SSS.6.85- Research Supplement: Unsuppressed Subguns; the HK SP5 and SP5K-PDW as Silencer Test Hosts (Public Article)
/Unsuppressed Muzzle Blast Phenomena Compared: 20-inch 7.62mm NATO, 8-in 300 BLK, 10.3-in 5.56mm NATO, and Subsonic 9x19mm from “PCCs” (Subguns)
Fig 1. PEW Science Suppression Rating Scale
Subsection 6.84 and previous Subsections of the Silencer Sound Standard have presented sound signature suppression behavior of silencer products with several small arm weapon systems. With the exception of semiautomatic handguns chambered in the 9x19mm cartridge (Subsections 6.6, 6.7, and 6.8), the remainder of published data and analysis has been generated using bolt-action weapon systems and reciprocating (automatic) rifle using the following cartridges:
Supersonic 7.62x51mm NATO
Supersonic 5.56x45mm 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).
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, additional weapon system variables have been introduced that influence the sound fields, as mapped by the measured muzzle and at-ear signatures. These sound fields continue to be incorporated into the database of suppressed small arm weapon system performance, summarized in Section 7 of the Standard.
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 so-called “pistol caliber carbine,” (PCC), or submachinegun, chambered in 9x19mm NATO.
Section 6.85.1 contains comparisons of unsuppressed subsonic 9x19mm muzzle blast from different barrel lengths, with other familiar unsuppressed small arm signatures.
Section 6.85.2 highlights PCC and submachinegun system silencer suppression performance factors.
Section 6.85.3 lists specific HK SP5, SP5K-PDW, MP5, and MP5-K performance factors.
Section 6.85.4 provides PEW Science HK SP5 and SP5K-PDW test host characteristics.
6.85.1 Unsuppressed Subsonic 9x19mm Muzzle Blast Waveform Comparison
The reciprocating system influence of an automatic submachinegun notwithstanding, the barrel length variation used when firing centerfire pistol cartridges can significantly influence both the measured precursor flow, subsequent pressure wave, and free field muzzle blast.
The muzzle blast signatures from five weapon systems are compared, below. The following were previously presented in Public Research Supplement 6.51:
Supersonic 7.62x51mm NATO (20-in barrel)
Subsonic 300 BLK (8-in barrel)
Supersonic 5.56x45mm NATO (10.3-in barrel)
This article introduces two more systems, now included in the Silencer Sound Standard:
Subsonic 9x19mm (8.9-in barrel)
Subsonic 9x19mm (5.8-in barrel)
7.62x51mm NATO is a full-powered rifle cartridge designed and intended for use in full-length rifle barrels. 300 BLK is an intermediate rifle cartridge that, when loaded subsonically, shares combustion similarities with pistol cartridges; it is designed and intended to be fired from short rifle barrels. The 5.56x45mm NATO round is an intermediate rifle cartridge that is not designed to be fired from short barrels. The 9x19mm NATO cartridge, when loaded subsonically, shares combustion similarities with subsonic 300 BLK and is intended to be fired from barrels of various length.
There are significant differences in muzzle blast and precursor flow generated by these five weapon system cartridges. The free field blast waves, measured 1.0 m left of the bare weapon muzzles, are compared in Figure 2a for the five systems, below (in the pressure regime).
Fig 2a. Early-Time Unsuppressed Rifle Muzzle Blast Pressure, 1.0 m left of the Weapon Muzzle, Free Field
The following are comparisons of muzzle blast combustion signatures between the three rifle cartridges from 6.51, noted from Figure 2a, above:
The precursor flow expelled from the 8-in 300 BLK barrel is visible. Precursor flow is not visible in the muzzle pressure signatures for the two supersonic rifle cartridges due to it coalescing with the blast wave. Note that the 300BLK waveform has been time-shifted such that its primary blast amplitude is easily compared.
The primary blast from 20-in barrel 7.62x51mm NATO and 10.3-in barrel 5.56x45mm NATO are coincidentally similar in both peak pressure amplitude, positive phase duration, and wave shape.
There is a significant secondary blast wave measured in the 10.3-in barrel 5.56x45mm NATO signature. 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, resulting in a peak amplitude almost as severe as the subsonic 300 BLK primary blast wave.
Additionally, below are comparisons of the combustion signatures between the three subsonic cartridge platforms:
The waveforms from the subsonic 9x19mm platforms, like the subsonic 300 BLK waveforms, have also been shifted in time such that their primary blast amplitudes are coincident, and easily compared. The precursor flow from the 8.9-in barrel subsonic 9mm platform is more severe than that from the subsonic 300 BLK platform due to the larger frontal area of the 9mm projectile moving more air than that of the 30 caliber projectile.
The precursor flow from the 5.8-in barrel subsonic 9mm platform is less severe than that from the 8.9-in barrel platform due to the precursor pressure wave prior to muzzle blast being velocity dependent, as the ammunition is identical (the 9mm bullet geometry is held constant). While both 9mm platforms are producing subsonic signatures, the longer barrel is still producing higher velocity, and therefore a higher pressure precursor pressure wave. The precursor flow from the shorter 9mm barrel is still more severe than the 300 BLK precursor flow due to projectile frontal area, despite its lower velocity.
Unlike in the case of the precursor pressure wave, the primary muzzle blast from both subsonic 9mm platforms are less severe than that from the subsonic 300 BLK platform.
It is important to note that the primary muzzle blast from the 5.8-in barrel 9mm platform is more severe than from the 8.9-in barrel 9mm platform. This is opposite to the relationship of precursor flow, as subsonic precursor flow is velocity dependent for a given projectile, whereas primary muzzle blast is combustion dependent.
The same five blast loads are presented again in Figure 2b, below (this time, in the impulse regime).
Fig 2b. Early-Time Unsuppressed Rifle Muzzle Blast Impulse, 1.0 m left of the Weapon Muzzle, Free Field
As stated above, when fired from a 10.3-in rifle barrel, 5.56x45mm NATO is unable to consume the entirety of its fuel load (propellant). The secondary blast wave generated by short-barrel 5.56x45mm NATO is severe. The secondary blast wave from the short 10.3-in barrel 5.56x45mm rifle results in a measured additive positive phase impulse that is almost 30% more severe than that measured from 20-in barrel 7.62x51mm NATO.
PEW Science Research Note 1: The additive positive phase impulse from external afterburning (a result of incomplete propellant combustion in the 10.3-in rifle barrel) 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.
Given the above, the severity of rifle muzzle blast from short barrels may significantly impact silencer system longevity, maintenance, and performance factors. Several suppressed rifle system performance factors are highlighted below.
PEW Science Research Note 2: The differing precursor pressure waves in the subsonic regime are not postulated to significantly influence silencer performance on PCC and submachine gun platforms. However, the primary muzzle blast may significantly influence total suppressed system performance. Some 9mm silencers may exhibit behavior that differs when subjected to varying amplitudes of muzzle blast. Furthermore, such silencer behavior has been shown in internal PEW Science testing to be nonlinear and a significantly influenced by silencer design. While some 9mm silencers may perform better when subjected to lower amplitude muzzle blast, other designs may favor a higher uncorking pressure. This phenomenon is the subject of continued PEW Science research.
6.85.2 PCC and Submachinegun System Silencer Suppression Performance Factors
The following performance factors influence the sound suppression performance of a silencer on a small arm automatic rifle or reciprocating PCC or subgun system:
Muzzle blast combustion pressure.
Muzzle blast combustion duration.
The proximity of the primary blast source (silencer endcap) to the shooter’s position.
Ancillary 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 and Omega Back Pressure Metric to aid the reader and industry in quantifying these phenomena.
6.85.3 Specific HK SP5, SP5K-PDW, MP5, and MP5-K Performance Factors
The following performance factors may significantly influence the sound suppression performance of a silencer on the HK MP5 automatic submachinegun system, as perceived by the weapon system operator:
All factors in the preceding section above.
Carrier mass.
Carrier travel distance.
Locking piece angle.
Roller diameter.
Trunion wear.
Measured bolt gap which is a function of (4), (5), and (6).
Stock length and stock position.
Recoil spring assembly 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.
6.85.4 PEW Science HK SP5 and SP5K-PDW Test Host Characteristics
PEW Science conducted a public and private polling program in order to solicit user feedback for initial 9x19mm PCC and subgun suppressed weapon system testing. The HK MP5 weapon system was selected due to significant community interest, its wide proliferation throughout the history of small arms, and its well known favor as a platform host for small arm pistol caliber silencers. A factory Heckler and Koch SP5 and a factory Heckler and Koch SP5K-PDW were purchased by PEW Science directly from Heckler and Koch (HK). The HK SP5 and SP5K-PDW weapon systems are the civilian semiautomatic analogs to the HK MP5 and MP5-K submachineguns.
The 8.9-in barrel HK SP5 and 5.8-in barrel HK SP5K-PDW subgun test hosts used by PEW Science possesses the following characteristics:
HK SP5:
Standard 8.9-in barrel (integrated 3-lug mount and 1/2-28tpi RH threading).
HK A2 fixed stock.
Standard SP5 bolt carrier group and recoil spring assembly.
HK 80-deg locking piece (atypical for the MP5/SP5).
Measured bolt gap within HK specifications.
HK SP5K-PDW:
Standard 5.8-in barrel (integrated 3-lug mount and 1/2-28tpi RH threading).
B&T folding stock.
Standard SP5K-PDW bolt carrier group and recoil spring assembly.
HK 80-deg locking piece (typical for the MP5-K/SP5K-PDW when suppressed).
Measured bolt gap within HK specifications.
Unless otherwise stated, all data generated, analyzed, and published by PEW Science with the SP5 and SP5K-PDW host weapon systems is done so with the above weapon system configurations. Note that the weapons function with the 80-deg locking piece, unsuppressed, and are therefore used in that configuration for all PEW Science testing.
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.
