridge and weapon system.  With an end cap installed, the silencer weighs 20 ounces and with a Rearden titanium direct-thread mount, weighs 20.8 ounces and is 6.8 inches long, as tested.  The Otter Creek Labs Infinity 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 analysis production for this Sound Signature Review was funded in part by PEW Science Project PEW-OCL-121-001-25. Therefore, data pertaining to the Infinity in this Sound Signature Review is published with the express written permission of Otter Creek Labs, LLC

The testing and analysis presented in this Sound Signature Review are of the Otter Creek Labs Infinity with three different end caps M4A1 Mid-Gas Automatic AR-15 rifle, chambered in 5.56x45mm NATO with a 14.5-inch barrel.. Federal XM193 55gr ammunition was used in the test. The standard PEW Science M4A1 test host weapon system is described in Public Research Supplement 6.127.

Past reports on the Otter Creek Labs Infinity:

  • Infinity testing and analysis with the 20-in 7.62x51mm NATO bolt-action is documented here.

  • Infinity testing and analysis with the 10.3-in 5.56x45mm NATO MK18 is documented here.

  • Infinity testing and analysis with the 8-in 300 BLK bolt-action is documented here.

Subject report:

  • Section 6.196.1 contains an abbreviated overall summary of Otter Creek Labs Infinity rifle silencer performance on the both the 14.5-in M4 and standard 10.3-in 5.56 MK18.

  • Section 6.196.2 contains Infinity Solid 5.56 End Cap test results and analysis.

  • Section 6.196.3 contains Infinity Vented 5.56 End Cap test results and analysis.

  • Section 6.196.4 contains Infinity Vented 7.62 End Cap test results and analysis.

  • Section 6.196.5 contains overall Suppression Rating comparisons of the Infinity with 3 end caps with other rifle silencers on the current market on the M4A1 Mid-Gas Rifle, including various configurations of the Dillon DRC556, CAT TSWIF, Sylvan Arms SA223TIS, BOE Mod 1, Echo Zulu Defense X1R556-Ti, PTR VENT Spiritus 556i, HUXWRX FLOW 556 Ti, Surefire SOCOM556-MINI2, KAC QDC/CRS-PRT, KAC QDC/MCQ-PRT, CAT/WB/A1, and Surefire SOCOM556-RC2.

  • Section 6.196.6 contains an article summary and PEW Science laboratory staff opinions.

Summary: When paired with the 14.5-in barrel M4A1 and fired with Federal XM193 with the solid 5.56 end cap, the the Otter Creek Labs Infinity achieved a Suppression Rating™ of 40.1 in PEW Science testing.  With the vented 5.56 end cap, the Otter Creek Labs Infinity achieved a Suppression Rating of 37.8.  With the vented 7.62 end cap, the Otter Creek Labs Infinity achieved a Suppression Rating of 36.1.

5.56x45mm NATO 10.3-in AR-15 rifle test results and analysis with the Otter Creek Labs Infinity is provided in report 6.179.

7.62x51mm NATO bolt-action rifle test results and analysis with the Otter Creek Labs Infinity is provided in report 6.178.

Subsonic 300 BLK bolt-action rifle test results and analysis with the Otter Creek Labs Infinity is provided in report 6.185.

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.

6.196.1 Otter Creek Labs Infinity 5.56 Automatic Rifle Performance Summary

The PEW Science Silencer Sound Standard includes the 5.56x45mm NATO evaluation of four rifle silencers designed and manufactured by Otter Creek Labs.  Comprehensive performance Rankings are tabulated in Section 7 of the Standard.  One of those silencers, the Infinity, has been previously evaluated on the the 10.3-in MK18 in three of its interchangeable end cap configurations.  The scope of this report contains standard 14.5-in M4A1 weapon system combustion suppression analysis with the same end cap configurations.  Figure 1 presents a performance comparison of the three configurations from the Infinity MK18 study (6.179) and the M4A1 subject study.  As a refresher, the three tested configurations of the silencer are classified in the taxonomy by the following:

  • Infinity (7.62 mm vented end cap): a hybrid design 30 caliber silencer with ancillary axial and annular venting at the distal end, with an over-bored 7.62-sized primary distal orifice.

  • Infinity (5.56 mm vented end cap): a hybrid design 30 caliber silencer with ancillary axial and annular venting at the distal end, with a 5.56-sized primary distal orifice.

  • Infinity (5.56 mm solid end cap): a hybrid design 30 caliber silencer with only axial bore venting at the distal end (5.56-sized primary distal orifice).

The three configurations are grouped in order, in accordance with their distal blast momentum propagation rate, from least restrictive to most restrictive.  The rate of distal external blast momentum accumulation is quantified from free field test measurements by the calculated PEW Science Omega Metric (6.40).  New M4A1 performance metrics are labeled in red.  Previously presented MK18 metrics are labeled in black.

Figure 1. Suppression Rating Comparisons of Otter Creek Labs Infinity Using PEW-SOFT 5.56x45mm Supersonic M18 and M4A1 Test Data and PEW Science Analysis

From the above semiautomatic short-barrel AR-15 rifle performance metrics, it can be concluded that signature suppression levels may increase or decrease in accordance with not only distal blast momentum propagation rate, but also in accordance with other signature components that vary based on silencer internal design.  Furthermore, it is extremely important to note that the early-time flow rate of a silencer may also influence performance, independent of late-time distal momentum control. 

With respect to the comparison of Infinity performance between the 10.3-in barrel carbine-length gas system (MK18) and 14.5-in barrel mid-length gas system (M4A1), the following observations are noted:

  1. As blast load input pressure decreases, configurations of the Infinity possessing distal venting and greater annular utilization (the vented end caps) become more efficient in pure muzzle suppression. This behavior is typical of many silencer designs when increasing barrel length.

  2. When distal vent area is decreased (moving from vented 7.62 end cap to vented 5.56 end cap), the trend of increased pure muzzle suppression with increased barrel length continues.

  3. When distal vent area is minimized (solid 5.56 end cap), no effective increase in pure suppression is noted by increasing barrel length. This phenomenon typically presents in hybrid design silencers having high early time flow rate and complex pressure stages. Below critical input pressure, ancillary flow paths (annular flow, for example) may not be as efficiently utilized. Similar behavior is noted in Total Signature Reduction Technology showcased in the Surefire SOCOM556-RC2 on the MK18 (6.52) compared with the SOCOM556-RC2 on the M4A1 (6.128).  It is important to note that this performance plateau with increased barrel length is not limited to the Infinity and the 556-RC2, nor should it be extrapolated to shorter or longer barrel lengths. Caution should be used when attempting such extrapolation due to not only differing critical pressure thresholds in various hybrid designs, but also complex shock dynamics that differ across barrel lengths. For example, it is postulated there is a barrel length increase at which pure suppression performance is realized. That threshold may differ across silencer designs. Another way for the reader to interpret these observations is that a reduction in barrel length (increase in input pressure) does not significantly influence the performance of the Infinity in the solid 5.56 end cap configuration. The Infinity can perform just as well on the MK18 as it does on the M4 in this configuration.

  4. The shooter’s ear Suppression Rating decreases (free field operator hazard increases) when moving from 10.3-in MK18 to 14.5-in M4A1, despite the vented end cap configurations of the silencer having higher muzzle suppression performance. This hazard increase is driven by the differences in the weapons’ gas systems. The M4A1 has both a longer dwell time and a larger gas port orifice size than the MK18, the combination of which increases ejection port blast hazards when suppressed.  This occurs regardless of the rifle manufacturer’s design intent.  This increase in free field operator hazard is relatively typical for silencers when transitioning from the MK18 to the M4A1, though there are exceptions.

The detailed 5.56x45mm NATO 14.5-in M4A1 evaluation of the Otter Creek Labs Infinity with 5.56 solid, 5.56 vented, and 7.62 vented end caps follows.

6.196.2 Otter Creek Labs Infinity Solid 5.56 End Cap Sound Signature Test Results

A summary of the principal Silencer Sound Standard performance metrics of the Otter Creek Labs Infinity with solid 5.56 end cap 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 public PEW Science testing, research, and development with a membership, here. State-of-the-art public firearm sound signature testing and research conducted by PEW Science is supported by readers like you.

 

Table 1. Otter Creek Labs Infinity Solid 5.56 End Cap Sound Metric Summary

 

6.196.2.1 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 Otter Creek Labs Infinity with solid 5.56 end cap are shown in Figure 2a. 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 3a. In Figure 3b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shots 2 and 3.

Fig 2a. Otter Creek Labs Infinity Solid 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Pressure Signature

Fig 2b. Otter Creek Labs Infinity Solid 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Sound Pressure Signature

Figure 3a. Otter Creek Labs Infinity Solid 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Impulse Signature

Figure 3b. Otter Creek Labs Infinity Solid 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Impulse Signature

Much of the analysis presented below is similar to that presented in the previous MK18 evaluation.  Performance deviations and pertinent comparisons are noted as applicable.  The primary behavioral mechanisms of the Infinity do not change when increasing AR-15 barrel length; however, performance scaling is nonlinear across both barrel length changes and end cap changes, as discussed in Section 6.196.1.  In that respect, there are secondary changes to the mechanisms.

As discussed in the preceding summary and in the 7.62x51mm NATO bolt-action Infinity analytical report (6.178), the Otter Creek Labs Infinity is a 30 caliber rifle silencer that is classified as a hybrid design in the PEW Science research taxonomy.  More specifically, the Infinity is classified as a staged hybrid.  Hybrid design silencers are those in which elements of both conventional and high flow rate silencers are used, coupled with other technology variations and staged elements to provide parametrically varied performance attributes. Such attributes include the ability to reduce early-time shock reflections while significantly altering the rate of proximal and distal venting. These silencers may allow for minimal reciprocating weapon functional influence, while at the same time significantly suppressing signature to the operator and bystanders. Hybrid designs span a large range of the market in both brand and performance in different combustion regimes.  There also exist subcategories of this class of silencers that have been shown to demonstrate efficacy in multiple flow regimes.  Discussion of these subcategories is outside the scope of this report.  Other hybrid designs and staged hybrids include those from PTR, CAT, CGS, SilencerCo, Surefire, and others.  For an overview of the three primary classes of rifle silencer designs, the reader is encouraged to review PEW Science Research Supplement 6.169. 

The Infinity hybrid design is characterized by the following physical design attributes:

  • Early venting in the first expansion (blast) chamber prevents excessive blast load impulse accumulation in early time.

  • Clearing off of the first reflector (blast baffle) occurs through venting on both the convex surface and boundary walls.

  • Multiple stages are utilized; high pressure baffle geometry in the first stage, lower pressure geometry in the second stage, and varied alternate flow path geometries that are utilized differently in accordance with user choice of end cap.

PEW Science Research Note 1: As discussed in the previous MK18 test report, the Infinity represents a large step forward in the Otter Creek Labs design pedigree.  When fitted with the solid 5.56 end cap, the Otter Creek Labs Infinity functions as a 30 caliber rifle silencer with a high early-time flow rate and a restricted late-time flow rate. This performance is demonstrated by the following characteristics shown in the above test data (differences between MK18 and M4 data are shown in bold):

  1. Relatively longer blowdown duration (Fig. 2a), relatively similar to the MK18.  Note bolt carrier group (BCG) speed is faster than measured on the MK18.

  2. Lack of decoupled ground reflection, per (1), though consistency increases begin to reveal the feature.

  3. Relatively short initial positive phase blast load duration with coincident rarefaction, throttled externally in early time (Fig. 2b); note significant rarefaction is now confined only to FRP on the M4 and initial jetting is now more consistent.

  4. Blast pressures reach maxima post-initial jetting (Fig. 2b), similar to the MK18.

  5. Relatively typical duration of rise to maximum positive phase blast impulse for the silencer’s size (Fig. 3a); note that FRP divergence onset is somewhat delayed when compared to the MK18.

Again, as revealed in the MK18 evaluation, the above gross behaviors are extremely similar to that observed on the .308 bolt-action rifle in report 6.178. However, a primary difference in the 7.62 NATO and 5.56 NATO performance of the infinity is how the silencer deals with combustion gas volume and jet diameter.  The Infinity is much more efficient in 5.56 suppression than it is in the suppression of 7.62 NATO.  This is particularly the case in first-round-pop (FRP) control, in which the relative severity on both the MK18  and M4A1 from the Infinity are significantly reduced compared to on the 20-in barrel bolt-action .308 rifle.  The early-time blast load venting in the first expansion chamber of the Infinity is well-suited to controlling 5.56 NATO jetting.  For all intents and purposes, users are likely to observe that the Infinity may be considered an advanced performance 5.56 rifle silencer that can also suppress 7.62mm cartridges at typical levels.  

PEW Science Research Note 2: To be clear, the solid 5.56 end cap configuration of the Otter Creek Labs Infinity is the most restrictive configuration of the silencer. Although the system still possesses a high early-time flow rate from expansion chamber venting to multiple stages, downstream orifice restriction does result in an elevated Omega Metric and compromise of operator protection. The exceptional muzzle (bystander) Suppression Rating of 40.0 is accompanied by the lower shooter’s ear (operator) Suppression Rating of 23.4. When such a wide differential in bystander and operator Suppression Rating is noted, the user should immediately examine the reasons as to why such divergence occurs.  In the case of the Infinity, its early-time flow rate is not high enough to forgo the required accompaniment of less downstream restriction; the Infinity’s hybrid design requires ancillary downstream venting to significantly lower ejection port blast to the weapon operator.  This is in contrast to some other hybrid designs that natively include such venting without compromising momentum control, such as technologies from PTR (Purposely Induced Porosity, or “PIP”) and CAT (Surge Bypass, SNIPER2, SB-SHOCK).

As discussed in the MK18 evaluation, even with downstream restriction, the solid 5.56 end cap configuration of the Otter Creek Labs Infinity outperforms the traditionally designed Otter Creek Labs Polonium and Polonium-K silencers in most signature suppression metrics. With its staged hybrid design, the Infinity is able to surpass the Polonium in overall pressure field suppression while at the same time providing enough early-time venting to reduce the propensity for severe blast load reflection into the weapon bore. The effective “backpressure” of the Infinity is lower than that of the Polonium and Polonium-K, regardless of end cap choice. It is extremely important to note that the reduction in back pressure between the Polonium and Polonium-K is driven only by downstream restriction from the use of less baffles; an isolated Omega Metric drop with a constant Alpha parameter from identical early-time blast load impulse accumulation in the first expansion chamber. The Otter Creek Labs Infinity lowers Alpha while allowing the user to vary Omega through the use of end cap choice.  Without early-time venting, semiautomatic rifle silencers may never achieve requisite performance efficacy.  For this reason, the Infinity should be considered a next generation silencer relative to the previous models offered from Otter Creek Labs.

PEW Science Research Note 3: With regard to performance plateaus with increased 5.56x45mm NATO barrel length (the M4A1 has a 40% increase in barrel length from a standard MK18), the muzzle Suppression Rating (free field bystander protection) with the Infinity does not increase with the solid end cap. It is postulated that with the downstream annular passages restricted with the solid end cap, the 14.5-in barrel does not provide blast load input amplitude that reaches a critical threshold in the silencer; annular flow is somewhat reduced when compared to operation on the shorter MK18 barrel. As discussed in Section 6.196.1, similar behavior is noted with the Surefire SOCOM556-RC2 on the MK18 (6.52) compared with the SOCOM556-RC2 on the M4A1 (6.128).  It is important to note that this performance plateau with increased barrel length is not limited to the Infinity and the 556-RC2, nor should it be extrapolated to shorter or longer barrel lengths. Caution should be used when attempting such extrapolation due to not only differing critical pressure thresholds in various hybrid designs, but also complex shock dynamics that differ across barrel lengths. For example, it is postulated there is a barrel length increase at which pure suppression performance is realized. That threshold may differ across silencer designs.

PEW Science Research Note 4: It is worth listing various other technologies in the hybrid design taxonomy. Differential flow rates in early and late-time are common characteristics of advanced hybrid designs in which elements of high flow rate designs are combined with more traditional geometries to throttle the rate of combustion gas propagation late enough in the time regime so weapon operation is influenced to a lower degree.  Other examples of hybrid technologies and hybrid designs examined in the Standard research pedigree include (but are not limited to):

  1. CGS Hyperion Technology - Hyperion (6.27), Hyperion K (6.28), Helios QD (6.13), SCI-SIX (6.101)

  2. Surefire Total Signature Reduction - SOCOM556-RC2 (6.128), SOCOM762-RC2 (6.26), SOCOM556-RC3 (6.151), SOCOM-556-MINI2 (6.144)

  3. CAT SURGE BYPASS - CAT ODB (6.120), CAT WB (6.130), CAT TSWIF (6.193)

  4. Liberty Precision Machine Tech - Torch (6.126)

  5. SilencerCo LBP - Velos LBP (6.134)

  6. PTR Purposely Induced Porosity (PIP) - VENT 3 (6.135), VENT Spiritus 556i (6.176), Dillon DRC556 (6.194)

  7. KAC Pressure Reduction Technology (PRT) - KAC 556 QDC/CRS-PRT (6.156), KAC 556 QDC/MCQ-PRT (6.156)

  8. FOR Systems Tech - Monarch 7.62 (6.155) and Monarch Recce (6.195)

  9. Sig Sauer Forward Flux - SLH300Ti (6.107)

Again, the above are classified as hybrid designs in the PEW Science research taxonomy.  None of the above technologies or silencers are “Flow-Through” designs. That technology is only available from HUXWRX, and its principles of operation differ significantly from all of the above. Although some of the above silencers may be marketed as “Flow-Through” silencers by dealers and distributors that sell products, the physical mechanisms and performance differentials between all of the technologies are significant.  The reader is highly encouraged to examine the in-depth laboratory testing and analysis of HUXWRX systems for a detailed description of pertinent behavioral factors.  For example, when distal gas velocity increases significantly, as it does in a true Flow-Through design, there are consequences to the sound field that may induce unfavorable frequency components and reflection phenomena. The Research Caution at the beginning of this FLOW 556 Ti test report elaborates on this phenomenon, with additional research references available.

PEW Science Research Note 5:  It is extremely important for the reader to understand the Suppression Ratings are computed with free field test data, and are therefore universal when translating the weapon and operator to different environments without reflections.  However, when the environment becomes complex with many reflecting surfaces, performance translation of different systems is not yet possible; sound field shapes and particle velocities result in varied blast wave time-of-arrivals, as well as changes of blast load angles of incidence; these variations change amplitude and phase waveform components.  As a result of these inevitable environmental complexities, user impressions will vary, as both the shooter and bystander.  Nonetheless, hazard reduction efficacy in the free field, as characterized by the Suppression Rating, holds.  The Suppression Rating comparisons are the current state-of-the-art, and the most “apples to apples” sound suppression performance comparisons that currently exist.  Further research is ongoing.

PEW Science Research Note 6: 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 85 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 7: The closing time of the AR15 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.

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 8: Note that the muzzle Suppression Rating of the Otter Creek Labs Infinity with solid 5.56 end cap is 39.9 and the at-ear Suppression Rating is 23.0; different zones on the Suppression Rating Dose Chart. The lower back pressure of the Infinity with solid 5.56 end cap does contribute to a less severe ejection port blast signature than silencers like the Otter Creek Labs Polonium or Polonium-K, but the back pressure is not low enough to substantially reduce the ejection port blast loads to the shooter on the standard untuned M4A1 weapon system. An end cap change may be required for the user’s particular weapon system.  And, accordingly, weapon tuning may influence hazard reduction efficacy, and is outside the scope of this study.

The signatures measured at the shooter’s ear are presented and analyzed in the Member Version of this report.

The evaluation in this section covered the performance of the silencer with high early-time flow rate and restricted / relatively low late-time flow rate. By changing the end cap to the 5.56 vented configuration, late-time flow rate is elevated; the blast load is able to further vent from the primary bore and also now vent from the annular space. Test results and analysis of that configuration are presented in the following section.

6.196.3 Otter Creek Labs Infinity Vented 5.56 End Cap Sound Signature Test Results

A summary of the principal Silencer Sound Standard performance metrics of the Otter Creek Labs Infinity with vented 5.56 end cap is shown in Table 2. 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 public PEW Science testing, research, and development with a membership, here. State-of-the-art public firearm sound signature testing and research conducted by PEW Science is supported by readers like you.

 

Table 2. Otter Creek Labs Infinity Vented 5.56 End Cap Sound Metric Summary

 

6.196.3.1 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 Otter Creek Labs Infinity with vented 5.56 end cap are shown in Figure 6a. The sound signatures of Shot 1 and Shot 2 are shown in Figure 6b, in early time. The real sound impulse (momentum transfer potential) histories from the same 6-shot test are shown in Figure 7a. In Figure 7b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shots 2 and 3.

Fig 6a. Otter Creek Labs Infinity Vented 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Pressure Signature

Fig 6b. Otter Creek Labs Infinity Vented 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Sound Pressure Signature

Fig 7a. Otter Creek Labs Infinity Vented 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Impulse Signature

Fig 7b. Otter Creek Labs Infinity Vented 5.56 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Impulse Signature

As was observed in previous Infinity 7.62 NATO testing and MK18 testing with a vented end cap change, when fitted with the vented 5.56 end cap the performance of the Otter Creek Labs Infinity changes significantly.  The varying stages of the silencer are utilized differently, and blast loads are now vented from both annular space and additional primary axial volume.  Utilizing annular venting in the Infinity design allows it to take more advantage of the longer barrel length of the M4.  The following is observed in the above data plots:

  1. More expedient blowdown, with ground reflection beginning to decouple (Fig. 6a); this is somewhat similar to with the MK18, though again with faster BCG velocity.

  2. More expedient and higher amplitude initial jetting with a more defined initial positive phase, signifying greater momentum propagation (Fig. 6b).  When compared to the MK18, the M4 lets the vented 5.56 end cap configuration of the Infinity provide more consistency in early load propagation.

  3. Faster rise to maximum positive phase blast load impulse (lower Omega Metric) in all shots (Fig. 7a).  FRP divergence is less severe on the M4 than on the MK18 and the previously observed latent accumulation in Shot 2 is forced to early time.

As previously discussed in the 7.62 NATO analysis, latent propagation inconsistency is relatively common when excess blast load vent area is introduced into a primary flow path.  It is likely that this adverse erratic combustion propagation originates from the ancillary venting of the primary bore.  The fact that this phenomenon decreases with an increased barrel length with this configuration of the Infinity indicates a very high degree of pressure sensitive annular utilization.  This further reinforces the performance plateaus with the solid 5.56 end cap configuration.  When changing to the vented 5.56 end cap from the solid 5.56 end cap on the M4, the muzzle (bystander) Suppression Rating increases by half a category compared with vented 5.56 end cap performance on the MK18.  See Figure 1 in Section 6.196.1.  This is a significant performance change driven by barrel length increase for this particular silencer in this particular configuration.

PEW Science Research Note 10:  The silencer has now changed function; the Infinity with a vented end cap is no longer a low-alpha, moderate-omega hybrid- it has been transformed into a low-alpha, low-omega hybrid.  While many positive attributes of its hybrid design are maintained, higher gas velocity and momentum propagation begin to push its performance envelope into a regime in which both bystander and operator risk may be compromised when using short barrels.  However, now on the 14.5-in M4, the muzzle (bystander) Suppression Rating does not drop as significantly. Operator protection does somewhat drop, but that is a result of the weapon’s gas system, as previously discussed.

This end cap change, and subsequent flow rate change resulting from different utilization of primary axial bore and annular blast load propagation, results in standard untuned 5.56 MK18 performance metrics that move the Infinity hybrid design into contention with systems such as the dedicated 5.56 Surefire SOCOM556-RC2 (6.128) on this host weapon.  The 5.56 vented Infinity approaches the operator protection (shooter’s ear Suppression Rating) of the 556-RC2, while nominally exceeding the bystander protection (muzzle Suppression Rating) of the 556-RC2.  This is a significant achievement for a 30 caliber silencer.

When compared with advanced hybrid designs like the CAT WB (6.129), the vented 5.56 end cap configuration of the Otter Creek Labs Infinity is competitive, but does fall short in raw performance, in consistency, and in size; the Infinity is much larger than the WB.  Nonetheless, the performance of the Infinity in this regard is notable.  When compared to the 5.56 PTR VENT Spiritus 556i (6.175), Dillon DRC556 (6.194), and BOE Mod 1 (6.183), the vented Otter Creek Labs Infinity is eclipsed in overall performance, but approaches the free field operator hazard reduction potential of the Spiritus.

The signatures measured at the shooter’s ear are presented and analyzed in the Member Version of this report.

The evaluation in this section covered the performance of the silencer with high early-time flow rate and elevated late-time flow rate. By changing the end cap from the 5.56 vented configuration to the 7.62 vented configuration, late-time flow rate is even further elevated; the blast load is now able to significantly vent from the primary bore in addition to the ancillary venting from annular space. Test results and analysis of that final configuration are presented in the following section.

6.196.4 Otter Creek Labs Infinity Vented 7.62 End Cap Sound Signature Test Results

A summary of the principal Silencer Sound Standard performance metrics of the Otter Creek Labs Infinity with vented 7.62 end cap is shown in Table 3. 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 public PEW Science testing, research, and development with a membership, here. State-of-the-art public firearm sound signature testing and research conducted by PEW Science is supported by readers like you.

 

Table 3. Otter Creek Labs Infinity Vented 7.62 End Cap Sound Metric Summary

 

6.196.4.1 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 Otter Creek Labs Infinity with vented 7.62 end cap are shown in Figure 10a. The sound signatures of Shot 1, Shot 2, and Shot 3 are shown in Figure 10b, in early time. The real sound impulse (momentum transfer potential) histories from the same 6-shot test are shown in Figure 11a. In Figure 11b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shots 2 and 3.

Fig 10a. Otter Creek Labs Infinity Vented 7.62 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Pressure Signature

Fig 10b. Otter Creek Labs Infinity Vented 7.62 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Pressure Signature

Fig 11a. Otter Creek Labs Infinity Vented 7.62 End Cap 5.56x45mm NATO M4A1 Automatic Rifle Muzzle Sound Impulse Signature

Fig 11b. Otter Creek Labs Infinity Vented 7.62 End Cap 5.56x45mm NATO M4A1Automatic Rifle Muzzle Sound Impulse Signature

As was observed above in the vented 5.56 end cap testing, and in the vented 7.62 end cap testing showcased in the 7.62 NATO evaluation, when fitted with the vented 7.62 end cap, the 5.56 suppression performance of the Otter Creek Labs Infinity changes significantly but less so on the M4 than it did on the MK18.  This is discussed below.

PEW Science Research Note 12: In addition to annular venting and ancillary axial bore venting, the primary bore vent itself is now enlarged. This is the highest vent area configuration of the Infinity and provides the highest distal flow rate possible.  And, as discussed in the PEW Science Silencer Sound Standard research pedigree ad nauseum, over-bore as a mechanism of flow rate increase (back pressure reduction) is seldom efficient.  However, one of the most significant findings of this study is the decrease in uncontrolled combustion propagation with the vented 7.62 end cap on the 14.5-in barrel M4 compared with that on the 10.3-in barrel MK18. On the MK18, significantly severe jetting manifesting as high amplitude blast overpressure was observed.  As shown with the M4 in Figure 10 of this report, the behavior has been mitigated by the barrel length increase.  This mitigation is further observed in the more controlled blast load impulse accumulation displayed in Figure 11.

PEW Science Research Note 13: The muzzle (bystander) Suppression Rating of the vented 7.62 end cap Infinity on the 14.5-in M4 jumps to 35.5 from the 25.4 achieved on the MK18. This is a significant change in behavior and highlights, again, the input pressure sensitivity of the annular flow paths of the Infinity’s design.  Vented end cap configurations of the Infinity are significantly more efficient with decreased blast load input.  This is not necessarily the case for its solid end cap configurations.

The performance increase with the M4 over that of the MK18 with this end cap configuration of the Infinity is very notable and a very significant finding. Such behavior provides insight into future design changes that may pay significant dividends in performance for varying blast load input parameters.

The signatures measured at the shooter’s ear are presented and analyzed in the Member Version of this report. Gross comparisons on the M4 system are provided below.

6.196.5 Suppression Rating Comparison (5.56x45mm from the MK18)

Figure 14 presents a comparison of the PEW Science Suppression Rating of the Otter Creek Labs Infinity, in three different end cap configurations, with that of several other silencers on the M4A1 weapon system. The standard PEW Science M4A1 test host weapon system is described in Public Research Supplement 6.127.

Figure 14. Suppression Rating Comparisons of the Otter Creek Labs Infinity and the Dillon DRC556 and the CAT TSWIF, Sylvan Arms SA223TIS, BOE Mod 1, Echo Zulu Defense X1R556-Ti, PTR VENT Spiritus 556i, HUXWRX FLOW 556 Ti, KAC QDC/CRS-PRT, KAC QDC/MCQ-PRT, Surefire SOCOM556-MIN2, CAT WB, and SOCOM556-RC2 with different mounts, Using PEW-SOFT 5.56x45mm Supersonic Test Data and PEW Science Analysis

Figure 14 presents an overall summary of the postulated hazards to the operator and bystanders when fielding an Otter Creek Labs Infinity with three of its different end cap configurations on the M4A1 weapon system with the included direct thread mount (shown in red).  Data is also presented for the fielding case of the Dillon DRC556 (6.194), CAT TSWIF (6.193), Sylvan Arms SA223TIS (6.187),  BOE Mod 1 (6.183), Echo Zulu Defense X1R556-Ti (6.181), PTR VENT Spiritus 556i (6.176), HUXWRX FLOW 556 Ti (6.168), Surefire SOCOM556-RC2 (6.128), SOCOM556-MINI2 (6.144), and KAC QDC PRT units (6.156) on different muzzle devices, along with fielding of the CAT WB (6.130).  Hazards are expressed with the Suppression Rating; a holistic parameter that captures human inner ear damage risk potential from a measured impulsive complex 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.  The PEW Science Suppression Rating is a damage risk criterion (DRC), a lower Suppression Rating indicates a higher personnel hazard in the free field - it is not a subjective quantity; it is an objective quantification of hearing damage risk potential.  Due to its true free field test data origin and complete waveform analytical calculation basis, it is the only known universal suppressed weapon system signature comparison metric available.

The following gross conclusions can be made from the above:

  1. The Otter Creek Labs Infinity with solid 5.56 end cap produces free field bystander hazard reduction on par with that of high performing designs like the CAT WB (6.130), HUXWRX FLOW 556 Ti (6.167), and Echo Zulu Defense X1R556-Ti (6.181).  This is a significant achievement, given that the Infinity is a 7.62 silencer.

  2. The free field operator (shooter) hazard reduction of the Otter Creek Labs Infinity with solid 5.56 end cap is competitive in operator protection, but bested by some other hybrid designs and flow-through designs, such as the aforementioned HUXWRX FLOW 556 Ti and CAT WB, as well as by the PTR VENT Spiritus 556i (6.175).  Nonetheless, the holistic performance evolution of the Infinity is notable. 

  3. When utilizing the vented 5.56 end cap, the Otter Creek Labs Infinity provides more balanced performance on the standard untuned 5.56 M4 weapon system.  In this configuration, the Infinity is competitive with the dedicated 5.56 Surefire SOCOM556-RC2 (6.128) on this host weapon.  The 5.56 vented Infinity approaches the operator protection (shooter’s ear Suppression Rating) of the 556-RC2, while nominally exceeding the bystander protection (muzzle Suppression Rating) of the 556-RC2.  This is a significant achievement for a 30 caliber silencer.  However, the performance of the vented 5.56 end cap configuration of the Otter Creek Labs Infinity is surpassed holistically by other aforementioned hybrid designs.

  4. The Otter Creek Labs Infinity can also utilize the vented 7.62 end cap on a 5.56 automatic rifle. In this chosen configuration on the M4, the Infinity will exhibit much more consistent combustion propagation than was observed on the MK18, and the performance can again be described as balanced. The user may experience even lower “gas blow back” than with the vented 5.56 end cap with little blast load hazard increase on this untuned weapon system.  The performance increase with the M4 over that of the MK18 with this end cap configuration of the Infinity is very notable and a very significant finding. Such behavior provides insight into future design changes that may pay significant dividends in performance for varying blast load input parameters.

  5. As discussed above and in preceding reports, the Otter Creek Labs Infinity has highly variable performance on a short barrel AR-15 weapon system, based on end cap selection. It is recommended that the user select the configuration appropriate for their needs, based on desired level of protection for the operator and bystanders, compatible with dynamics of their particular weapon system.  It is likely that once the user chooses an appropriate configuration for their use case, the performance achieved will be significantly higher than legacy designs.

PEW Science Research Note 14: The muzzle Suppression Rating of the Otter Creek Labs Infinity with solid 5.56 end cap is in the top 7 of the PEW Science laboratory M4A1 Rankings, and the shooter’s ear Suppression Rating of the Infinity with the vented 5.56 end cap, characterizing operator risk in the free field, is entering the current top 5 in the Rankings.  Overall, the adaptability and versatility of the Infinity configurations represents a step forward in the Otter Creek Labs design pedigree, and places their new generation hybrid design among many top performing models.  PEW Science highly encourages the reader to examine the previous 7.62x51mm NATO bolt-action Infinity analytical report (6.178) and 5.56x45mm NATO 10.3-in MK18 Infinity analytical report (6.179), in which the performance of the silencer with its multiple end caps is examined in both a higher-volume combustion regime and longer barrel length blast load input scenario.

As detailed in this report, 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. 

Small arm weapon system suppression performance is a spectrum. The PEW Science Suppression Rating and the Silencer Sound Standard help quantify this spectrum for end users and industry, objectively.

6.196.6 Review Summary: Otter Creek Labs Infinity on the MK18 5.56x45mm AR15 with 10.3-in Barrel

When paired with the 14.5-in barrel M4A1 and fired with Federal XM193 with the solid 5.56 end cap, the the Otter Creek Labs Infinity achieved a Suppression Rating™ of 40.1 in PEW Science testing.  With the vented 5.56 end cap, the Otter Creek Labs Infinity achieved a Suppression Rating of 37.8.  With the vented 7.62 end cap, the Otter Creek Labs Infinity achieved a Suppression Rating of 36.1.

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 Infinity is a severe-use mid-size 30 caliber rifle silencer with an advanced but flexible hybrid design.  In the role of a 5.56 rifle silencer, it is considered full-size.  The silencer is user-configurable for different blast load flow states and allows the operator to adapt the system to a variety of host weapons.  With a high early-time flow rate, the silencer may perform well on semiautomatic platforms even in its most restrictive configuration.  For sensitive platforms, the user may fully vent blast loads from multiple stages of the silencer, lowering back pressure to a minimum for this design.  The Infinity is 3D-printed from Inconel alloy and is expected to offer significant durability for hard use. The vented end cap configurations of the Infinity produce performance that scales well with increasing barrel lengths.

The Otter Creek Labs Infinity is a 30 caliber rifle silencer that is classified as a hybrid design in the PEW Science research taxonomy.  More specifically, the Infinity is classified as a staged hybrid.  Hybrid design silencers are those in which elements of both conventional and high flow rate silencers are used, coupled with other technology variations and staged elements to provide parametrically varied performance attributes. Such attributes include the ability to reduce early-time shock reflections while significantly altering the rate of proximal and distal venting. These silencers may allow for minimal reciprocating weapon functional influence, while at the same time significantly suppressing signature to the operator and bystanders. Hybrid designs span a large range of the market in both brand and performance in different combustion regimes.

The Otter Creek Labs Infinity with solid 5.56 end cap produces free field bystander hazard reduction on par with that of high performing designs like the CAT WB, HUXWRX FLOW 556 Ti, and Echo Zulu Defense X1R556-Ti.  This is a significant achievement, given that the Infinity is a 7.62 silencer.

The free field operator (shooter) hazard reduction of the Otter Creek Labs Infinity with solid 5.56 end cap is competitive in operator protection, but bested by some other hybrid designs and flow-through designs, such as the aforementioned HUXWRX FLOW 556 Ti and CAT WB, as well as by the PTR VENT Spiritus 556i.  Nonetheless, the holistic performance evolution of the Infinity is notable. 

When utilizing the vented 5.56 end cap, the Otter Creek Labs Infinity provides more balanced performance on the standard untuned 5.56 M4 weapon system.  In this configuration, the Infinity is competitive with the dedicated 5.56 Surefire SOCOM556-RC2 on this host weapon.  The 5.56 vented Infinity approaches the operator protection (shooter’s ear Suppression Rating) of the 556-RC2, while nominally exceeding the bystander protection (muzzle Suppression Rating) of the 556-RC2.  This is a significant achievement for a 30 caliber silencer.  However, the performance of the vented 5.56 end cap configuration of the Otter Creek Labs Infinity is surpassed holistically by other aforementioned hybrid designs.

The Otter Creek Labs Infinity can also utilize the vented 7.62 end cap on a 5.56 automatic rifle. In this chosen configuration on the M4, the Infinity will exhibit much more consistent combustion propagation than was observed on the MK18, and the performance can again be described as balanced. The user may experience even lower “gas blow back” than with the vented 5.56 end cap with little blast load hazard increase on this untuned weapon system.  The performance increase with the M4 over that of the MK18 with this end cap configuration of the Infinity is very notable and a very significant finding. Such behavior provides insight into future design changes that may pay significant dividends in performance for varying blast load input parameters.

As discussed above and in preceding reports, the Otter Creek Labs Infinity has highly variable performance on a short barrel AR-15 weapon system, based on end cap selection. It is recommended that the user select the configuration appropriate for their needs, based on desired level of protection for the operator and bystanders, compatible with dynamics of their particular weapon system.  It is likely that once the user chooses an appropriate configuration for their use case, the performance achieved will be significantly higher than legacy designs.

The muzzle Suppression Rating of the Otter Creek Labs Infinity with solid 5.56 end cap is in the top 7 of the PEW Science laboratory M4A1 Rankings, and the shooter’s ear Suppression Rating of the Infinity with the vented 5.56 end cap, characterizing operator risk in the free field, is entering the current top 5 in the Rankings.  Overall, the adaptability and versatility of the Infinity configurations represents a step forward in the Otter Creek Labs design pedigree, and places their new generation hybrid design among many top performing models.  PEW Science highly encourages the reader to examine the previous 7.62x51mm NATO bolt-action Infinity analytical report (6.178) and 5.56x45mm NATO 10.3-in MK18 Infinity analytical report (6.179), in which the performance of the silencer with its multiple end caps is examined in both a higher-volume combustion regime and longer barrel length blast load input scenario.

The Otter Creek Labs Infinity retail package ships without a mount.  While this lowers cost and gives the user maximum flexibility, it also requires the user to remain diligent and be responsible for their mounting system choice. Appropriate mount for both practical field use case and blast load dynamics is paramount; the Infinity has a significant degree of early-time venting.  This beneficial performance trait may be compromised through the use of mounts that induce undue blast load reflection, such as compensators like the "Cherry Bomb” and others.  Use of such devices may increase the effective back pressure of a system by elevating the Alpha parameter; the early-time blast load impulse accumulation inside the first expansion chamber of the silencer.  PEW Science encourages the user to contact the silencer manufacturer for a recommended mounting system for their requisite application(s).

The silencer is primarily constructed from Inconel 725 alloy; users may subject the silencer to abusive firing schedules without significant damage. PEW Science has not performed durability testing or evaluation of the Infinity.  The user is encouraged to contact the manufacturer for use guidance.

In this review, the Otter Creek Labs Infinity 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.