SSS.6.200- Research Supplement: Unsuppressed 7.62×39mm AK-Pattern Rifles as Silencer Test Hosts (Public Article)

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Unsuppressed 7.62×39mm AK-Pattern Rifle Muzzle Blast Phenomena Compared: Zastava ZPAPM70 (16.3-in barrel), Arsenal SAM7R (16.3-in barrel), and Arsenal SAM7C (12.6-in barrel)

Fig 1. PEW Science Suppression Rating Scale

Subsection 6.199 and previous Subsections of the Silencer Sound Standard have presented sound signature suppression behavior of silencer products with a multitude of small arm weapon systems. Published data and analysis has been generated with:

As demonstrated, 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 breech 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; AK-pattern rifles chambered in 7.62×39mm.

  • Section 6.200.1 contains comparisons of unsuppressed AK-pattern rifle muzzle blast.

  • Section 6.200.2 highlights AK-pattern rifle system silencer suppression performance factors.

  • Section 6.200.3 lists specific AK-pattern rifle performance factors.

  • Section 6.200.4 provides PEW Science AK-pattern rifle test host characteristics.

6.200.1 Unsuppressed Rifle Muzzle Blast Waveform Comparison

Unsuppressed .22LR muzzle blast waveforms from bolt-action rifles were introduced in Section 2 of the Standard. Further unsuppressed muzzle blast was characterized in:

This current supplement examines 7.62x39mm combustion propagation.  Like 5.56x45mm NATO and supersonic 300 BLK, 7.62x39mm is an intermediate rifle cartridge.

Three examples of early-time free field muzzle blast overpressure measurements are shown in Figure 2a, below. These examples were generated with Hornady American Gunner 123gr 7.62x39mm ammunition fired from the following test host weapons:

  1. Zastava M70, Yugoslavian (Serbian)-pattern stamped received AK-pattern rifle with 16.3-in barrel (bare muzzle)

  2. Arsenal SAM7R, Bulgarian-pattern milled received AK-pattern rifle with 16.3-in barrel (bare muzzle)

  3. Arsenal SAM7C, Bulgarian-pattern milled received AK-pattern rifle with 12.6-in barrel (bare muzzle)

When viewing the below data, it is important to note that the above imported rifles have bore specifications that differ from the U.S.-manufactured ammunition used in the tests:

  • CIP (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives) bore groove diameter is 0.312-in.

  • SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) bore groove diameter is 0.0309-in to 0.311-in; Hornady bullet measured as 0.310-in.

These differences, along with other factors, result in noted phenomena.

Fig 2a. Early-Time Unsuppressed AK-Pattern 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:

  • Despite being supersonic events in the fielded test systems, the ammunition generates precursor shocks prior to primary muzzle blast (between 29.25 ms and 29.5 ms, above).  It is proposed that these precursor events are different than those previously observed in the measured test data with subsonic 9x19mm combustion and subsonic 300 BLK combustion in Research Supplement 6.85 and early in the Silencer Sound Standard with .22LR rimfire bolt-action combustion. In those subsonic tests, this precursor is formed by air column compression.  In these supersonic 7.62x39mm tests, the precursors are postulated to instead be formed by the following combined factors:

    • Obturation delay from differing bore diameter and projectile diameter, inducing blow-by (see CIP and SAAMI specification, above)

    • Longer throat, or freebore, in the CIP-spec chambers, inducing further delay.

    It is important to note that the precursor events are not from the piston system of the AK-pattern rifle. In addition to their consistency in timing and amplitude with barrel-generated precursor shocks in literature (Klingenberg & Heimerl, 1992), PEW Science laboratory measurements of suppressed AK-pattern rifles show precursor suppression; a phenomenon that is not plausible for gas system-generated vented blast loads.

  • The primary blast from the two 16.3-in barrel systems are similar in peak pressure amplitude, positive phase duration, and wave shape.

  • The primary blast from the 12.6-in barrel system is of similar shape, but higher amplitude, and contains several secondary shocks, including significant secondary blast after 30.75 ms.

  • As is the case in systems in which truncated barrel length does not allow for adequate propellant burn time (short barrel 5.56 systems as previously examined), the significant secondary blast wave described above 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 more severe than the blow-by precursor(s).

To further illustrate the relative severity of the above 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 AK-Pattern Rifle Muzzle Blast Impulse, 1.0 m left of the Weapon Muzzle, Free Field

As stated above, when fired from shorter rifle barrels, 7.62x39mm ammunition is unable to consume the entirety of its fuel load.

PEW Science Research Note 1: The additive positive phase impulse from precursor flow (in this case blow-by) and external afterburning (a result of incomplete propellant combustion) are real, measured phenomena occurring from a bare barrel muzzle in the free field. When a silencer is attached to the barrel muzzle, it is reasonably postulated that blow-by and afterburning still occur. 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 16.3-in and 12.6-in 7.62x39mm 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 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.

As typically cautioned, the severity of rifle muzzle blast from these types of systems may significantly impact silencer system longevity, maintenance, and performance factors. Several suppressed rifle system performance factors are highlighted below.

6.200.2 Rifle System Silencer Suppression Performance Factors

The following performance factors influence the sound suppression performance of a silencer on a small arm rifle system:

  1. Muzzle blast combustion pressure.

  2. Muzzle blast combustion duration.

  3. The proximity of the primary blast source (silencer endcap) to the shooter’s position.

  4. Ancillary overpressure sources (ejection port or breech of an automatic weapon system, gas system vents, and potential mechanical shock).

  5. Silencer muzzle blast suppression performance.

  6. Silencer flow restriction and its interaction with the weapon system, influencing sources in (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 Research Parameter to aid the reader and industry in quantifying these phenomena.  Public education is further facilitated by laboratory staff on a weekly podcast.

6.200.3 Specific AK-Pattern Rifle Performance Factors

The following performance factors may significantly influence the sound suppression performance of a silencer on an AK-Pattern rifle small arm system, as perceived by the weapon system operator:

  1. All factors in the preceding section above.

  2. Bolt carrier mass.

  3. Receiver length, stock type, and stock position.

  4. Gas-system length.

  5. Gas port size.

  6. Gas piston characteristics (the rifles operate with a long-stroke gas piston system).

  7. Recoil system 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, ejection port or breech blast, piston system vent blast, and mechanical shock in early and late time all coalesce. All phenomena contribute to the shooter’s at-ear signature.

6.200.4 PEW Science AK-Pattern Rifle Test Host Characteristics

PEW Science conducted a public and private polling program in order to solicit user feedback for initial 7.62x39mm AK-pattern rifle suppressed weapon system testing. Two barrel lengths were selected due to significant community interest; the “standard” 16.3-in rifle and the 12.6-in carbine. Two factory Arsenal Bulgarian-import AK-pattern rifles were procured by PEW Science.  One factory Zastava Yugoslavian (Serbian)-import AK-pattern rifle was procured.  In similar fashion to previous test host systems selected with user feedback, the following conclusions resulted from the polling program:

  1. Factory AK-pattern rifles were preferred. Factory configurations allow for standardized result presentations from which the user can glean the maximum amount of data utility. End user modifications such as modified gas system pistons, and other operating system mechanical modifications, typically result in higher personnel risk reduction, due to lower amplitude breech blast or piston vent blast, or both. Therefore, test data and analysis presented with standard factory configurations should present a reasonable lower bound of system suppression performance, and therefore provide a reasonable upper bound of personnel risk, in the free field.

  2. Imported receiver models were preferred. While US-manufactured AK-pattern rifles exist, AK community enthusiasts in the U.S. market typically view Yugoslavian stamped variants and Bulgarian-pattern milled variants as reference standards, representing examples of quality and durability.  PEW Science recognizes that there are mechanical variations with AK-pattern rifle systems across brands and subtypes, as there are with any small arm weapon system type distributed worldwide.

  3. It is likely that PEW Science will maintain testing and public data presentation with the two Bulgarian-pattern (Arsenal) systems. As the Yugoslavian-pattern system was selected to investigate system variation and also per laboratory client request, it will remain in test inventory for future evaluation(s) as appropriate. Note it is likely that suppression performance and personnel risk, as quantified by the PEW Science Suppression Rating, will be similar for a given silencer between the 16.3-in barrel Bulgarian and Yugoslavian systems. However, the 12.6-in barrel system performance will most certainly be significantly different.

The three AK-pattern rifle test hosts used by PEW Science possesses the following characteristics:

Factory Zastava ZPAPM70 Rifle (Serbian import to U.S., procured directly from Zastava Arms USA):

  • 16.3-in 7.62x39mm barrel, cold hammer forged, chrome-lined

  • 1.5mm bulged trunnion stamped receiver, Yugoslavian pattern

  • Fixed wooden stock and furniture

Factory Arsenal SAM7R Rifle (Bulgarian import to U.S., procured directly from Arsenal, Inc.):

  • 16.3-in 7.62x39mm barrel, cold hammer forged, chrome-lined

  • Milled & forged steel, hot/hard die hammer forged blank, then milled; Bulgarian pattern

  • Fixed polymer stock and furniture

Factory Arsenal SAM7C Rifle (Bulgarian import to U.S., procured directly from Arsenal, Inc.):

  • 12.6-in 7.62x39mm barrel, cold hammer forged, chrome-lined

  • Milled & forged steel, hot/hard die hammer forged blank, then milled; Bulgarian pattern

  • CNC Warrior SAM7K tubular folding stock on 1913 rail, 8-in model

  • Polymer furniture

Unless otherwise stated, all data generated, analyzed, and published by PEW Science with AK-pattern host weapon systems is done so with the above weapon system configurations.

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.