SSS.6.210- Research Supplement: Unsuppressed Rifles; the 14.5-in KAC SR-25 as a Silencer Test Host (Public Article)
/Unsuppressed Rifle Muzzle Blast Phenomena Compared: 20-inch 7.62mm NATO, 10.3-in 5.56mm NATO, 14.5-in 5.56mm NATO, and 14.5-in 7.62mm NATO
Fig 1. PEW Science Suppression Rating Scale
Subsection 6.209 and previous subsections of the Silencer Sound Standard have presented sound signature suppression behavior of silencer products with several small arm weapon systems. Published data and analysis has been generated with semiautomatic handguns and submachineguns chambered in the 9x19mm and .22LR cartridges, the MK18 automatic rifle and M4A1 automatic rifle chambered in 5.56x45mm, as well as with bolt-action weapon systems using the following cartridges:
Supersonic 7.62x51mm NATO
Supersonic 6.5 CM
Subsonic 300 BLK
Subsonic .22LR
Bolt-action weapons allow for the elimination of variables to study pure sound signature suppression phenomena. Other than sound transmission through the weapon system itself, there is one primary source of overpressure to atmosphere (the bare muzzle or silencer endcap, if equipped).
Reciprocating (automatic) weapons introduce additional sources of overpressure to atmosphere, namely from their breach during function, which is necessary for ejection. Since the introduction of automatic rifle signatures into the Standard in Public Research Supplement 6.51 and Public Research Supplement 6.127, and automatic subgun signatures in Public Research Supplement 6.85, additional weapon system variables have been introduced that influence the blast overpressure and sound fields, as mapped by the measured muzzle and at-ear signatures. These fields continue to be incorporated into the database of suppressed small arm weapon system performance, summarized in Section 7 of the Standard. Hazard Map Briefs are now presented in Section 8 of the Standard to give users a visual spatial depiction of small arm personnel hazards.
PEW Science is continuing the aforementioned bolt-action and automatic rifle weapon testing, analysis, and research. The purpose of this Public Research Supplement is to present a first-look at muzzle blast phenomenology for a new addition to the published host weapon systems in the Standard; the 14.5-in KAC SR-25 CC automatic rifle chambered in 7.62x51mm NATO.
Section 6.210.1 contains comparisons of unsuppressed rifle muzzle blast.
Section 6.210.2 highlights rifle system silencer suppression performance factors.
Section 6.210.3 lists specific automatic rifle performance factors.
Section 6.210.4 provides PEW Science SR-25 test host characteristics.
6.210.1 Unsuppressed Rifle Muzzle Blast Waveform Comparison
The reciprocating system influence of an automatic rifle notwithstanding, the barrel length variation used when firing centerfire rifle cartridges can significantly influence the measured free field muzzle blast.
The muzzle blast signatures from three rifle cartridges are compared on four platforms, below:
Supersonic 7.62x51mm NATO (20-in barrel, bolt-action)
Supersonic 5.56x45mm NATO (10.3-in barrel and 14.5-in barrel, semiautomatic)
Supersonic 7.62x51mm NATO (14.5-in barrel, semiautomatic)
7.62x51mm NATO is a full-powered rifle cartridge designed and intended for use in full-length rifle barrels. The 5.56x45mm NATO round is an intermediate rifle cartridge that is also intended to be fired from full-length rifle barrels. Nonetheless, shorter barrels are commonly used. There are significant differences in muzzle blast generated by rifle cartridges and intermediate rifle cartridges as barrel lengths are varied. The free field blast overpressure waves, measured 1.0 m left of the bare weapon muzzles, are compared in Figure 2a for the four cases, below (in the pressure regime). The same data is plotted in Figure 2b in pressure units of [psi].
Fig 2a. Early-Time Unsuppressed Rifle Muzzle Blast Pressure [Pa], 1.0 m left of the Weapon Muzzle, Free Field
Fig 2b. Early-Time Unsuppressed Rifle Muzzle Blast Pressure [PSI], 1.0 m left of the Weapon Muzzle, Free Field
The following comparisons of muzzle blast combustion signatures between the systems are noted from Figure 2, above:
The primary blast from 20-in barrel 7.62x51mm NATO and 10.3-in barrel 5.56x45mm NATO are coincidentally similar in peak pressure amplitude, positive phase duration, and wave shape. The primary blast from 14.5-in barrel 5.56x45mm NATO is of slightly less amplitude and positive phase duration, with a similar wave shape. The primary blast from 14.5-in barrel 7.62x51mm NATO is higher amplitude than from the other systems.
There is a significant secondary blast wave measured in both the 10.3-in barrel and 14.5-in barrel 5.56x45mm NATO signatures. This blast wave is produced by the rapid afterburning of fuel (propellant) not consumed during the projectile push through the gun barrel. Upon muzzle exit, this propellant contributes significantly to the measured overpressure. The secondary blast from the 14.5-in barrel 7.62x51mm NATO system occurs later in time (see below).
The variation in secondary combustion timing highlighted in (2) is due to the following factors:
The 7.62 NATO cartridge has significantly more propellant mass and generates a much higher volume of gas than the 5.56 NATO cartridge. In a 14.5-in barrel, there is a significant amount of residual high-pressure, unburnt, and fuel-rich gas remaining in the barrel at the moment of bullet uncorking. The time required for this larger mass of gas to physically exit the larger bore and expand into the environment is longer. The gas phase of the signature is extended in time.
Secondary combustion doesn’t happen instantaneously at the muzzle; it occurs once the expanding gas plume has decelerated and mixed with enough ambient oxygen-rich air to reach a combustible fuel-air ratio. The core of the jet from the 14.5-in 7.62 NATO system stays “fuel rich” longer, as it takes more time for the larger volume to expand, disperse, and mix. This places the ignition point physically further from the muzzle and therefore the blast wave arrives at the pressure sensor later in time.
The additional length of the 20-in 7.62 barrel allows the gas to expand and cool more significantly inside the bore; the pressure and temperature at exit are below the threshold required for auto re-ignition.
The same blast waves are presented below in Figure 3a and Figure 3b in the impulse regime, with blast impulse units of [Pa-ms] and [psi-ms], respectively.
Fig 3a. Early-Time Unsuppressed Rifle Muzzle Blast Impulse [PA-MS], 1.0 m left of the Weapon Muzzle, Free Field
Fig 3b. Early-Time Unsuppressed Rifle Muzzle Blast Impulse [PSI-MS], 1.0 m left of the Weapon Muzzle, Free Field
As stated above, when fired from shorter rifle barrels, both 5.56x45mm NATO and 7.62x51mm NATO are unable to consume the entirety of their fuel load. The secondary blast waves from the short barrels result in measured early-time additive positive phase impulse.
PEW Science Research Note 1: The additive positive phase impulse from external afterburning (a result of incomplete propellant combustion) is a real, measured phenomenon occurring from a bare barrel muzzle in the free field. When a silencer is attached to the barrel muzzle, it is reasonably postulated that afterburning still occurs. However, due to the confinement of the silencer blast chamber environment, the increase in blast impulse inside the silencer may occur earlier in time and thus may be even more significant than that measured in the free field. There are also secondary and tertiary barrel shock loads contributing to additive impulse both in the unsuppressed and suppressed state that should not be neglected in system analysis nor in system design.
PEW Science Research Note 2: It is likely that the differing blast pressure amplitudes and durations in both the positive and negative phase produced from different barrel lengths result in varying suppressed small arm weapon system signatures. Different silencer designs produce different external signatures when presented with varied blast overpressure input history. Therefore, PEW Science postulates, with a high degree of confidence, that suppressed small arm signatures from these hosts may differ for each silencer, and the relationships between suppression performance for two silencers on each host may not scale linearly. Research is ongoing.
PEW Science Research Note 3: The volumetric flow demand from the 14.5-in barrel 7.62 NATO system is significantly higher than from 5.56 NATO systems. This is expected to play a role in system back pressure phenomena, as well as ejection port blast phenomena.
Several suppressed rifle system performance factors are highlighted below.
6.210.2 Rifle System Silencer Suppression Performance Factors
The following performance factors influence the sound suppression performance of a silencer on a small arm rifle system:
Muzzle blast combustion pressure.
Muzzle blast combustion duration.
The proximity of the primary blast overpressure source (silencer endcap) to the shooter’s position.
Ancillary blast overpressure sources (ejection port of an automatic weapon system).
Silencer muzzle blast suppression performance.
Silencer flow restriction and its interaction with the weapon system, influencing (4).
As PEW Science continues to test, analyze, and publish data on suppressed small arm weapon systems, the reader is encouraged to remain mindful of the above six performance factors. Factors (1) and (2) are a function of the host weapon and ammunition. Factor (3) is a function of the silencer length and host weapon. Factor (4) is only a concern on semi- and automatic weapon systems. Factors (5) and (6) are silencer and host weapon dependent. PEW Science publishes the Suppression Rating, Omega Back Pressure Metric, and Hazard Maps to aid the reader and industry in quantifying these phenomena. Public education is further facilitated by laboratory staff on a weekly podcast.
6.210.3 Specific Automatic Rifle Performance Factors
The following performance factors may significantly influence the sound suppression performance of a silencer on an AR-15 or AR-10 automatic small arm rifle system, as perceived by the weapon system operator:
All factors in the preceding section above.
Buffer mass.
Receiver extension length and stock position.
Gas-system length.
Gas port size.
Bolt carrier mass.
Bolt lock mechanics.
Buffer spring resistance and stiffness.
All of the above factors influence the sound signature generated from the weapon system ejection port, measured at the shooter’s head position. Muzzle blast and ejection port blast coalesce. Both phenomena contribute to the shooter’s at-ear signature.
6.210.4 PEW Science KAC SR-25 CC Test Host Characteristics
PEW Science conducted a public and private polling program in order to solicit user feedback for initial 5.56x45mm MK18 short-barrel rifle suppressed weapon system testing. The MK18 weapon system was selected due to significant community interest. A factory Daniel Defense MK18 upper receiver assembly was purchased by PEW Science from a vendor. PEW Science manufactured its own lower receiver assembly with which to combine the MK18 upper receiver assembly for testing. The configuration of that test host was presented in Public Research Supplement 6.51.
The above exercise was repeated for the 14.5-in barrel 5.56x45mm NATO system and resulted in the selection of the Factory Daniel Defense M4A1RIII upper receiver assembly as presented in Public Research Supplement 6.127.
For the 7.62x51mm NATO semiautomatic test host selection, the following logic was used for selection:
The 14.5-in barrel length provides the greatest degree of data utility for 7.62 NATO, in conjunction with the existing 20-in barrel bolt-action 7.62 NATO rifle dataset. The 14.5-in length may be equipped with a “pinned-and-welded” (P&W) muzzle device to achieve a total 16-in barrel system length, thus removing the necessity for United States Federal registration of a Title II (NFA) firearm and confining regulatory compliance to that of a Title I (GCA) firearm. Furthermore, due to combustion phenomenology, it is likely that signatures from a 14.5-in system will be conservative when compared with those from a 16-in or 20-in system. Finally, 14.5-in and 16-in systems are more likely to be suppressed than, for example, 20-in barrel semiautomatic 7.62mm (AR-10) systems, due to overall length and handling characteristics with an installed silencer.
The KAC SR-25 is one of the longest running “standardized” AR-10-type automatic rifle systems in common use. Unlike the AR-15, there exist several “AR-10” and “AR-308” design platforms that possess components that are not interchangeable. In addition to receiver differences, there may be differences in bolt carrier groups (BGCs), recoil buffer and spring assemblies, receiver extension dimensions, gas system types, lengths, and dimensions, and other differences that are somewhat spurious to catalog and track across all the system variations on the market. The KAC SR-25 pattern magazines and receiver dimensions are, with industry-wide agreement, the most standardized “bridge” between original military use and the civilian market. The gas system of the SR-25, though proprietary, is non-adjustable. This is a positive attribute for a standard test host, as there is no gas system variation between tests.
The 14.5-in KAC SR-25 Combat Carbine (CC) automatic rifle test host used by PEW Science possesses the following characteristics:
Factory KAC SR-25 upper receiver assembly (procured directly from Knights Armament Company):
Complete KAC SR-25 CC system upper assembly (KAC Part Number 111468)
KAC 14.5-in factory 7.62x51mm NATO barrel.
KAC fixed propriety gas block
Mid-length gas system (KAC proprietary gas tube and fitting).
KAC enhanced SR-25 bolt carrier group (BCG).
KAC standard charging handle.
PEW Science KAC SR-25 lower receiver assembly (procured directly from Knights Armament Company):
Complete KAC SR-25 system lower assembly (KAC Part Number 116858)
KAC SR-25 carbine buffer mass.
KAC SR-25 carbine buffer spring.
KAC SR-25 carbine-length receiver extension.
Magpul MOE SL-K buttstock placed in extension position 3 of 8.
KAC A2 pistol grip.
Unless otherwise stated, all data generated, analyzed, and published by PEW Science with the KAC SR-25 host weapon system is done so with the above weapon system configuration.
This article is part of ongoing PEW Science suppressed small arm weapon system research. This research is funded by PEW Science Members. PEW Science thanks you for your support.
