SSS.6.127- Research Supplement: Unsuppressed Rifles; the 14.5-in M4A1 Mid-Gas as a Silencer Test Host (Public Article)
/Unsuppressed Rifle Muzzle Blast Phenomena Compared: 20-inch 7.62mm NATO, 8-in 300 BLK, 10.3-in 5.56mm NATO, and 14.5-in 5.56mm NATO
Fig 1. PEW Science Suppression Rating Scale
Subsection 6.126 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 subguns chambered in the 9x19mm cartridge, the MK18 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).
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 automatic subgun signatures in Public Research Supplement 6.85, 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 14.5-in M4A1 Mid-Gas automatic rifle chambered in 5.56x45mm NATO.
Section 6.127.1 contains comparisons of unsuppressed rifle muzzle blast.
Section 6.127.2 highlights rifle system silencer suppression performance factors.
Section 6.127.3 lists specific automatic rifle performance factors.
Section 6.127.4 provides PEW Science M4A1 test host characteristics.
6.127.1 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 and 14.5-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 peak pressure amplitude, positive phase duration, and wave shape. The primary blast from the 14.5-in barrel 5.56x45mm NATO is of slightly less amplitude and positive phase duration, with a similar wave shape.
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, resulting in a peak amplitude almost as severe as the subsonic 300 BLK primary blast wave.
The secondary blast wave generated by both 10.3-in and 14.5-in 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 shorter rifle barrels, 5.56x45mm NATO is unable to consume the entirety of its fuel load. The secondary blast wave from the 5.56x45mm rifle barrels result in measured early-time additive positive phase impulse that is still 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) 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 10.3-in and 14.5-in 5.56x45mm barrels result in varying suppressed small arm weapon system signatures. Different silencer designs produce different external signatures with varied overpressure input history. Therefore, PEW Science postulates, with a high degree of confidence, that suppressed small arm signatures from these two 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.
Given the above, the severity of rifle muzzle blast from even 14.5-in barrels may significantly impact silencer system longevity, maintenance, and performance factors. Several suppressed rifle system performance factors are highlighted below.
6.127.2 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. Public education is further facilitated by laboratory staff on a weekly podcast.
6.127.3 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.
6.127.4 PEW Science M4A1 Mid-Gas 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 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 configuration of that test host was presented in Public Research Supplement 6.51.
PEW Science repeated the above exercise for the next 5.56x45mm semiautomatic rifle host system. This public and private polling program was conducted over a time period greater than one year. Significant feedback was received. Two primary conclusions resulted from the polling program:
The 14.5-in barrel length provides the greatest degree of data utility, in conjunction with the 10.3-in barrel length. 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 slightly conservative when compared with those from a 16-in system. Finally, 14.5-in and 16-in systems are more likely to be suppressed than, for example, 20-in barrel AR15 systems, due to overall length and handling characteristics with an installed silencer.
The mid-length gas system length provides the greatest degree of data utility and future applicability, when compared to that of the carbine-length gas system for a 14.5-in barrel AR15. PEW Science met with several rifle manufacturers, including Daniel Defense, Aero Precision, LMT, and others. The weapon manufacturers made it clear that the shorter dwell time of the mid-length gas system, when compared to that of the carbine-length gas system, was the future for 14.5-in barrel AR15 weapon procurement. PEW Science recognizes that at the time of this article publication, the carbine-length gas system is fielded in greater numbers than the mid-length gas system. However, as the dwell time from a mid-length gas 14.5-in barrel is still longer than that from a carbine-length gas 10.3-in MK18 barrel, PEW Science made the decision to continue with the more modern mid-length gas system choice due to the propensity of suppressed system ejection port blast to still be high. During internal laboratory research, PEW Science has determined that silencers with significant back pressure may produce more hazardous ejection port blast with a 14.5-in barrel mid-length gas system than with a 10.3-in barrel carbine-length gas system. It is postulated that 14.5-in barrel carbine-length gas systems may produce even more severe ejection port blast with such silencers. This phenomenon notwithstanding, the mid-length gas system will most likely produce the greatest degree of future data utility.
The 14.5-in M4A1 automatic rifle test host used by PEW Science possesses the following characteristics:
Factory Daniel Defense M4A1RIII upper receiver assembly (procured directly from Daniel Defense):
Daniel Defense 14.5-in factory 5.56x45mm NATO barrel.
0.076-in diameter barrel gas port.
Daniel Defense fixed MK12 gas block.
Mid-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 M4A1 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.
