SSS.6.78 - KAC QDSS-NT4 and the MK18 5.56x45mm Short Barrel Automatic AR15 Rifle (Free Version)
/KAC QDSS-NT4 on the MK18 5.56x45mm AR15 with 10.3-in Barrel
The QDSS-NT4 is manufactured by Knight’s Armament Company. It is a 223 caliber centerfire rifle silencer, intended to suppress the 5.56x45mm NATO cartridge. It has a 1.5-inch diameter and is 6.4 inches in length. The QDSS-NT4 possesses a crimped cone baffle core constructed from Inconel steel alloy, which is press-fit and intermittently welded to the primary tube. A secondary perforated steel girdle tube shrouds the primary tube. The silencer possesses an integral gate latch enabling mounting to the host weapon with the M4QD flash hider, the M4QD NT-4 MAMS muzzle brake, or the Triple Tap Compensator. A secondary rotating c-clip retains the gate latch. The silencer weighs 21.2 ounces and the M4QD flash hider weighs 2.3 ounces, for a total system weight of 23.5 ounces, as tested. Although it may be considered a legacy silencer, the QDSS-NT4 is still being produced. Inquiries for acquisition should be directed to Knight’s Armament Company.
This Sound Signature Review contains single-test results using the KAC QDSS-NT4 with the M4QD flash hider 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 6.78.1 contains the QDSS-NT4 test results and analysis.
Section 6.78.2 contains Suppression Rating comparisons of the QDSS-NT4 with dedicated 223 and 30 caliber silencers on the current market, including the Rugged Razor556, Otter Creek Labs Polonium, Surefire SOCOM556-RC2, HUXWRX HX-QD 556, Q Trash Panda, CGS Helios QD, SilencerCo Saker 556, Rugged Razor762, and others.
Section 6.78.3 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 KAC QDSS-NT4 mounted with the M4QD flash hider mount achieved a composite Suppression Rating™ of 29.1 in PEW Science testing. As with all weapon systems, the user is encouraged to examine both muzzle and ear Suppression Ratings.
Relative Suppression Rating Performance is Summarized in SSS.7 - PEW Science Rankings
6.78.1 KAC QDSS-NT4 Sound Signature Test Results
A summary of the principal Silencer Sound Standard performance metrics of the QDSS-NT4 tested with the M4QD flash hider 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. KAC QDSS-NT4 Sound Metric Summary
6.78.1.1 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 QDSS-NT4 are shown in Figure 1a. The sound signatures of Shot 1, Shot 2, and Shot 3 are shown in Figure 1b, in early time. The real sound impulse (momentum transfer potential) histories from the same 5-shot test are shown in Figure 2a. In Figure 2b, a shorter timescale is shown comparing the impulse of Shot 1 to that of Shot 2 and Shot 3.
Fig 1a. KAC QDSS-NT4 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Pressure Signature
Fig 1b. KAC QDSS-NT4 5.56x45mm NATO MK18 Automatic Rifle Sound Pressure Signature
Figure 2a. KAC QDSS-NT4 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
Figure 2b. KAC QDSS-NT4 5.56x45mm NATO MK18 Automatic Rifle Muzzle Sound Impulse Signature
The KAC QDSS-NT4 is a dedicated-bore 5.56x45mm silencer and exhibits some signature characteristics similar to that of the Surefire SOCOM556-RC2 (Review 6.52), and less similar to the SilencerCo Saker 556 (Review 6.53) and the Otter Creek Labs Polonium (Review 6.75). The behavior of the QDSS-NT4 shares some commonality with over-bored silencers exhibiting higher flow rates. This behavior, along with some other unique characteristics, is illustrated by the following key signature differences:
Unlike the Saker 556 and Polonium, the QDSS-NT4 exhibits greater direct-coupling of muzzle blast with bullet shock (Fig 1b).
Unlike the SOCOM556-RC2, the QDSS-NT4 produces more severe gas jetting in early time, which becomes progressively delayed during the shot string (Fig 1b).
Unlike all three of the aforementioned silencers, the QDSS-NT4 produces significantly steep initial positive phase impulse slopes after Shot 1, which gradually decay, and then significantly decrease during and after Shot 4 (Fig 2a).
PEW Science inner ear response analysis indicates that the silencer also gains suppression performance as the shot string progresses. The silencer has minimally perceptible first-round-pop (FRP) and the analysis indicates the silencer may not stabilize in signature suppression performance after the first shot until later shots in the string. The KAC QDSS-NT4 signature is more severe after the first shot and is measured to normalize after the fourth shot.
The above four differentiating signature characteristics of the KAC QDSS-NT4 indicate that its signature suppression performance may vary in the shot sequence to a degree that is different from the aforementioned silencers.
PEW Science Research Note 1: PEW Science postulates that one consequence of the high FRP suppression performance of the QDSS-NT4, in the manner achieved, is relatively unstable signature suppression performance early in the shot string. The suppression performance is measured to take longer to normalize than with the aforementioned silencers. PEW Science data and analysis indicates that the suppression of subsequent shot signatures may not normalize until at least four shots have been fired though the silencer. This phenomenon has occurred in multiple PEW Science tests of the KAC QDSS-NT4.
PEW Science Research Note 2: The variation in rate of rise to maximum initial positive phase impulse, later in the shot string, impacts the computation of the simplified PEW Science Back Pressure Metric, Omega. On this weapon system, the KAC QDSS-NT4 possesses an Omega Metric very similar to that of the Surefire SOCOM556-RC2, early in the shot string. Later in the shot string (after Shot 3 in this test) the QDSS-NT4 is measured to possess an Omega Metric that is higher, but not as high as possessed by silencers like the SilencerCo Saker 556 or Otter Creek Labs Polonium. The mechanism producing this change in Omega is ancillary combustion during Shot 2 and Shot 3, and is postulated not to be a change in flow rate.
PEW Science Research Note 3: Although not postulated to occur in this test, it is worth noting that dynamic back pressure increase was noted in the testing of the Rugged Razor556 (Review 6.76), (albeit not severely). The Razor556 possesses a relatively large and concentrated interstitial expansion volume (large single baffle spacing), combined with over-bore, which produces minor flow rate variation on this weapon system. PEW Science postulates that the KAC QDSS-NT4 limits complete combustion propagation through its interstitial baffle volumes during the first shot, which has the benefit of high FRP suppression performance but the detriment of possible ancillary combustion occurring in subsequent shots. The contrast in combustion dynamics between the two silencers highlights two volumetric baffle stack design differences and their drastically different effects; the sensitivity of the simple, large volume expansion chamber in the Razor556, and the higher performance of the complex geometric volumes in the QDSS-NT4 formed by the crimped cone baffles.
Multiple features may contribute to prolonged ancillary combustion in the QDSS-NT4, including non-uniform interface between the baffles and tube wall, as well as porting presence in the first baffles.
PEW Science Research Note 4: The duration over which the QDSS-NT4 depressurizes is longer compared to over-bored silencers for the platform, such as the CGS Helios QD (Review 6.66), and it is therefore more sensitive to so-called gas stack, the phenomenon in which a silencer’s flow rate is overwhelmed by pressurization during continuous fire. Adverse effects of gas stack are highly dependent upon rate of fire. This is a continued subject of internal PEW Science research.
The measured first-round-pop (FRP) from the QDSS-NT4 is visible in both the pressure regime (Figure 1b) and the impulse regime (Figure 2b), in peak amplitude, timing, and wave shape, as is typical from a suppressed rifle. Again, the FRP measured at the muzzle from the KAC QDSS-NT4 is postulated to not be immediately noticeable to bystanders, in accordance with PEW Science inner ear modeling.
PEW Science Research Note 5: 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 81 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 6: The closing time of the MK18 bolt is directly related to the flow restriction of a silencer for a given weapon system. PEW Science has determined bolt closing time variation from the unsuppressed state to be a reliable indicator of silencer back pressure, with strong correlation with the PEW Science Back Pressure Metric, Omega. However, PEW Science has also determined that the indicator is unreliable upon upper receiver fouling. Sound signatures are not influenced by this fouling, as these kinematics occur in late time, after gas venting to atmosphere. Momentum transfer, weapon condition (upper receiver fouling), and other factors, can significantly influence bolt closing time. PEW Science urges the reader to exercise extreme caution if using the published bolt closing time to make determinations regarding silencer flow restriction (back pressure) or weapon system kinematics. This type of calculation may provide erroneous results, as the weapon condition at the time of each test is not published data. The time-scale duration showing bolt closing time is only published by PEW Science such that the signature data pedigree may be verified.
The shape, timing, and magnitudes of the early-time pressure pulses and overall shape of the impulse waveforms measured at the muzzle, from shot-to-shot, are relatively consistent. The consistency of the waveform amplitudes highlight the silencer’s overall sound performance consistency at the muzzle after the FRP, as well as the relative consistency of the tested automatic rifle firearm configuration.
As typically indicated, first-round sound signatures always differ from subsequent shots, as the atmosphere within the silencer changes. The FRP phenomenon cannot always be shown by viewing only the peak sound pressure. This is one of the reasons why The Silencer Sound Standard requires examining multiple sound signature metrics. Ammunition consistency can play a role in the determination of FRP, however, the close examination of measured pressure and impulse waveforms typically excludes ammunition from the possible factors influencing true FRP, due to the relative consistency of most high quality factory ammunition.
PEW Science Research Note 7: Note that the muzzle Suppression Rating of the KAC QDSS-NT4 with the M4QD flash hider mount is 27.2 and the at-ear Suppression Rating is 23.2; the same zone on the Suppression Rating Dose Chart. The higher flow rate of the QDSS-NT4, when compared with that of some other dedicated bore 5.56x45mm silencers, allows it to produce a more balanced sound signature, between the muzzle and ear, on the standard MK18 weapon system. Regardless of this relatively balanced signature, as with the use of many silencers on the standard MK18, caution should be exercised by the weapon operator.
6.78.2 Suppression Rating Comparison (5.56x45mm from the MK18)
Figure 5 presents a comparison of the PEW Science Suppression Rating of the KAC QDSS-NT4 with the M4QD flash hider 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 KAC QDSS-NT4 and other silencers, Using PEW-SOFT 5.56x45mm Supersonic Test Data and PEW Science Analysis
The relatively balanced signature of the KAC QDSS-NT4 on the MK18 weapon system is apparent in the above data. The QDSS-NT4 exhibits a Suppression Rating at the shooter’s ear that is on-par with that of the Surefire SOCOM556-RC2 (6.52) and KGM R556 (6.60); both dedicated 5.56x45mm bore silencers with higher than average flow rates. The sound signature of the KAC QDSS-NT4, to bystanders, is on par with the HUXWRX HX-QD 556 (6.54) and Q Trash Panda (6.61). The Surefire SOCOM556-RC2 is significantly quieter to bystanders than the KAC QDSS-NT4, as reflected by the Suppression Rating at the muzzle.
Both the SilencerCo Saker 556 (6.53) and Otter Creek Labs Polonium (6.75) also exhibit a less severe signature to bystanders than the KAC QDSS-NT4, but at the expense of higher flow restriction (higher back pressure), which adversely impacts the signatures of those two silencers to the shooter on the MK18 weapon system.
PEW Science Research Note 9: The data in Figure 5 contains test results using silencer technology spanning multiple decades and represents a variety of methods by which signature reduction is possible. Four silencers stand out as performing most similarly to each other in sound signature on the MK18, despite possessing radically different technologies:
The HUXWRX HX-QD 556: a silencer with extremely high flow rate, with overall suppression performance dependent upon relatively high pressure and duration.
The KGM R556: a silencer that achieves a relatively high flow rate through significant distal porting and suppression through more conventional baffle technology.
The Q Trash Panda: an extremely simple silencer, geometrically, that is able to achieve balanced performance with short-barrel 5.56x45mm weapons through over-bore and volume, at the expense of erratic signature and flash.
The KAC QDSS-NT4: a legacy silencer that achieves performance comparable to modern designs using steel crimped cones to form the entirety of its baffle stack; imprecisely fit into a tube, resulting in differing wall offsets. This primitive construction, coupled with minor porting of the initial baffles, may be one of the reasons for its aforementioned delay in signature stability.
The balanced performance of these four silencers highlights the importance of both gross suppression and overall flow rate. 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.
6.78.3 Review Summary: KAC QDSS-NT4 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 KAC QDSS-NT4 mounted with the M4QD flash hider mount achieved a composite Suppression Rating™ of 29.1 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 KAC QDSS-NT4 is a full-size 5.56mm machine gun rated rifle silencer that possesses competitive sound signature suppression performance with many silencers on the current market, and achieves balanced performance on the MK18. The QDSS-NT4 may be considered a legacy design. The silencer’s mass is significant and possesses a total system weight of 23.5 ounces.
Despite the age of its crimped cone baffle design, the QDSS-NT4 exhibits sound suppression performance that meets or exceeds that of some 5.56x45mm silencers on the current market, on the MK18. The first-round-pop (FRP) suppression performance of the KAC QDSS-NT4 is particularly impressive. However, certain aspects of the legacy design may contribute to some performance detriments. For example, while the crimped cone baffles result in relatively efficient performance, their non-uniform fit within the tube, coupled with some initial porting, may result in erratic post-FRP signature that takes time to stabilize. This nuance is not a significant performance detractor overall, but in PEW Science’s opinion, is an artifact of a simple legacy design that was ahead of its time. The same geometric features that enable the silencer’s above-average performance bring challenges of their own. Regardless, for a silencer to be constructed in such a simple manner, and exhibit such longevity and proliferation, its place in silencer history is nontrivial.
Several design features of the KAC QDSS-NT4 are present due to final design of the system not being complete at the time of contract performance evaluations. For example, the perforated steel outer girdle tube is welded to the main tube in order to increase internal pressure capacity; an addition that was made after the silencer was developed, due to inadequate tube strength to resist the peak dynamic loads within the silencer at elevated temperatures. Another example of post-design retrofit is the c-clip retention mechanism for the gate latch. Without the c-clip, the so-called “tombstone” gate latch may become disengaged during use. Intended to be a stop-gap measure during performance evaluation to retain the latch, the c-clip remained in the design during final fielding of the silencer, and is present on production models to this day.
Mounting of the silencer to the various compatible muzzle device options is relatively simple. The gate latch easily secures the silencer to the mount. The c-clip, however, can be arduous to actuate for gate latch retention or disengagement. The user may find it easier to actuate the c-clip with a solid object like a brass casing, depending on the condition of the silencer. Operation by hand can often prove difficult. Users should note that there is significant rotational “play” in the assembly when the silencer is mounted. This is considered to be within nominal operational limits.
The QDSS-NT4 uses four crimped cone Inconel steel baffles. The baffle design has been scaled, both smaller and larger, for use in various Knight’s Armament silencer products through the company’s history. The silencer is extremely durable and may be used with aggressive semiautomatic and automatic firing schedules.
In this review, the QDSS-NT4 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.
