AD

MATERIEL

COMMAND MEMORANDUM REPORT BRL-MR-3476

N AERODYNAMIC AND FLIGHT DYNAMIC,

CHARACTERISTICS OF THE NEW

FAMILY OF 5.56MM NATO AMMUNITION

Robert L. McCoy

October.1985 DTICscz' ~DECO 08~

tI.D.

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1. REPORT NUMBER 12. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

"MEMORANDUM REPORT BRL-MR-Nf7 ___-_________-__-__

4. TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED N.

AERODYNAMIC AND FLIGHT DYNAMIC CHARACTERISTICS .OF THE NEW FAMILY OF 5.56MM NATO AMMUNITION FINAL

" .. 6. PERFORMING ORG. REPGRT NUMBER

7. AUTHOR(.) 6. CONTRACT OR GRANT NUMSER(s)

ROBERT L. McCOY.. PERFORMING ORGANIZATION NAME AND ADDRESS W0. PROGRAM ELEMENT. PROJECT. YASK

U.S. Army Ballistic Research Laboratory AREA WORK UNIT NUMBER,ATTN: SLCBR-LFAberdeen Proving Ground, MD 21005 -5066 RDTE IW263607D640

ii. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

U.S. Army Ballistic Research Laboratory October 1985ATTN: SLCBR-DD-T 19. NUMBER PAGESAberdeen Proving Ground, MD 21005-5066 .,-

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Aerodynamic Characteristics Yaw Limit-Cycle

Aerodynamic Drag Accuracy.,Gyroscopic Stability

12t AIK!YXrACs?~~srv sl~ N eto, ea didwntityby block numbot.) (mf

Free flight spark photography range tests, Mann barrel accuracy tests, and long-range limit-cycie •aw tests were conducted with the U.S. M855/M856 ammunitions,and the Belgian (FNB) SS-109/L110 ammunition. Tolerances in bullet jacket wall"thickness and bullet seating alignment were identified as contributing to the

i dispersion problem in U.S. ammunition. The aerodynamic characterisitics of thefour projectiles were determined, and the gyroscopic stability was found to besufficient. All four projectiles are dynarmically stable at supersonic speeds.

(continued)"~~~" DO 7,•1•3 -nou• or 1 .ov. as is E• "'~xrD. 1473 Eot UNCLASSIFIED

, SECURIY CLASSIFICATION OF TNI% PAGE (•mem note Sneeed)

UNCLASSIFIEDSECURITY CLASSIFICATION OF TnIqS PAGt.(W. en LW.R Entoted)p-The M855/SS-109 projectiles •,how limit-cycle ,yaw levels rowing to approximatelyl

six dEgrees a% 800 metres r ,ye. \('i.,-7YK,- . .' -,

0 a-,.

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UNCLASSIFIEDSECUNITY CLASSIFICATION OF YWIS PAO M 3h.4g D "I floetdt

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"TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS ...................................... ... 5

LIST OF TABLES .... 7

I. INTRODUCTION ........... ......, 9..,,.......,,.,.., .-. 9

IeI TEST MATERIEL AND PROCEDURE......... ,.,. • • •. • •, •.o............. 9

III. ACCURACY FIRING TEST RESULTS...,e•..... • ................ ••• 11

IV. AEROBALLISTIC TEST RESULTS .................................... 11

A . D r a g C o e f f i c i e n t . . . . . , . , , , , . , , , . . , . . . , . , . 1 2

B. Overturning Moment Coefficient ................. 13

C. Gyroscopic Stability Factor........ ............. ,. 13

D. Lift Force Coefficient...1..,,.. 163E. Magnus Moment Coe iclent.,....,.,,.., 14

F. Pitch Damping Moment Coefficient.. ,.....,....,..... ,. 14

G. Damping Rates ....... .,,,... . .. ,,, 14

V. LIMIT-CYCLE YAW TEST RESULTS......... ................. ,.,..... 15

* ~~VI. COMMENTS ON AMMUNITION DISPERSION ...... ,................. 15

SVII. CONCLUSIONS AND RECOMMENDATIONS,.... ,. ,. ,.,. ,......,,,,. ....... 17

REFERENCES,......., 56

•LIST OF SYMBOLSe ........ 5

', ~ ~D I S T R I B U T I O N L I S T o o. . .. ... . . . . , . , , . , , o , , . , o .... . 6 1

Accesion [-orNTiS CRA&I," WIC TAG L

LU a 1, ic ed L l

. .. . ............

Di.t ib i ..... ,*-•--.--.Av. .. ~ Y",ds "

- Avui;,;:,ity 'is

• Dit 12,W 11 r"!, U ',

3'Il

LIST OF ILLUSTRATIONS •-"-

Figure Page

1 1 PHOTOGRAPH OF THE MANN BARREL AND FRANKFORD REST ................. 19

2 SKETCH OF SS-109 AND M855 BALL PROJECTILES ........................ 20

3 SKETCH OF Ll10 AND M856 TRACER PROJECTILES.... ................... 21

4 SHADOWGRAPHS OF M855 AND SS-109 PROJECTILES AT MACH 1.9.,... 4.. 22

5 SHADOWGRAPHS OF M855 AND SS-109 PROJECTILES AT MACH 1.2 ........... 23

6 SHADOWGRAPHS OF M855 AND SS-109 PROJECTILES AT MACH 0.9........... 24

7 SHADOWGRAPHS OF M856 AND Ll10 PROJECTILES AT MACH .9............. 25

8 SHADOWGRAPH OF L110 PROJECTILE AT MACH 1.4.......... 26

9 ZERO-YAW DRAG FORCE COEFFICIENT VERSUS MACH NUMBER, LM855/SS-109....,..... ............ ..... ... ......... ........ .... ..... 27,...

1 10 ZERO-YAW DRAG FORCE COEFFICIENT VERSUS MACH NUMBER,

M856/LIO.. M........ ........................T......T...E...... ...... 28

11 OVERTURNING MOMENT COEFFICIENT VERSUS MACH NUMBER,• M855/SS-109 ............................ 29 •

,'12 OVERTURNING MOETCOEFFICIENT VERSUS MACH NUMBER, .'

"M856/L ,......... ........ .. ...................................... 30

13 GYROSCOPTC STABILITY FACTOR VERSUS MACH NUMBER, M855/SS-109 ....... 31

14 GYROSCOPIC STABILITY FACTOR VERSUS MACH NUMBER, M856/L110......... 325 LT F E .- '

"15 LIFT FORCE COEFFICIENT VERSUS MACH NUMBER, M855/SS-09............. 33

17 16 LIFT FORCE COEFFICIENT VERSUS MACH NUMBER, M856/LSS ............. 345

* 17 MAGNUS MOMENT COEFFICIENT VERSUS MACH NUMBER, M855/SS-I09 ......... 35

" 18 MAGNUS MOMENT COEFFICIENT VERSUS MACH NUMBER, M856/L11O....6 ... "3

* 19 PITCH DAMPING MOMENT COEFFICIENT VERSUS MACH NUMBER,.M855/ S-109 ....................................................... 37

20 PITCH DAMPING MOMENT COEFFICIENT VERSUS MACH NUMBER,3M856/8110......................................................... 38

21 FAST ARM DAMPING RATE VERSUS MACH NUMBER, M855/SS-.109 ......... ,.., 39 __

22 SLOW ARM DAMPING RATE VERSUS MACH NUMBER, M855/SS-109........... 40

5

"LIST OF ILLUSTRATIONS (Continued)

"Figure Page

23 FAST ARM DAMPING RATE VERSUS MACH NUMBER, M856/L1IO ............... 41

24 SLOW ARM DAMPING RATE VERSUS MACH NUMBER, M856/L11O............... 42

25 PHOTOGRAPH OF LIMIT-CYCLE YAW TEST EQUIPMENT ...................... 43

"26 STRIKING VELOCITY AND STRIKING YAW VERSUS RANGE, SS-109........... 44

27 STRIKING VELOCITY AND STRIKING YAW VERSUS RANGE; M855............. 45

"28 STRIKING VELOCITY AND STRIKING YAW VERSUS RANGE, M856/L11O........ 46

29 STRIKING VELOCITY AND STRIKING YAW VERSUS RANGE, SS-109,FROM THE XM249EI WEAPON ........... 47

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F:?6S.

LIST OF TABLES

Table Page L1 AVERAGE PHYSICAL CHARACTERISTICS OF 5.56MM NATO-:' ~PROJECTILES ........................... ,........,... 48

2 ACCURACY FIRING TEST RESULTS ...................... .... .......... * 9 *"

3 AERODYNAMIC COEFFICIENTS OF 5.56MM NATO BALL PROJECTILES ........... 50

"4 AERODYNAMIC COEFFICIENTS OF 5.56MM NATO TRACER PROJtCTILES ......... 51

5 FLIGHT MOTION PARAMETERS OF 5.56MM NATO BALL PROJECTILES ........... 52

6 FLIGHT MOTION PARAMETERS OF 5.56MM NATO TRACER PROJECTILES......... 53

7 LIMIT-CYCLE YAW TEST RESULTS ........................... , ........... 54

8 DISPERSION SENSITIVITY FACTORS FOR 5.56MM AMMUNITION ............... 55

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I. INTRODUCTION

On 24 March 1983, the author attended a meeting with personnel of the, Squad Automatic Weapon (SAW) Project Office at Dover, New Jersey. The subject

of the meeting was the unacceptably high lot rejection rates of early produc-1 | tion 5.56mm M855 (Ball) and M856 (Tracer) ammunition manufactured at Lake City

A;niy Ammunition Plant (LCAAP). The rejected lots failed to meet tVe accuracyspecification, and LCAAP had indicated to the SAW Project Offire '.hat theybelieved the government-furnished test barrels might be contributing tu theproblem.

The result of the meeting was a joint recommendation, by the BallisticResearch Laboratory (BRL) and the SAW Project Office, to conduct a three-parttest at the BRL free-flight range facility. The first part of the test was tobe an accuracy check, using the Kart-manufactured barrels supplied to LCAAP.The accuracy test was to include rejected lots of M855/M856, control lots ofthe Belgian (FNB) counterpart SS-109/LI1O ammunitions, and handloaded ammuni-tion using 52 grain Sierra Benchrest bullets, in both Lake City cartridgecases, and commercial match grade cases. All accuracy firing was to be doneat 100 yards, in the BRL Aerodynamics Range. 1 The second part of the testconsisted of aeroballistic range firings to determine the aerodynamic andflight characteristics of the LCAAP and FNB ammunitions, using down-loadedpropellant charges to simulate ranges out to 800 metres. The third test phasewas a real-range determination of striking velocity and limit-cycle yaw forthe four ammunition types, using the limit-cycle test equipment in the BRLTransonic Range.

Test materiel and funding were provided to BRL by the SAW Project Office,and the first phase of testing began on 31 May 1983. The accuracy tests werecompleted on 27 June 1983, and the second part of the test schedule was con-

* ducted in September-October 1983. The third phase, limit-cycle yaw testingwas conducted in March 1984. This report covers the results from all threephases of the SRL tests.

II. TEST MATERIEL AND PROCEDURE

,O Two of thu Kart accuracy barrels, chambered and threaded to fit aRemington M700 action, were supplied to the BRL by the SAW Office, The Kartbarrel serial numbers were 014 and 018, and both barrels had previously beenin use at LCAAP; these two barrels were among those suspected by LCAAP person-nel of contributing to the accuracy problem. One goal of the Phase I testingwas to determine which of these two barrels was the more accurate, and select

S it for the remainder of the tests.

W1 . F. Braun, "'The Fr'ee FZight Aeocyiinio.a Range, " Ballistic ResoalchLaboratores, Aberdeen Proving Ground, Vav•,yand, BRL Repo•,t No, 1048,

SAugust 1958. (AD 202249)

9

Figure 1 is a photograph of the accuracy test set-up. The barrelledaction is mounted in a Frankford rest, wilh a Leupold M8, 16X scope to checkreturn-to-battery alignment between shots. By using a systematic procedurefor returning the recoil cradle to battery, it was determined that shot-to-shot scope alignment within 1 1/16" at 100 yards range could be nmaintained.

The U.S. lot acceptance accuracy specification for 5.56mm NATO Ballammunition is as follows: the mean radius of each group of ten shots shallnot exceed 2 inches at 200 yards. Tracer ammunition must meet a less strin-gent requirement; less than 4 inch mean radius at the same range. For the BRL100 yard indoor firings, the required accuracy criteria translate to 1-inchmean radius (14R), for the M855 and SS-109, and 2-inch MR for the M856 and L110Tracers.

Initial accuracy testing during Phase I showed Kart barrel number 014 togive slightly smaller round-to-round dispersion than did barrel 018; hence,barrel 014 was used for all subsequent BRL testing. The Phase II tests were £conducted at four Mach numbers; 2.75, 1.9, 1.1, and 0.7, which roughly corre-spond to ranges of zero (muzzle), 300 metres, 700 metres, and 1000 metres,respectively, for the Ball ammunition. Ten data rounds of each typ'. werefired, for a total forty round test program; of these, thirty-six roundsyielded useful aerodynamic and flight performance data. A half-muzzle typeyaw inducer was used on some of the Mach 1.1 and Mach 0.7 firings, in anattempt to determine any significant aerodynamic non-linearities at transonicand subsonic speeds. All Phase II aeroballistic tests were fired in the BRLAerodynamics Range, using the same Phase I weapon mounting shown in Figure 1.

- The Phase III testing also used the weapon mounting system of Figure 1,but with the gun moved to one of the three firing positions at the BRL Tran-

. sonic Range facility. The real-range limit-cycle yaw tests were conducted atranges of 300, 600, and 800 metres; ten data rounds of each ammunition typewere fired at the 300 and 600 metre ranges, and fifteen data rounds of eachtype were fired at the 800 metre range. Unfortunately, the tracer projectilesfogged the photcgraphic film at 800 metres range, due to the long residence"time of the subsonic bullets over the instrumentation; hence, no tracer limit-cycle data were obtained at 800 metres.

Figures 2 and 3 show sketches of the U.S. and FNB Ball bullets, and theSU.S. and FNB tracers, respectively. The sketches reflect bullet contour

measurements, made on the BRL Mann optical comparator. Table 1. lists theaverage measured physical characteristics of the four projectile types.

Selected prints of spark shadowgraphs of the four bullet types, from thelimit-cycle yaw tests, are shown in Figures 4 through 8. Figure 4 shows theflowfield around the i4855 and SS-109 projectiles at 300 metres range :.(Mo 1.9), figure 5 shows similar shadowgraphs for the same projectiles at600 metres (Mo . 1.2) and figure 6 shows 800 metre results (M - 0.9). Figure7 is a comparison of flowfields at 300 metres range (M,,, 1.9= for the M856and LI1O tracers, and figure 8 shows a 600 metre shadowgraph (14 1.4) forthe Li1O tracer projectile. No 600 metre data were obtained for the M856tracer, due to the extremely large anununition dispersion at that range.

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III. ACCURACY FIRING TEST RESULTS

The ammunition provided to the BRL for testing consisted of Ball, SS-109,Lot FNB-83A-O01-O01, and Tracer, LIlO, Lot FNB-83A-O01-O01, for the Belgian-produced rounds; and Ball, XM855E1, Lot LC-83E300-5230, plus Tracer, XM856EI,Lot LC-83E300-5231, for the U.S. production ammunition. In addition to theammunition provided by the SAW Office, the BRL procured, through open commer-cial sources, a sufficient quantity of Federal .223 Remington unprimed cases"(LOT 8A-2132), CCI #450 small rifle primers, Sierra, .224", 52 grain Benchrestbullets, and IMR 3031 propellant (LOT 232AA09A), for use in checking the accu-"racy of the Kart test barrels.

The accuracy firing test results are summarized in Table 2. All groups"fired were ten-shot groups, with one exception, as noted. The accuracyfirings showed that Kart barrel No. 014 was slightly superior to No. 018,based on the performance with 52 grain Sierra Hollow Point Boattail (HPBT)bullets, although both barrels showed very small dispersion with the SierraHPBT bullets. The FNB manufactured SS-109 Ball and LI1O Tracer ammunitionalso easily met accuracy requirements, from either Kart barrel tested.

The first test fired with LCAAP produced M855 Ball ammunition, from Kartbarrel No. 014, failed to meet specifications. The only group fired withLCAAP manufactured M856 Tracer ammunition did meet the accuracy requirement.Additional testing was performed, in which the M855 bulletc were pulled, thenloaded in commercial Federal cases, first with IMR 3031 piropellant, then withthe standard charge of LCAAP propellant. Both groups met the accuracyrequirement. Finally, the Sierra HPBT bullets were loaded in LCAAP primedcases, with IMR 3031 propellant, and the results duplicated the commercialcase results, which suggested that the LCAAP catridge case was not a signifi-cant contributor to the accuracy problem.

Although the BRL accuracy firing test results are based on small samplesizes, the indications are that the act of pulling the M855 bullet and reseat-ing it into the cartridge case, using straight-line seating dies, improved itsperformance markedly. Neither the cartridge case nor the propellant used

*- appeared to have any significant effect on dispersion.

' The conclusions reached from the accuracy test firings were (1) The Kart-manufactured Mann test barrels were nct a significant contributor to cheobserved M855/M856 dispersion problem, (2) The lot of M855 Ball ammunitiontested failed to meet accuracy specifications, (3) The bullet seating opera-tion ir LCAAP ammunition assembly was at least part of the cause of theobserved M855 dispersion.

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"IV. AEROBALLISTIC TEST RESULTS

"The free-flight spark photography range data were fitted to solutions ofS.the linearized equations of motion and these results used to infer linearized

Sm

k; , 3".

aerodynamic coefficients, using the methods of Reference 2. The actualprojectile aerodynamic force-moment system often is not strictly linear.Given sufficient data the actual non-linear behavior can be determined fromthe range results. 3 For the four 5.56mm NATO projectiles, sufficient datawere obtained to permit determination of the effect of yaw level on the dragcoefficient; no statistically significant values of the non-linear terms couldbe found for any of the transverse aerodynamic force or moment coefficients. K',

The round-by-round aerodynamic data obtained are listed in Tables 3 and4, and the measured flight motion parameters are given ir Wbles 5 and 6.Three rounds of SS-109 ammunition previously fired in an Z•aermeyer Mann barrelare also included in Tables 3 and 5.

A. Drag Coefficient -

The drag coefficient, CD, is determined by fit'ting the time-distance

measurements from the range flight. CD is distinctly non-linear with yaw

level, and the value determined from an individual flight reflects both thezero-yaw drag coefficient, CD, and the induced drag due to the average yaw

0level of the flight. The drag coefficient var'ation is expressed as an evenpower series in yaw amplitude:

Co 0 D+ 662 +

where C0 is the zero-yaw drag coefficient, C0 is the quadratic yaw-drag00 D62

coefficient, and 62 is the total angle of attack squared.

Analysis of the SS-109 and M855 drag coefficient data showed the tworounds to have essentially equal yaw-drag characteristics, but significantlydifferent zero-yaw drag levels. Values of CD 7.0 at supersonic speeds,

D62and CD = 9.8 at subsonic speeds were used to correct the range data to

* * zero-yaw conditions, for both the SS-109 and the M855. Figure 9 shows the"variation of the zero-yaw drag coefficient, CO , with Mach number for the two

0projectiles. The M855 design has about 8% more drag than the SS-109 at

*Q supersonic speeds, and this difference increases to approximately 20% at* subsonic speeds. Since the M855 and SS-109 projectiles do riot reach subsonic

speeds except at ranges beyond 800 metres, the effect of the subsonic dragdifference between the two designs would be observed only at extremely longranges.

2. C. 11. Murphy, "Data Reduction for the Free Flight Spark Ranges,"Ballistic Research Laboratories, Aberdeen Proving Ground, Maryland,BRL Report No. 900, February 1954. (AD 35833)

3. C. H. Murphy, "The Measnurement of Non-Linear Forces and Moments by Meansof Free Flight Tests," Ballistic RHsearch Laboratories, Aberdeen ProvingGround, Marylana, BRL Report No. 974, February 1956. (AD 93521)

0o

Figure 10 is a-similar plot of the zero-yaw drag coefficients for the4M856 and LI10 tracer projectiles. A least squares fit of the range data

yielded the values CD = 5.7 at supersonic speeds, and C 6.4 at

subsonic speeds, for both tracer projectiles. Note that all tracers wereessentially nnn-burning over the 50 metre flight observed in the BRL Aero-dynamics Range, thus Figure 10 represents the drag coefficient behavior ofnon-burning tracers. Figure 10 also shows that the M856 and the Ll10 tracershave essentially identical zero-yaw drag, although thE larger observed round-to-round variation in trace;- drag coefficient could mask a slight differencein average drag level. The larger round-to-round scatter in Figure 10 isprimarily due to the ogival boattail shape of the tracer bullets; minor round-to-round variations in surface finish along an ogival boattail lead to a vari-able boundary layer separation point, which in turn leads to larger than usual

* round-to-round variations in base drag.

B. Overturning Moment Coefficient

The range measured overturning moment coefficienit, CM is plotted

against Mach number in Figure 11, for the Ball projectiles, and Figure 12 forthe Tracer ammunition. Figure 11 shows that the SS-109 projectile has anoverturning moment coefficient approximately 5% higher than does the M055, atsupersonic speeds. Figure 12 shows no significant difference in overturningmoment coefficient between the M856 and the L1I0 tracers.

C. Gyroscopic Stability Factor

The launch gyroscopic stability factors (Sg) for the Ball and Tracer -.

"projectiles, fired from the 7-inch twist Kart barrel, are shown in Figures 13and 14. Figure 13 indicates equivalent launch Sg for the M855 and the SS-109

projectiles, which shows that differences in the physical characteristics ofthe two designs essentially offset the overturning moment coefficient differ-ence observed in Figure 11. Figure 14 shows that launch gyroscopic stabilityfactors for the M856 and the Li1O tracer designs are equivalent.

Note that the decreased launch Sg at lower launch velocities, shown in

both Figures 13 and 14, will never be observed in field firings, since the5.56mm NATO projectiles are never fired at reduced muzzle veiocities. Thegyroscopic stability factors shown at the highest velocities tested arerepresentative of actual ammunition performance at ambient field conditions.

All four projectiles tested have sufficient gyrosc.opic stability topermit firing at extreme cold weather (high air density) conditions, with nosignificant degradation in performance.

0. Lift Force Coefficient

"The range values of lift force coefficient, CL , are shown for the Ball

and Tracer projectiles respectively, in Figures 15 and 16. Figure 15 showsequivalent values of CL for the SS-109 and the M855 projectiles, and Figure

13

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16 shows no significant difference in lift force coefficient between the M856

and the LiO tracer designs.

E. Magnus Moment Coefficient

The Magnu, moment coeffcient, CM is plotted against Mach number inpa

Figures 17 and 18, for the Ball and Tracer projectiles, respectively, and forsmall angles of attack. The Magnus moment coefficient for the M.855 and SS-109projectiles is a small positive quantity at supersonic speeds, and a suL,.•-.r-tial negative quantity at transonic speeds. The Magnus moment coefficient for

. the tracer projectiles is positive at supersonic speeds. Unfortunately, theepicyclic damping rates were poorly determined for the tracer designs at tran-sonic and subsonic speeds; hence, no Magnus moment or pitch damping momentdata were obtained for the tracers at lower velocities.

F. Pitch Damping Moment Coefficient

"The pitch damping moment coefficient sum, (CM + CM. ) is plottedq

against Mach number in Figures 19 and 20, for the Ball and Tracer projectiles,respectively, and for small angles of attack. The pitch damping momentcoefficient sum for the M855 and SS-109 projectiles is substantially negative

* at supersonic speeds, and tends toward zero at transonic speed. The pitchdamping moment for the tracer projectiles is a relatively large negativequantity at supersonic speeds. Since a negative value of (CM + CM.) causes

q adamping of the yawing motion, Figures 19 and 20 show generally favorable pitchdamping properties of all the 5.56mm NATO projectiles at ý,personic speeds.

G. Damping Rates

The damping rates, and XS, of the fast and slow yaw modes indicate

- the dynamic stability of a projectile. Negative x's indicate damping; aS"positive x means that its associated modal arm will grow with increasing time.

Figures 21 and 22 show the fast and slow arm damping rates for the M855and SS-109 Ball projectiles, at small angles of attack. Figure 21 shows thatthe fast arm is damped at all velocities tested. Figure 22 shows the slow armto be damped at high supersonic speeds, but tending toward a weak undamping atlow supersonic and transonic speeds. Thus, Figure 22 suggests the possibilityof a slow-mode limit cycle yaw at low supersonic and transonic speeds, forboth the M855 and SS-109 projectiles.

Figures 23 and 24 show the fast and slow arm damping rates for the M856and Li1O Tracer projectiles, at small angles of attack. Both yaw modes appearto be strongly damped at supersonic speeds. Unfortunately, the damping ratesof the tracer projectiles fired at transonic and subsonic speeds were poorlydeterminec, thus no data were obtained for the lower velocity regions.

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-V. LIMIT-CYCLE YAW TEST RESULTS

The final phase of the BRL tests of 5.56mm NATO ammunition was the real-range, limit-cycle yaw and striking velocity determination. The limit-cycleyaw testing was conducted during March 1984, at the BRL Yransonic Rangefacility, using three of the Aerodynamics Range spark photography stations.Figure 25 is'a photograph of the experimental facility used for limit-cycleyaw testing. The direction of bullet travel in Figure 25 is from upper left,through the barricades and spark photography stations, and into the bullettrap at lower right.

If a projectile successfully negotiates the instrumentation shown inFigure 25, and all stations are triggered, three measurements of striking yawand two determinations of striking velocity are obtaincl. Occasionally, twoof the three spark stations will trigger, and two determinat'|ons of yaw andone velocity are obtained. No data were obtained for the Ll10 tracer at 800metres range, since the long residence time of the bright tracer over thephotographic plates caused excessive fogging of the film. No data wereobtained for the M856 tracer at either 600 metres or 800 metres range, due tothe large round-to-round dispersion of the M856 ammunition. A tabulatedsummary of all the data obtained from the limit-cycle yaw testing is given in.Table 7.

* The striking velocity and limit-cycle yaw behavior of the 5.56mm NATOammunition is shown in Figures 26 through 29. Figures 26 and 27 show strikingvelocity and striking yaw for the SS-109 and M855 projectilas, respectively,frcri the Kart Mann barrel, No. 014. The vertical bars shown on the strikingyaw plots represent limits of plus and minus one standard deviation. Figure28 is a similar plot of striking velocity and striking yaw for the two tracerprojectiles, also from the Kart Mann barrel. The 856 tracer projectile shows"approximately 6 metres/second higher striking veiocity than does the L11O, outto 300 metr-es range. Note that the striking yaw histories of the L110 andM856 projectiles appear to be essentially identical, out to 300 metres.

Figure 29 is a plot of striking velocity and striking yaw versus range,for the SS-109 projectile, fired from the XM249E1 Squad Automatic Weapon(SAW), in June 1981. The ammunition used was Lot 01 FNB 81, and the SS-109/XM249EI test was conducted with funding provided by the U.S. Army MaterielSystems Analysis Activity (AMSAA). Note the excellent agreement in strikingvelocity hi-tury between the two weapons, as indicated in Figures 26 and 29.The limit-cycle yaw, at ranges beyond 300 metres, appears to be slightlygreater from the Kart Mann barrel than that observed three years earlier fromthe XM249FI. It is not possible to infer from the present data if thedifference if observed limit-cycle yaw behavior is due to weapon/rifling

Sdifferences, or the lot-to-lot differences in the SS-109 ammunition.

! VI. COMMENTS ON AMMUNITION DISPERSION

One of the principal problems encountered in U.S. production of the.5.56mm NATO arinunit'on has been excessively high rejection rates of LCAAP,,idnufactured ammunitions lots. In 1982-83, several lots of M855 Baliammunition failed to meet the accuracy specification. The M855 accuiaazyproblem has since been corrected; however, current lots of LCAAP produced M856Tracer ammunition are showing excessive rejection rates, again due to failure on

15

the accuracy specification. In nearly all cases, the FNB manufactured SS-109Ball and LI0O Tracer ammunition, fired simultaneously as control rounds, meetor exceed the U.S. accuracy specifications.

The largest contributing factors to dispersion in modern small arms ammu-nition are the lateral throwoff ard the aerodynamic jump, produced by projec-tile static and dynamic unbalance, respectively. Of these two, the aerody-namic jump due to dynamic unbalance is generally the predominant effect, andwve will examine some of the causes and consequences of dynamic unbalance inmodern small arm projectiles.

If a bullet jacket varies in wall thickness around its circumference, andthe jacket and core are of different density materials, an unbalance is intro-duced in proportion to the jacket wall eccentricity. If the lateral meridianplane containing the eccentricity is not held constant, both a static and adynamic unbalance are introduced. In addition, if the bullet has a two-piececore, and the front and rear core sections are of widely different densitymaterials, evan a relatively small jacket wall eccentricity can introduce alarge dynanmic unbalance in the bullet.

In their classical text, Exterior Ballistics ' McShane, Kelley, and Renoderive the aerodynamic jump effect, due to either an in-bore yaw, or an anal-agous dynamic unbalance in the projectile. The aerodynamic jump is the amountby which the direction of motion of the projectile is changed, and is given asan angle in radians. The result, from Chapter XII of Exterio; Billlstlcs, isconverted to the modern aeroballistic nomenclature: "_--

CB

Jump -(-L) (kt 2 -ka 2 ) (C)

where Jump - magnitude of trajectory deflection (radians)

n - twist of rifling (calibers/turn)

Skt2 - y/m d2

k 2 Ix/m d2

•y • projectile transverse moment of inertia

Ix - projectile axial moment of inertia

m -projectile mass

d - projectile reference diameter

CL lift force coefficient

' C Q overturning moment coefficient

S'.dynamic unbalance angle, or in-bore yaw due to bullet tilt

in cartridge case (radians)

*

4. E. J. McShane, J. L. Kelley, and F. V. Reno, Extez-ior Balli6tics,University of 0enver Pra8a, 1953.

16

2 2 )2CL , "For a given dynamic unbalance angle, the quantity [ (.)(kt k] a)

can be considered as a dispersion "sensitivity factor,* and we will nowevaluate this factor for several 5.56mm projectiles. The M193 Ball and M196

"* Tracer rounds, fired from the H16A1 rifle with 12-inch twist of rifling, willbe compared with the M855 Ball and M856 Tracer projectiles fired from the 7-inch twist rate of the M16A2 or the M249. The results are given in Table 8.

Comparison of the dispersion sensitivity factors in the las1 column ofTable 8 shows that the dynamic unbalance of the Mi855 bullet must be held toapproximately 60% of that present in M193 projectiles, in order to equal the

4M193 dispersion. The same comparison of old and new tracer ammunition showsthat the M856 dynamic unbalance cannot exceed 50% of that present in M196tracers, if comparable dispersions are to be obtained. Since the dynamicunbalance in all the above bullets is approximately proportional to jacket r.•wall eccentricity, it is apparent that tolerances in jacket wall thickness forthe new 5.56in, NATO projectiles need to be held at half the levels permittedfor the older M193/M196 family of 5.56mm ammunition. The aerodynamic jump dueto bullet tilt in the cartridge case is analagous to that of dynamicunbalance; thus, bull2t tilt in the seating operation of M855/14856 ammunitionshould be held to half that allowed for the M193/M196 family.

VII. CONCLUSIONS AND RECOMMENDATIONS

The Kart manufactured Mann test barrels supplied to LCAAP are not asignificant contributor to the observed M855/M856 dispersion problem.

The bullet seating operation in LCAAP ammunition assembly is part of thecause of observed M855/M856 dispersion, and some effort needs to be made inthe direction of reduced bullet tilt in assembled ammunition.

Tolerances in bullet jacket wall thickness for the M855/M856 need to be

held to approximately half those permitted for the older M193/M196projectiles, to insure satisfactory accuracy with the new 5.56mm NATOammunition,

The zero-yaw drag coefficient of the M855 Ball projectile isapproximately eight percent higher than that of the SS-109 design, atsupersuinic speeds. The drag coefficients of the M856 and L11O tracerprojectiles appear to be essentially identical at supersonic speeds.

All four 5.56mm NATO projectiles tested have sufficient gyroscopicstability, when fired from the 7-inch twist of' rifling, to permit firing atextreme cold weather (high air density) conditions, with no significantdegradation in performance.

The M855/SS-109 Ball projectiles show good yaw damping properties forboth the fast and slow yaw modes, at supersonic speeds. The slow yaw modeshows weak undamping at small yaw levels for both Ball projectile designs attransonic speeds.

17

The t-855/SS-109 Ball projectiles show limit-cycle yaw levels growing ?romipproximately 0.5 degree at 3b0 metres range, to approximately 6 degrees at800 metres range. The tracer projectiles show yaw levels damping toapproximately 0.5 degree at 300 metres range, and remaining at this level outto 600 metres range.

18

6"

..................................................

a0

LL

CU-

64-

19

SS-109

9045'

R=85

4.07

M855

-~.40 -o - .7

4.0

ALL DIMENSIONS IN CALIBERS

(1 CALIBER 5.69 mm)

Figure 2. Sketch of SS-109 and M855 Ball Projectiles

20

2.14

jER=8.4r

5.13

M856

2.3

5.18

ALL DIMENSIONS IN CALIBERS(1 CALIBER =5.69 mm)

Figure 3. Sketch of 1110 and M856 Tracer Projectiles

21

W7 w

W-~ .- A- -v w,-

WHl W

- " I i- M-p -- 47 1;

22

IFI-

t i~

0Figure 5. Shadowgraphs of M855 and SS-109 Projectiles at Mach 1.2

23

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SS10

* Figuve 6. Shadowgi-aphs of M855 arid SS-109 Projectiles at M'ach 0.9

24

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* . ,.A�..A..........* A * - A A A A A - -. a ',-. A .h - -. .k .. AflSA' A �

E1000-K

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RANGE (n

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ix 10-

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Figure 26. Striking Velocity and Striking Yýaw versus Range, SS-109

44

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0 200 400 600 800

RANGE (n

S10-

a 8-

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Figureý 27. Striking Velocity and Striking Yaw versus Range, M855

45

oM856 TRACERE 00 El 11I10 TRACER

H800-U0-J600-

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S200-

0 200 400 600 800

RANGE )N

LU 0 M856 TRACERS10- E011I10 TRACER

8-

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Figure 28. Striking Velocity and Striking Yaw versus Range, M856/L11O

46

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RANGE (n

~' LULUx 10-

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Figure 29. Striking Velocity and Striking Yaw versus Range, SS-109,from the XM249EI Weapon

47

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53.

Table 7. Limit-Cycle Yaw Test Results

Average Standard Average StandardProjectile Range Total No. Striking Deviation Striking Deviation

Type of stations Yaw in Yaw Velocity in Velocity(M) Measured (Deg) (Deg) (MIS) (MIS)

SS-109 300 30 .50 .29 648.5 2.6

600 30 2.41 .35 415.5 5.1

800 57 5.54 1.43 315.1 3.1

M855 300 33 .55 .37 656.2 13.5

600 41 3.29 1.39 406.2 8.4

800 51 6.32 3.70 312.7 2.1

LIlO 300 39 .40 .33 638.0 5.3

600 11 .35 .21 463.0 9.6

M856 300 28 .43 .22 644.2 15.8

540

Table 8. Dispersion Sensitivity Factors for 5.56mm Ammunition

2CL 2 2 CLProjectile n (kt 2 - kaL ) [ (2 7r) (kt- ka a)

(cal/turn) nM nM M

K193 Ball 53.6 .56 1.57 .10

M855 Ball 31.3 .77 1.13 .17

M196 Tracer 53.6 .75 1.83 .16

M856 Tracer 31.3 1.35 1.22 - .33

55

B-.

S•

9.'

Ri55

REFERENCES

1. W. F. Braun, "The Free Flight Aerodynamics Range," Ballistic ResearchLaboratories, Aberdeen Proving Ground, Maryland, BRL Report No. 1048,August 1958. (AD 202249)

2. C. H.-Murphy, "Data Reduction for the Free Flight Spark Ranges," Ballistic"- -" Research Laboratories, Aberdeen Proving Ground, Maryland, BRL Report No.

900, February 1954. (AD 35833)

"3. C. H. Murphy, "The Measurement of Non-Linear Forces and Moments by Meansof Free Flight Tests," Ballistic Research Laboratories, Aberdeen Proving

"* Ground, Maryland, BRL Report No. 974, February 1956. (AD 93521)

S4. E. J. McShane, J. L. Kelley, and F, V. Reno, Exterior Ballistics,University of Denver Press, 1953.

56

S'. .. . . . .a - .

LIST OF SYMBOLS

- I CD = Drag Force

(1/2) p V S

C zero yaw drag coefficient

"CD2 = quadratic yaw drag coefficient

Lift Force Positive coefficient: Force in planeC (1/2) p L of total angle of attack, cat,..L

to trajectory in direction of at"

"•t directed from trajectory to

missile axis.) 6 = sin a

CN Normal Force Positive coefficient: Force in plane-" a (i/2) p 2 of total angle of attack, (t-l

to missile axis in direction of x.

C C + CD

C Static Moment Positive coefficient: Moment increase"M(1/2) p d 6 angle of attack at""

* =Magnus Moment Positive coefficient: Moment rotates

M(12) p V2 S d (D\) 6 noseLto plane of at in direction

of spin.0

SMinus Force Negative coefficient: Force acts inCN 12T d direction of 900 rotation of the(1/2) P V2S * positive lift force against spin.

.3

57

-S - :=- , - .-. • .. . . . . ..."' " "" " " "...... . ."" "- "....A . '- * . '. *

LIST OF SYMBOLS (continued)

For most exterior ballistic uses, where & = q, 6 = -r, the definition of thedamping moment sum is equivalent to:

C Damping Moment Positive coefficient: Moment increasesCM + CM. 2 qtd angular velocity.q a (1/2)p Sd (-VS -)

C Roll Damping Moment Negative coefficient: Moment decreasesp(1/2) p V S d ) rotational velocity.

*" CPN = center of pressure of the normal force, positive from base tonose

*' a, 6 = angle of attack, side slip

"2 21/2at 2( + a2) = sin"I 6, total angle of attack

= fast mode damping rate

negative X indicates damping

xs slow noae damping rate

Sp air density

0 1fast mode frequency

slow mode frequency

CG center of gravity

d body diameter of projectile, reference length

x =axial moment of it".rtia

y =transverse moment of inertiay

580

, - ' .. , • ,. . . ., ," .•, :, • .• , ." ,, •- : • ", ,,. , • • , • '• ... , , .'. , ,: ,' ' '. ' ',,,•. ," , ' ,',,-,.,." , . ."' .' .,

LIST OF SYMBOLS (continued)

ka 21x /

2 2t y

K F - magnitude of the fast yaw mode

K5 magnitude of the slow yaw mode

2.length of projectile

m = mass of projectile

M = Mach number

p- roll rate

q~, r transverse angular velocities

2 2 1/2t ~ r

R = subscript denotes rdnge value

s = dimensionless arc length along the trajectory

= ~ 2

S reference area

Sd dynamic stability factor

S = gyroscopic stability factor

V velocity of projectile

Effective S guared Yaw Parameter

ý2 2 2

K F+ Ks

59

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