\ ~ ,\ J 'I! DEPARTMENT OF THE ARMY FRANKFORD ARSENAL

Philadelphia, Po. 19137

SILENCERS PRINCIPLES AND EVALUATION ~.

REPORT R-1896 AUGUST 1968

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.1 and Cunctional descriptions and mestic and Coreign sil~ncer" and :luded are cross-sectional draw- . ) of all systems t~sted. An acoush· .ven in the form of far field sound r constituents oC sowld sigl¥tures with their sources in the system.

a record of silencing principles -arious noise generating phenom-

ii

r TABLE OF CONTENTS

INTRODUCTION. • . • • •

SILENCER PRINCIPLES. •

DESCRIPTION AND EVALUATIONS OF SILENCERS TESTED •.••..••••••••..••.

Caliber. ZZ Hi-standard Pistol/French Silencer •.

Caliber. ZZ Silenced Hi-standard Pistol . •

Caliber. ZZ Silenced AAI Experimental Test Fixture. • • • • • • • • . • • • • • •

Caliber. ZZ Hi-standard Pistol/FA Silencer.

Caliber. 30 M1903 Rifie/Maxim Silencer. Caliber. 30 Silenced Ml Carbine. • •

Caliber • 3Z Silenced Sleeve Gun. • •

Caliber • 3Z Silenced Welrod Pistol.

9 mm Silenced Welrod Pistol • • • •

9 mm Silenced Sten Submachine Gun •

Silenced Sten Barrel, Type I. Silenced Sten Barrel, Type n . . . .

9 mm P38 Walther Pistol/ AAI Silencer.

9 mm Walther MPK Submacblne Gun/West German Silencer. • • • • • • • • • • • •

Caliber • 45 Silenced M3 Subma~hine G\1Il • • • • • • • • • • • • • •

Bell Laboratories Silenced Barrel • AMF Silenced Barrel. • • • •

CONCL USIONS • • • • •

RECOMMENDATIONS. . . . . . APPENDIX A - Ballistic.: Crack Field. •

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Page No •

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7

10

16

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31

37

47

5Z

58

66 7Z

76 8Z

88

94

100

100 llZ

119 120-1 * lZ3

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by the background noise, presently there is no satisfactory means for predicting this masking effect. However. it is known that whe n the sound pressure level (SPL) of a continuous signal exceeds the SPL of background noise by more than 10 db, the eHects of baCkground noise, [or all practical purposes, can be neglected. If the same c rite rion is asswned for transient noises, then masking of the weap­on noise by the backgorund noise should become significant only when the baCkground sound pressure exceeds about a t hi r d of the weapon sound pressure. All Frankford Arsenal SOW1d traces wer e made with the baCkground noise well below this limit.

Caliber. 22 Hi_standard Pistol / French Silencer

The "French" silencer is a recently manufactured item, designed for a caliber. 22 or a Slightly larger caliber weapon. In all respects other than tha t its baUles are n ot perforated , thi s si l encer of French origi n i s ident i cal to a Parker-Hale "SoW1d M odera t o r " pre sently being manufac tured commercially in Britai n . History and exact origin o f the "French" silencer are unknown .

The all-metal silence r tested (Figures 1 and 2) was adapted t o a caliber. 22 Hi-standard semi-automatic pistol by threading the barrel. Inside, the silencer contained a series of metal baHles, spaced 0.43 inc h apart. The first baffle wa s located 2.31 inches from the gun barrel muzzle, pres\.UT'lably to reduce stresses on the bafHes and to provide an initial expansion c hamber. The projec tile passage diameter throughout the silencer was 0.28 inch. Outside, the silencer diame,ter and length wer e , respectively, 0.94 and 7.31 inches. Table I lists some of the more importan t phy s ical and func­tional parameters o f the Hi-standard pistol and the "French" si ­

lencer.

Figures 3 and 4 show scope traces of the sound pressure-time history of the pistol without the silencer firing a Long Rifle cartridge. Thetraceswererecorded five meters fr om and direcly t o the side o f the pistol muzzle. The three primary a c oustical effects - primer initiation, precursor s hock exit. and propellent gas bla s t ~ arc distinc tly visible on the traces. The highest sound impulse and peak sound pressure level (136 db) were due to the propellent gas discharge occ urring aft er the projectile exited the barrel. The peak sound pressure level due to exiting of the precursor shock was 113 db. The primer initiation sound pulse was approximately 98 db.

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TABLE I. Caliber. ZZ Hi-standard Pistol/French Silenc er

P rojectile Wei ght (Long Rifle) Diameter Velocity (at silencer exit) Ener gy (at silencer exit) Travel at peak ballistic pre88ur~ (e stimat e d) Travel in barr e l Travel time in ba rrel Travel time in silencer

Propellant Weight (double base, flake, web _ O. 003 in.)

Chamber volwn'e

Ballistic pressure Peak At barrel muzzle (estimated)

Silencer Passage diameter (for projectile) Weight Free volume

Piatol weight (without eilencer)

Time betwe en precursor and projectile exiU from s ilence r (estimated)

13

40 gr 0.225 in 1050 Ipo 98 It -Ib 0.4 inch 7 . a inch 0.65 rns (approx) 0 . 55 rna

1.7 gr (+0.2 gr prim e r)

0.016 in. 3

24, 000 psi I, 000 psi

0 . 28 in. 0.25 Ib 2.38 in. 3

2.75 lb

0.95 mo

171) B Jy OP

II~ dh \:~

The so und signature of a Hi-standard pistol with the French sile nce r and usi ng a L ong Rifle cartridge i s shown in Figur e 5. I n this c a se the sound pressur e s were s ubstantially lower than with the un s ilenced pistol. The first distinct sound perceived during the firing cycle was a pulse (pt I, Figure 5) generated about the time the firing pin hit the prim e r . Sinc e at this t ime four successive functions ... hammer fall. firing pin striking primer, primer explosion, and gas leakage aro und the cartridge case - occurred, the exact source of the first sound pulse is not definite. H owever, experiments with other systems indicate that, generally, by far the loudest pulses are generated by the gas leakag e around the cartridge case and by the hamme r fall . The firi ng pin striking the primer is generally s omewhat louder in weap-ons without a hammer.

The next sound after primer initiation was the precursor wave exiting £rom the silencer muzzle. Thi s sound pulse (pt 2) , be cause of its low amplitude , is barely distinguishable in the trace. Shor tly after t he precursor, the blow- by pres sure wave (generated by the leakage of prope ll ent gases past the projectile) exited. Exit of this pressure wave gave rise to a pulse of 117 dp peak SPL. The pro ­jectile exited the s ilencer 0.3 m illisecond after the b low-by wave . Its exit was followe d by the main efflux of gases, which resulted in the positive pul se of 11 9 dp peak SPL. Following the projectile exit and initial gas efflux , the s t ead y discharge of propellent gases ga ve rise to turbulence which, combined with reverberations wi thin the silencer, generated a pro longed noi se (pt 5) of approximately 105 db peak S PL.

Sound s i gnature o f the caliber. 22 Hi-standard pistol witho ut silenc er, firing a Short cartridge, is shown for reference in Figure 6.

Caliber. 22 Silenced Hi ... s tandard Pi etol

During World War II , the U. S. Infantry Board established interest in silenced weapons. 37 A variety of weapons , i ncluding the sile nced calibe r. 22 Hi-standard pistol (Figures 7, 8 . and 9 . and Table 11), were given consideration. It was c oncluded that all si ... lenced weapons were bulky and still detectable at close range s. Be ­cause o f low lethality, it is doubtful if the Hi-standard pistol described herein found very wide application.

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CD Precursor hit from Barrel L2 - 113 db

CD Projectile Exit from Barrel L3 '" 135 db

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Figure 6 . Sound Pressure-Time History, five meters to side, Caliber .22 Hi-standard Pistol (Unsilenced), Using Short Cartridge (29-gr projectile; velocity, 1050 fpl; peak pr88Bure. 16000 psi)

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Figure 7. CroBs section. Caliber. 22 Silenced Hi-stand a rd Pistol

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TABLE II. Caliber. 2~ Silenced Hi-standard Pistol

Projectile Weight (Long Rifle) Diameter Velocity (at silencer exit) Energy (at silencer exit) Travel at peak ballistic pressure (estimated) Travel in barrel Travel time in barrel Travel time in silencer

Propellant Weight (double base, flake, web _ O. 003 in.)

Chamber volwne

Ballistic pressure Peak At barre l muzzle (estimated)

Silencer Passage diame,ter (for projectile) Weight (excluding gun barrel and pistol) Free volume

Around gun barrel (including barrel hole s) In front of gun barrel

Wire mesh volume Rolled (around gun barrel) Discs (front of barrel)

GWl barrel and pistol weight

40 gr D. 225 in. 930 (ps 75 It_Ib 0.4 in. 6.2 in. O. 65 ms (approx) 0.22 ms

1.7 gr (+ O. Z gr primer)

0.016in. 3

24,000 psi 90 psi

0.234 in. 0.63 Ib

1. 84 in. 3 0.76 in. 3

0.79in.3 0.35in. 3

Z. 37 Ib

,3d. deoP

Silencing of the Hi-standard pistol has been accomplishe~ essentially by drilling the barrel and enclOSing it in a silencing tube. The barrel has four longitudinal rows of lIbleed" holes, 0.12S inch in diameter and spaced 0.2:50 inch apart. The primary function of the 'holes is reduction of the ballistic pressure which, in turn, also reduces the velocity of a lIupersonic Long Rifle cartridge below the speed of sound.

ZZ

The silencing tube surrounds and e barr e l. At the rear it is attached to the while at the front it terminate s with a thr tube contains a roll of brass wire mesh 1

a stack of wire mesh discs extending bey wire screening is presumably int e nded fe gases. Proj ectile passage through the fr tube is 0.234 inch in diameter. The sile length (beyond gun barrel) are, respecth

Figure 10 shows the sound pres sur Hi_standard pistol. The s ound trace, likf was taken five meters to the side of the from the scope trace, the pistol'smain s initiation (pt 1, Figure 10), bleed hole bI (pt 3). The primer initiation pulse , whi( propellent g':-s leakage around the cartri( of 98 db. The next sound pulse (pt 2) wa occurring thr ough the bleed hole s exite '

this sound p"lse had a relatively large i r. only 101 db. Shortly afte r, the blow-by barrel muzz.le, exited. This event occur with the projec tile exit. The combined t

discha.rge follOwing the projectile exit r e (pt 3) o( III db peak SPL. Following thi (pt 4) due to propellent ga.s discharge tt: within the silencer.

The magnitude of these sound pulsf In the majority of cases it was somewhal The general negative trend of sound pre ! was more consistent, representing the e gas discharge from the weapon. In gene sound scope tra ce of the silenced calibel with ita quiet performance.

Caliber. 22 Silenced AAl Experimen!

The AAl caliber. ZZ siienced te st , and Table 'III) was designed by Aircraft J

in 1965. The study was conducted as pa contract issued to investigate unconventi

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1. 7 gr (+ O. Z gr primer)

0.016 in. 3

Z4,000 psi 90 psi

O. Z34 in. 0.631b

1.84 in. 3 0.76 in. 3

0.79 in. 3 0.35 in. 3

Z. 37 Ib

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:lard pistol has been accomplishec1 rei and enclosing it in a silencing tube. 1al rows of "bleed" holes, 0.125 inch inch apart. The primary function of allistic pressure which, in turn, also rsonic Long Rifle cartridge below the

22

The silencing tube surrounds and extends beyond the pistol barrel. At the rear it is attached to the threaded receiver extenSion, while at the front it terminates with a threaded cap. Inside, the tube contains a roll of brass wire mesh surrounding the barrel and a stack of wire mesh discs extending beyond the barrel muzzle. The wire screening is presumably intended for cooling of the propellent gases. Projectile passage through the front portion of the silencing tube is 0.234 inch in diameter. The silencing tube diameter and length (beyond gun barrel) are, respectively, 1.0 and 2.5 in.

Figure 10 shows the sound pressure-time history of the silenced Hi-standard pistol. The sound trace, like that of the French silencer, was taken five meters to the side of the pistol muzzle. As can be seen from the scope trace, the pistol's main sound sources were: primer initiation (pt 1, Figure 10), bleed hole blow-by (pt Z). and projectile exi (pt 3). The primer initiation pulse, which was predominatly due to propellent gas leakage around the cartridge case, had a peak SPL of 98 db. The next sound pulse (pt 2) was generated when the blow-by occurring through the bleed holes exited from the muzzle. Although

this sound pulse had a relatively large impulse, its peak SPL was only 101 db. Shortly after, the blow-by wave, originated at the gun barrel muzzle, exited. This event occurred almost simultaneously with the projectile exit. The combined effect of blow-by and gas discharge following the projectile exit resulted in a positive pulse (pt 3) of 113 db peak SPL. Following this, several sound pulses occurred (pt 4) due to propellent gas discharge turbulence and reverberations within the silencer. '--

The magnitude of these sound pulses varied from round to round. In the majority of cases it wa. somewhat lower than that of Figure 10. The general negative trend of sound pressure after sound pulse (pt 3) was more consistent, representing the eventual decrease of propellent gas discharge from the weapon. In general, the relatively uncluttered sound scope trace of the silenced caliber. ZZ pistol correlated well with its quiet performance. .

Caliber. 22 Silenced AAI Experimental Test Fixture

The AAl caliber. ZZ silenced test fixture (Figures 11, IZ, 13, and Table III) was designed by Aircraft Armaments, Incorporated, in 1965. The study was conducted as part of a Frankford Arsenal contract issued to investigate unconventional use of small arms.

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sound pulse (pt Z) was generated when the precursor exited from the silencer. Its peak SPL was 105 db. The blow-by wave generated at the gun barrel muzzle exited from the silencer after the precursor. Thill blow-by pulse, with its peak SPL of 120 db, represented the system's loudest effect. The projectile exited about O. 2 ms after the blow~by wave. Gas flow at the silencer exit evidently was altered little by the prOjectile since the event is not discernable on the Bcope trace. This, of course, was to be expected with the large blow~hy clearances and volume. To a subjective listene r , the system sounded unquestion~ ably loud.

9 mm Walther MPK Suhmachine Gun/West German Silencer

The MPK silencer (Figures 56, 57, and 58) was r eportedly developed in West Germany s ometime after 1963, when the 9 mm MPK submachine gun itself was introduced. Although somewhat bulky and internally elaborate, it is ruggedlyconstructed, well finished, and readily adaptable to a standard MPK weapon. The construction o( the silencer, its size and acoustical performance suggest that it was built as an experimental model-shop item.

Internally, the MPK ailencer (Fi gure 56) consists of a 10 inch long by 2 inch diameter expansion chamber containing an elaborate assortment of geometrically complex tubes, vanes, and conea. Exc ept (or two steel end pieces and the steel turbine - like vanes, the silencer is made entirely of aluminum. Beginning at the rear, the silencer starts with a simple 1. 5 inch long expansion chamber, followed by a tubular spool , out aide_vaned at the rear and closed at the (ront. The steel vanes forming one end of the s pool are brazed to the aluminum tube . After the spool come two conea, an expansion chamber, and another cone,

All Frankford Arsenal sound tests on the MPK were made with special rounds loaded for subsonic velocities (Table Xll). The sound history of the MPK without the silencer is shown in Figure 59 . The weapon's main no18e, like that of most othe r unsilenced systems, was due to the precursor wave and the projectile exit blast. The two positive pulses five meters to the side of the MPK were, respectively. 131 and 140 db peak SPL.

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IIben tbe precursor exited (rom the db. The blow-by wave generated at m the silencer after the precursor • . t SPL of 120 db, represented the jectile exited about O. 2. mS after the ilencer exit evidently was altered little is not discernable on the scope trace. :ted with the large blow-by clearances tener, the system sounded unquestion-

:hine Gun/West German Silencer

°el 56, 57. and 58) was reportedly etime after 1963, when the 9 mm MPK !uced. Although somewhat bulky and · Jlyconstructed, well finished, and A.PK weapon. The construction of the performance suggest that it was built

ltem.

(lcer (Figure 56) consisU of a 10 inch m chamber containing an elaborate .lplex tubes, vanes, and cones. Except steel turbine ... like vanes, . the silencer Beginning at the rear, the silencer 19 expansion chamber, followed by a I rear and closed at the front. The the spool are brazed to the alwninwn coneS, an expansion chamber, and

ound tean on the MPK were made ublonic velocities (Table XW). The 11 the lilencer is ahown in Figure 59. hat of moat other unsilenced iY8tems. Ind the projectile exit blast. The two te .ide of the MPK were, respectively.

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TABLE XlII. 9 mm MPK Walther Submachine Gun/German Silenc er

Projectile Weight Diameter Velocity (at s ilencer exit) Energy (at silencer exit) Travel at peak ballistic pressure (estimated) Travel in barrel Travel time in barrel (estimated) Tr ave l time in 9ilencer

Propellant Weight (M9, double base, flake, web ..... O. 003 in. )

Chamber volwne

Ballistic pressure Peak At barrel muz.z.le (estimated) ¥

Silencer Passage diameter (for projectile)

Weight Free volwne

Unsi1enced submachine gun weight (without magazine)

Time between precursor and projectile exits [rom silencer (estimated)

98

115 gr O. 356 in. 1000 Cps 255 ft- Ib 0 . 4 in . 6. 32 in . 0.62 ms 0.79 mB

3 gr (+ 0.3 gr primer)

0.038 in. 3

22,400 psi 1,400 psi

0 . 41 in., 0 . 44 in. and 0.55 in. l. 5 Ib (approx) 15 . 0in. 3

6. Z lb.

0. 69 m.

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Although not visible in the trace of Figure 59, the MPK haa a relatively loud, prolonged, mechanical noise. This noise is due primarily to the forward slap of the bolt against the breech and consequent vibration of the weapon's mechanical components. The sound trace of the weapon, Cired dry, without a cartridge, is shown in Figure 60. The peak SPL of the MPK1s mechanical noise was approximately 106 db five meters from the weapon.

The sound history of the silenced MPK,firing a subsonic car­tridge,is shown in Figure 61. The ,ystem" first significant sound (pt I, Figure 61) corresponded to the time of primer initiation. The first po,itive pulse of this sound (approximately 106 db), five mete r s from the weapon, was due to the forward slap of the bolt against the gun breech. Approximately 0.2 ms after the bolt bottomed, there was a second positive pulse of 114 db peak SPL due to rearward gas leakage around the cartridge case. FollOwing prim er initiation came the first noise emitted from the silencer muzzle. Thi s was a pulse (pt Z) of 108 db peak SPL due to the exiting of th e precursor wave generated in gun barrel. The blow-by occurring in the s ilencer ex­ited from the muzzle approximately 0.4 rns later. This gave rise to a positive pulse (pt 3) of 116 db peak SPL. The pr ojectile exited about 0.4 m s later. This generated the system's large s t pulse - 118 db peak SPL. Following this there was a series of smaller random sound pulses due t o reververations inside the silencer.

Figure 62 shows. e s sentially, only the muzzle noise of the silenced MPK. The r ecord was obtained by wrapping the weapon with a suede coat. The effect can be seen most readily in the barely visible primer initiation noise (pt I, Figure 62) and the more clearly defined precursor pulse (pt Z).

In comparison to other systems, the silenced MPK sounded somewhat louder than the 9 mm Stens and quieter than the caliber. 32 and 9 mm Welrods. Olalitative1y, the MPK sound Signature could be described as a somewhat muffled clap, immediately followed by a diminishing hiss.

Caliber.45 Silenced M3 Submachine Gun (Figure 63)

Bell Laboratories Silenced Barrel

During World War II, Bell Laboratories investigated si­lencing of various weapons, including the caliber. 45 M3 submachine gun.

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Figure 74. Sound Pressure-Time History. five meters to aide, Ca11b~r .45 AMP Silenced M3 Submachine Cun Barrel in Test Fixture

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