SSS.6.51- Research Supplement: Unsuppressed Rifles; the 10.3-in MK18 as a Silencer Test Host (Public Article)
/Unsuppressed Rifle Muzzle Blast Phenomena Compared: 20-inch 7.62mm NATO, 8-in 300 BLK, and 10.3-in 5.56mm NATO
Fig 1. PEW Science Suppression Rating Scale
Subsection 6.50 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 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). The previous 50 articles in Section 6 of the Standard have illustrated how different muzzle combustion pressure amplitudes, durations, and flow rates influence silencer sound suppression performance. The sound fields, as mapped by the measured muzzle and at-ear signatures, have been used to generate a database of suppressed small arm weapon system performance, summarized in Section 7 of the Standard.
PEW Science is continuing bolt-action 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 short-barrel automatic rifle chambered in 5.56x45mm NATO.
Section 6.51.1 contains comparisons of unsuppressed rifle muzzle blast.
Section 6.51.2 highlights rifle system silencer suppression performance factors.
Section 6.51.3 lists specific automatic rifle performance factors.
Section 6.51.4 provides PEW Science MK18 test host characteristics.
6.51.1 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, below:
Supersonic 7.62x51mm NATO (20-in barrel)
Subsonic 300 BLK (8-in barrel)
Supersonic 5.56x45mm NATO (10.3-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. There are significant differences in muzzle blast generated by these three cartridges. The free field blast waves, measured 1.0 m left of the bare weapon muzzles, are compared in Figure 2a for the three cartridges, 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 comparisons of muzzle blast combustion signatures between the three cartridges are 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.
The secondary blast wave generated by short-barrel 5.56x45mm NATO is severe. To illustrate the relative severity of this phenomenon, the reader is encouraged to examine the same blast loads 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 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: 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.
6.51.2 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 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.
6.51.3 Specific Automatic Rifle Performance Factors
The following performance factors may significantly influence the sound suppression performance of a silencer on the AR15 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.
6.51.4 PEW Science MK18 Test Host Characteristics
PEW Science conducted a public and private polling program in order to solicit user feedback for initial 5.56x45mm 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 the vendor Charlie’s Custom Clones. PEW Science manufactured its own lower receiver assembly with which to combine the MK18 upper receiver assembly for testing.
The 10.3-in MK18 automatic rifle test host used by PEW Science possesses the following characteristics:
Factory Daniel Defense MK18 upper receiver assembly:
Daniel Defense 10.3-in factory 5.56x45mm NATO barrel.
0.070-in diameter barrel gas port.
Daniel Defense fixed MK12 gas block.
Carbine-length gas system (standard gas tube).
Daniel Defense MIL-SPEC automatic bolt carrier group (BCG).
Daniel Defense standard charging handle.
PEW Science select-fire AR15 lower receiver assembly:
H2 buffer mass.
Standard carbine buffer spring.
MIL-SPEC carbine-length receiver extension.
Magpul MOE SL-K buttstock placed in extension position 4 of 6.
Geissele Super Select Fire (SSF) select-fire trigger group.
Colt M16 autosear, bushing, and spring.
Magpul MOE-K2 pistol grip.
Unless otherwise stated, all data generated, analyzed, and published by PEW Science with the MK18 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.
