UNCLASSIFIED

AD 295 754

ARMED SERVICES TECHNICAL INMATIN AECARLINGTON HALL STATIONARLINGTON 12, VIRGINIA

UNCLASSIFIED

NOTICE: When government or other drawigs, speci-fications or other data are used for any purposeother than in connection with a definitely relatedgovernment procurement operation, the U. S.Government thereby incurs no responsibility, nor anyobligation whatsoever; and the fact that the Govern-ment may have foxmiated, fumished, or in a waysupplied the sai d-rawings, specifications, or otherdata is not to be regarded by implication or other-wise as in any manner licensing the holder or anyother person or corporation, or conveying an rightsor permission to manufacture, use or seli anypatented invention that may in any way be relatedthereto.

298 754AS'DmTD-43-

* Exterior Ballistics of theAR -4S RiflIe

by Robert W, Cross

ASD Technical Domuauintry Report No., ASD-41DR-63-2

JANUARY 1963 o Project No. 912A-OOO-97205

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WEAPONS LABORJliW 7 --DIRECTORATE OF ARMAMENT DEVELOPMENT

Dot 4, AERONAUTICAL SYSTEMS DIVI-SION

AIRFORCE SYSTEMS COMMAND* UNITED STATES AIR FORCE

EGLIN AliR FORCE BASE, FLORIDA

Qualified requ'Sters. may obtain Copies from ASTIA. Orders will beexpedited if piaced through the ilbrarian or other person designated torequest documents ,from, ASTIA.

When US Goverhnment drawings, specifications, or other data are usedfor any purpose other thln a definitely related government procurementoperation, the government thereby iticurs no responsibility nor any obli-gation Whatsoever; and the fact that the government ray have forrnulated,fturiiished, or in any way supplied the said drawings, specifications, orother data is not to be regarded, by implication or otherwise, as in anymanner licensing the holder or any otherperson or corporatiot, or con-veybinga&ny rights or permission to manufacture, use, or -ell any patentedinvention that may in any way be related thereto.

Do tnot return this copy. Retain or destroy,

FOREWORD

This report was pre-pared by the Weaponis Laboratory, Dietachment No.4, Aeronautic-al Sysateft DiVibion of the Departmenit of the A-ir Force.All workc was performed by the Weapons Laboratory persotnel under thedirect suptrvisioni of Mr. Gerald A.. Gustafisont 'Technical Director.

Thi work was sccomiplished-unider project No. 012A4OOOO-97205i Atthel request of Headquarter! USAF.

ABSTRACT

This, study wa conducted to provide inforiation which could serveat a basis for certain design decisions involving the hl-15 rifie.Bullet stability, lethality, penetration, and deflection were subjectsof the physical testing conducted. In addition, bullet shape andmuzzle velocity with regard to. stability are discussed,

The exterior ballistic theory concerning the Remington 0.223 cal-lber bullet predicts that bullet instability can be expected if thebullet is fired through a barrel with a one-turn:-in-14-in. twist inan air density greater than that which occurs at 00 F at sea level.Further, the theory predicts that the bullet will be stable if firedthrough a barrel with a one-turn-in-12-in. twist in an air density upto that which occurs at -600 F at sea level, Confirmatory tests con-ducted at the Climatic Laboratory indicate the predicted instabilitywhen using the one-turn-in-14-in. barrel and indicate the stabilityof bulletS, fired from the one-turn-in-12-in. barrel in conditions downto i65° P at se: level.

Other tests during this study indicate that there is no significant difference in the lethality of bullets fired fro either barreliAlto, no severe deflection or break-up problem were noted when thecaliber 0.223 bullet was fired through brush or other natural cover,and physical testing proved that the bullet is capable of penetratingand killing a wide variety of soft-material targets.

A study indicates that substitution of a f-latibase bullet forthe boat-tail bullet will result in a stable projectile when firedfrom a one-turn-in-14-in. barrel; however, there will be an unaccept-

able attendant degradation of the exterior balliltics.

PUBLICATION REVIEW

This technical documentay report has been reviewed and is approved.

Colonel, USAFDirector

LIST OF ILUSTRATIONS

Frigure

1A-1, Rifle and Test Setup, at the Climatic 4Lab-oratory..

2 Remington Model 700 ADL AMf le and Tesit Setup, 5at the Citmatie Laboratory

RelAtive Density of Dry Air at Sea Level to TAir at Altitude at a Constant 7O F Temper-ature

4 RelAtive Density of Dry Air at 700 F to Ai-r 7at Lower Temperatures at a Pressure Constantof 301 in. Hg

5 Caliber 0.223 Remin*gton, Bull eit Computed Sta- 8biity Fact6r vs Relative Density of Air atSea Level and 70OF

6 Maximum Spreads vs Air Density and Temperature 17

7 Average Dispersion vs Air Density and Tem. -8perature

8 Average Maxcifirn Spreads and Dispersions vs 19Air Density and Temperature

9 Typical Flash X-ay Shoving Bullet During Z 3Impaet in Gelatin Block

10 Angle of Deflection vs Ratio of Impact Velo- 26city to Minimum Penetration Velocity, SteelCylindersr Impacting Mild Steel Plate at 450Obliquity

11 Penetrat-ion, Through k-in. Armor Plate (Exit 28Hole),

12 Penetration Through 55-Gal Drum 28,

13 Penetration Through Three., Spaced, Skin 29Thicknesses of a Jeep Body. Entrance ThroughHood, Exit Through Rear Weel Well.

14 Penetration of Fuel Cell., High, Angle of Oh, 2.9'liquity of Impact. Note S platter on Mud Shield.

iv

TABLE OF CONNTS

SECTION 1 - INTRODUCTION, 1

SECTION 2 - DESCRIPTION 3

SECTION. 3 - TEST PROCEDURS,, RSULTS, AND, DISCUSION 6

BULKT STAB ILiY 6

Confirmat6ory Tbests 9

Test Set-Up 9

Test Proceduires 10

Data Redutiton 11i

Results 11

BULET LETHALITY 20-

BULLET PENETRATION,, DEFLEC-TON, AD BREAK-UP 215

SECTION 4- CONCLUSIONS 30

REFENCEs 31

APPENDIX - BULLET STABILITY COMPUTATION TECHNIQUES 32

V

SECT ION I . INTODUCTI~oN

The purpose of this study was to provide information which couldbe used as a basis for eertainL destign ddesions involving the, AR-15,rifle. Tzhe majior portion of this study is devoted to bullet stability,heever, other considerations such as lethality, pienetation, and bullttdeflec-tion are also covered.

The exterior ballistic theory given in reference 6 concerning the:stability of the Remigton Caliber 0.223 bullet prediets that bulletifnstability can be expected at temperatures below 00F at sea levelwhen the bullet is fired from, a barrel with rne-tun-in-4in. rifling.Further , the theory predicts that a modification of the barrel to arifling of one turn, in 12 in. would stabilize the bullet sufficiently.Confirmatory tests were carried, outt in the Climatic Laboratory atEglin Air Force Base, Florida. A detailed discusstion of the ballisttic,theory appears in the Appendix to this report.

While factors such as stability can be predicted from; the theory,it is not possible to determine by theory alone any change in lethality,which might have been brought about by the modification. Therefore,a series of impact tests in which both a modified AR-15 rifle and anunmodified AR-15 rifle were fired into 20 percent gelatin was conducted

at the Aerojet-General Corporation test facility in Chino Hills, Cali-fornia. The results of these tests show no degradation of performancewas caused by the modification.

A possible fix to the, stability problem would be to substitute a55-grain, flat-base bullet in place of the 551-grain boat-,tail bulletnow, in use. The flat-base bullet should stabilize the bullet to -659Fas predicted. It is further true that the flat-base bullet wil1 prob-ably result in a more accurate ammunition for short ranges. For someapplications then, the flat-base bullet can be used. One of theseapplicaltions might be f or competitive shooting at shor-t ranvges.

However, there are also some disadvantages in using a flat-basebullet. One of these is that a lower velocity at any given range isbrought about by the higher drag, Another fact or is that flat-basebullets are affected more 'by crowns thnre boat-talblesThis aresults in a gr eater dispersion on long-range shots in combat.The lower velocity will result in reduced lethality on impact, and thelarger Vdispersion wl result in fewer targets hit. Both of thesecontribute to a reduction in the combat effectiveness of the weapon.

Fo these ame r4esons, a boat-tail, bullet is nov standard in, the14.4, 7.6ig r ffIe.

Ote possible consequence to increasing the rate of t is t in thebarel is a, reducition in muzzle velocity. the amounot of energy in-volved in the spinning of the projectile (rotational energy) is lessthan 0.2% rof the translational ene-gy (muzzle etergy). The variationin, rotational energy involved in, hanging from one turnt in 14 in. toone turn, in 12 in. is less than 0.051o%. The 'resultant velocity changepredicted as a result of changing the twist ifs less than 1 ft per sec.This variation is far below the velocity variationi expectedi fromround to round. with the highest quality amunition,, and thus, is notc6tide-red to 1 be of importance.

Tests Vere conducted to determne petformance charateristics ofthe Caliber 0.223 bullet with respect r to penetration, defleetion, andbreak-up, Tests performed by persontnel of Detachment 4-, ASD, using arifle with a one-turn in 14 inch, barrel indicate the following:

a. ihe Calber 0,223 bullet will pe etrate through 0.25 in. ofsteel plates at ranges sligh.tly greater than 100t yd.

b. The CAliber ;0.2,23 bullet will penetra te throug more, thanthree layers of spaced sheet mtal, such as that uiedIn the bodiesof jeeps and trucks.

c. The Caliber 0.223, bullet will penetra te through more than1"0 in. of pine boards.

d. The 'Caliber 0.223 bullet does not br eak up under norma Iusage.

2

SECTION 2 - DESCRIPTION OF ITEMS TESTED

A total of nine rifles were used in this test: four AR-15's, twoM-14's, and three Remington 700 ADL's. AR-15 Nos. I and 2 were rifleswith one-turn-in-14-in. twists, and AR-15 Nos. 3 and 4 were rifles withone-turn-in-12-in. twists. The M-14 rifles were manufactured by theSpringfield Armory and were identical except for serial numbers. Thethree Remington 700 ADL rifles were identical except for the barrels--one had a one-turn-in-10-in, twist, one had a one-turn-in-12-in. twist,and one had a one-turn-in-14-in. twist.

The AR-15 rifles (Fig. I) were equipped with three-power Inergascopes, and the Remington rifles (Fig. 2) were equipped with LymanSuper Targetspot scopes. The caliber 0.223 Remington ammunition wasissue ammunition from Lot No. R. A. 500. The 7.62mm NATO ammunitionused was from Functional Lot No. RA-L85057. Two 10-shot groups were firedutilizing Caliber 0.223 Winchester-Western anmunition with an unknownlot number.

3

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44

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SECTION 3 - TEST PROCEDURES, RESULTS, AND DISCUSSION

BULLET STABILITY

It should be noted that air density is a function of temperatureand atmospheric pressure and to some extent, humidity. Figs. 3 and4 show how relative density varies with altitude and temperature, re-spectively. The ordinate values in these graphs may be used asmultipliers to find new stability factors when si is given for the700 F, sea level condition.

Fig. 4 shows the caliber 0.223 Remington bullet stability factoras a function of air density for bullets fired from both the one-turn-in-14-in. and the one-turn-in-12-in. barrels at the same muzzle velo-city (3250 fps). The density of standard air is assumed to be 0.0749lb per ft3 at 700 and 29.9 in. Hg. These curves are all based on anassumed value for a equal to 1.16 at 700 and sea level.

From Figs. 3, 4, and 5, it can be predicted that the caliber0.223 Remington bullet from the one-turn-in-14-in. barrel will becomeunstable at a temperature of 00 F at sea level and that the samebullet from the one-turn-in-12-in. barrel will become unstable at atemperature of about -1000 F.

The stability factor for the caliber .223 Remington bullet hasbeen determined experimentally and theoretically with reasonableagreement. The value for the stability factor (a) as given in refer-ence I is 1.13. This value was obtained by firing through yaw cards.Another estimate of s was made by the General Electric Company usingcarefully determined dimensions of the bullet. From these dimensionsthe axial and transverse moments of inertia of the bullet were com-puted and from these the stability factor was determined. This methodyielded a value of 1.14.

It has been verbally stated by personnel at the Ballistics ResearchLaboratories at Aberdeen Proving Ground, Maryland, that a spark rangedetermination of stability factor yielded a value at 1.20. The sparkrange determination is inherently the most accurate method. Anotherdetermination of stability factor also performed at Aberdeen using adynamic ballance technique for determining the moments was said toyield a value of 1.16. All of these values are for a standard 700 Ftemperature and 29.9 in. Hg atmospheric pressure. These variations(1.13 to 1.20) are within the lot-to-lot variations encountered inammunition production.

The 7.62 (NATO) 1480 bullet when fired from an M-14 rifle is re-ported to have a stability factor in excess of 2. As tests bear out,there is no stability problem with this bullet in air.

6

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Fi; 3Rltive Den-sityt 61 Dry Air at Se~a Level tot A.'ir at Altitude at acolns1tanit 700 F T empje rat.ur &.

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8

CONF IMTORY TESTS. in order to confirm the stability values derivedfrom the computations, it was necessary to conduct a series of firingtests., A climaticalily controled, properly istrumented,, aero-balisticrange was not available, for these experiments. Because of this, otherthan the usual techniques for stabiity testing were applied.

A ya"ing bullet will be subjected to two forces not experiencedby a well-stabilized bullet. First, the drag will be increasedbecause of the increased area presented. This increase is caled the"yaw-drag.' Second, the bullet will experience lift because of theresultant force on the projectile foteing it away from its instantane-ous trajectory. Both o f these forces introduce impact errors. Forthis reason, a greater bullet dispersion, can be expected when, usingunstable bulliets or marginally s-table bullets than wahen, stable bulletsare used.

A fitr-ing test pr ogram was devised, to demonstrate the effect ofair density on the dispersion ofr the bulletis. These experiments wereconducted, in, the Climatic Laboratory at Eglin Air Force Base,, Florida.By conducting these tests concurrently with other tests, it was, pos-

ble to obtain data in environmental conditions ranging from 06OF to-65°F which represent a significant change of air density and spansthe temperature range of interest- Data from these tests were sup-

plemented by firings outside the Climatic Laboratory with a, gooddegree of correlation.

TEST SET-UP. Limitations of tifme and resources did not allow acompletely pure scientific expertment to be performed. Many unmeasuredfeactors, have crept into the data, some of which are discussed. Itshould be pointed out, however, that for the most part the experiments

yielded results which are consistent with the theory and whitch aredirectly applicable to real combat situations. For this reason it isfelt that the experiments repo ted have even higher value than if they

had, been generated in an idealized set of conditions.

The maximum range posslble within the Climatic Laboratory withoutinterfering with other tests was 220 ft. This distance has been used

throughout the ballistiC experiments and from the data it is seen tobe adequate.

The firing-n butt was constructed of steel similar to but largerthan the small commercially available bullet stops. It was roealizedthat these bullets would crater the back plate on impact even thoughit was set at an obliquity of 450 and that back splatter would be asever e problem. Therefore., a 2-in. wooden shield was placed in font

of the bullet stop to catch tplattger.

9

TEST PROCEDURES. All firing was performed under identical conditionsusing bench-rest techniques. Such a technique yields the minimaiming errors with the equipment available for the experiments.

The temperature of all equipment used in the experiments wasbrought to the chamber temperature by soaking for at least eight hours.

Cold-soaking the ammunition may have caused different group sizes thanwould have been obtained by using ammunition at constant and highertemperature, e.g., +600F.

It was possible to run only one experiment to ascertain how muchthe cold ammunition contributed to the total dispersion. This trialusing ammunition at +60oF in a test chamber at -2 50 F produced asmaller group than awmnition at -25° F. The margin of reduction waswithin the expected limits for group-to-group variation. Based onpast experience, changes in group sizes with changes in ammunition tem-

perature is more likely to be due to improper matching of ammunition tothe dynamic characteristics of the gun and particularly the barrel.

The resolution of this problem is not possible without much more

experimentation, and one can question the military value of such astudy. There are several good texts which discuss the matching of

ammunition to gun dynamics in order to reduce bullet dispersion. Pro-bably the best among these is "The British Text Book of Small Arms."

Ten-shot groups were fired on each target. Every shot was care-fully aimed under as near identical conditions as possible. The firingrate was not carefully controlled; however, at least 10 sec. and notmore than 30 seconds elapsed between shots. The firing rate wasstrongly dependent upon conditions in the chamber not under the controlof the test personnel. Such things as fogged lenses, arctic clothing,guns improperly degreased, etc., all contributed in some measure tothe lack of complete control of the firing rate.

It could be argued that the rate must be erratic in order to haveuniform aiming conditions. The shooter is not likely to arrive atthe optimum time to pull the trigger in a regular cadence. However,the error contributed by changing gun temperature conditions (caused

by the irregular firing cadence) is small compared to the total dis-persion throughout most of the testing.

Measurements of temperature, humidity, barometric pressure, and

chamber pressure were taken continuously during the experiments to

obtain the exact density. In all experiments in the chamber exceptthose on 8 December 1962, the chamber was held at a constant tempera-

ture for several hours prior to the firing. Thus, the chamber tempera-

ture was uniform and the temperature reading was taken at one point in

the chamber. On 8 December, when the chamber temperature was chanfLng

10

during the tests, temperature readings were taken at several pointsalong the bullet trajectory, and readings were taken before and aftereach group firing.

All firing of guns, except on 9 November and on 13 December, wasperformed by the same shooter. A single well-defined aiming pointwas used for each 10-shot group. As will be observed, an excessivelylarge number of targets were taken only on 8 December. Shooter fatigueshould not be a significant factor even on 8 December because of moreagreeable temperatures on that date.

In general, it can be said that every attempt was made to removeall errors except the ballistic errors. Where this was not possible,firings were performed to assess the magnitude of the extraneous con-tribution or the data have been manipulated to remove the effect.Data variation was treated by averaging or by normal curve fittingtechniques.

DATA REDUCTION. Each impact in every ten-shot group was recorded withits coordinate values in an orthogonal coordinate system with arbitraryorigin. Measurements were made with respect to the centroid of thesignature left by the bullet in passing through the paper.

From the coordinate values, the following measures were determinedfor each group: maximum spread, maximum lateral spread, maximum ver-tical spread, and the standard deviation of the group about the meanpoint of impact.

In some instances, one or several of the bullets were in a yawingattitude as they passed through the target. Where this yaw was dis-cernible, measurements of the major axis and minor axis of the signa-ture were made, and from this and the known geometry of the bullet, acomputation of angle of yaw was made.

RESULTS. Table 1 gives the results for all targets fired during thisexperiment.

The data from Table I have been plotted in Figs. 6, 7, and 8 sothat the variation in group size is shown as the firing conditionschange. Several methods have been chosen in order to emphasize certainfactors which might evade casual observation. Each of the graphs willbe discussed in subsequent paragraphs with the object of presentingthe data in the manner used and the conclusicns one can reach from thedata.

Fig. 6 shows the average maximum spread as a function of airdensity for the Remington rifles, the AR-15 rifles, and the H-14 rifles,respectively. Shown also are the temperatures at which the data weretaken.

11

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19

In Fig. 6 it can be seen that there is a severe dispersion pro-blem with the Remington caliber 0.223 bullet in the Sne-turn-in-14-in.barrels when the air density reaches 0.085 lb per ft , which corres-ponds to 50 F at sea level. This corresponds to a computed stabilityfactor for these guns and bullet of 1.02, as determined in the compu-tational techniques (Fig. 5). The same bullets from the one-turn-in-12-in. barrels show no stability problems down to -650 F. The computa-tionally derived stability factor for this condition is 1.18, quitesufficient for a stable bullet.

Fig. 7 shows the average dispersion (standard deviation) for theRemington, AR-15, and M-14 rifles as a function of air density. Aswould be expected the same results as pointed out above are apparentand the same comments are appropriate.

As a matter of interest, all of the groups for the caliber 0.223rifles were averaged for the one-turn-in-14-in. barrels and one-turn-in-12-in. barrels and plotted against air density. Fig. 8 shows theaverage maximum spread and the average standard deviation. From thesecomposite curves, it now appears that excessive dispersion occurs ata density of about 0.0865 lb per ft3 . This corresponds to a computedstability factor of 1.01. This air density occurs at approximately00 F at sea level and is almost exactly the computed critical point.

It can be seen from the graphs that the group sizes for the one-turn-in-12-in. barrels increase at -650 F. This is not considered tobe an inherent stability factor, since all group sizes increased at-650 F. These increases are believed to have resulted from discomfortof the shooter, fogging of the scope, and poor performance of theammunition at this temper&ture. However, such a temperature occurs sorarely and over such small land regions on the earth that the militarysignificance of the -650 F data is slight for that reason alone.

From the above data it can be determined that there is a severestability problem using the Remington caliber .223 ammunition in theone-turn-in-14-in. barrel. It can be further determined that the one-turn-in-12-in. barrel satisfactorily stabilizes the bullet far below-650 F. Computations show that a one-turn-in-13-in. barrel would bea sufficient twist for the Remington bullet for all practical combattemperatures; however, no adverse effects result from using a one-turn-in-12-in. barrel, and it affords an over stable margin that will permituse of possible future bullets of improved form.

BULLET LETHALITY

One possible consequence of changing the twist of the AR-15 barrelto improve stability would be a reduced bullet lethality. To determine

20

if this change would result, a series of impact tests was conductedby Aerojet-General Corporation using both the one-turn-in-14-in. barreland the one-turn-in-12-in. barrel.

In order to provide a more comprehensive coverage of the subject,the following discussion on the mechanism of wounding is quoted fromWound Ballistics (pp 129, 130, 132, 136, 166) as published by theMedical Department of the United States Army. The following applicablequotations are extracted from this book:

1. "Pitch of rifling through determining the rate of spin is afactor in controlling the stability of the bullet in flight and in turnthe degree of yaw on impact. The rate of spin in the usual militaryrifle is high. With the .30 caliber flat-base bullet at a muzzlevelocity of 2,700 f.p.s. and a rifling pitch of 30 calibers, the spinis more than 3,500 revolutions a second. This spin is only adequateto stabilize the 150-grain bullet in air flight. The spin has anegligible effect in maintaining the bullet in a point-on position indenser mediums, such as water or tissue."

2. "However, density of resistant materials is a direct factoron the retardation and other motions of a missile. Water with a den-sity 800 times that of air and tissues of slightly greater densitiesact much as a magnifying of all of the retardations, yaw, and gyrationsof the bullet 800 or more times. A very slight tip or yaw will becomeone of more than 500 by the time a .30-caliber 110-grain solid bullethomologous in shape with the 150-grain flat-base bullet has traversed3 in. of water. Not infrequently, the increase in yaw will exceed1000. Changing from one density to another also induces marked varia-tions in degree of yaw.

"This, of course, immediately changes the area of presentation;a bullet enters tissue point on but in a few inches may be tipped upto 900 or more and the presentation area is broadside. The forces ofspin are still operating, however, through the overturning couple andtend to stabilize and maintain the bullet in point-on flight. Conse-quently, in another few inches, the bullet is again point-on and mayleave the body through a small exit wound. Neither entrance nor exitwounds give any idea regarding the extensive interior destructionoccasioned by the extreme tip and periodic bullet gyrations within thetissues."

3. "Yaws of more than 1700 have been observed in bullets in pass-ing through 6 inches of water. Theoretically, yaw can be of any valueto just under 1800. A yaw of 1700 increases the retardation factors172 times and a yaw of 1790, 190 times. This readily explains why asuperspeed bullet is stopped in a very few feet of a homogeneous mediumsuch as water.

21

"This also explains why a super-velocity bullet is retarded sogreatly in producing a casualty. The extreme retardation of suchbullets can result in a wound with comparatively enormous destruction,

tissue pulping, bone shattering, and other extreme manifestations only

possible with the modern, fast-moving military bullet."

4. "The situation (cavity formation) in soft tissues of livinganimals appears (from tests) to be very similar to that described fora gel."

5. "Knowing the relationship between the permanent cavity, zoneof extravasation, and the temporary cavity, the military surgeon canuse this knowledge in determining the extent of the wound."

Based on the above referenced book and particularly from theabove quotations, an experimental program was planned to demonstratethe relative lethality of the AR-15 rifle and the AR-15 rifle with a

modified barrel. In the program, 20 percent gel was used and high-speed photographic shadow-graphs were taken of the transient or tem-porary cavity caused by bullets from each gun.

Two factors were felt to be important in the formation of thecavities. First, the cavity volume is important because it is ameasure of the energy expended in the formation of the wound. Second,the depth at which the center of the cavity is located is importantsince it indicates the depth at which the maximum damage can beexpected in a homogeneous target. It was assumed that if these measure-ments were essentially the same for both guns, then their lethalitiesshould be the same.

Fig. 9 shows a flash X-ray composite with the bullet at fourpoints during impact in gelatin. The bullet yaw and deformation areclearly evident from the X-ray. The point at which tumbling occurredcorrelates with the evidence obtained from the high-speed shadowgraphs.

This bullet was fired from the modified AR-15 rifle. Note thatthe bullet impacted at almost exactly zero degrees yaw. The rangefrom rifle to impact was 50 feet. The gelatin block (12 in. x 6 in. x6 in.) was completely disrupted during this impact and the last fourexposures of the X-ray picture were lost as a result of splatteringgelatin. Initial transient cavity formation is evident.

The original plan was to have flash X-ray and high-speed photo-

graphic histories of the bullet yaw and cavitation. Instrument cali-bration tests indicated that 12-in. cubes of gelatin were required todisplay the full cavitation capability of the bullet.

The low X-ray power available made it impractical to obtain X-raycoverage of the impacts. Cavitation data collection was limited to

high-speed photographic methods alone.

22

0 M0

43

Four valid, tests were performed, wth each oftwo A-iS rilsGun, Mo. 1 with, the stanidard oeunn-4i.barrel and Out N61, 3with, the.oetr'i'l-n barrel were used in, thdee tests.

Tablelt 2' thout the results of these tesrts - easurtinents taken fromfthe, fim were maxrimum, cavity diameter and depth of cavity, Caviltydiamheter wat defined: to be the maximumn lateral ex-panslion o-f 'the cavity.lCavity depth, was defined to, be 'the di1stance, fbrom, the point of entryinto, the gelatin, to the enfter Of the cavity at maximmadiamfeter. Also!shown are the impacting veltocity as taken, from chrtoniograph sereensplaced, immediately int front of the geai:n h iefo entry tomaximum cavity. AllA shotas referred to in Table 2 were fi ,ed at , t rangeof 2:00 f.,

TABLE 2. RAESULTS OF# tHPACT TESTS

Stri'kinag Rkifling,Maimm av tDpth to Time After Velocity (iniperl

~Tea~t No. Date Diameter (fi) 'Cen (in.4) Enrtry (flec), fps) torf)

1 12-12-62 9.4 6 1 ,5x10L3 2958 12

2 12-12-62 9.0 5 lrO0 3!062 4

4 12,-162 9.0 6 1. 7x10 3 r 3007 12

5 12-13 -62 8.x5 i6 w5x03 3015 14

9 12,- 14-6.2 8.5 6.3 l.5gx10 3 2946 12

10 12-14-62 8.2 6 l.5x0 3 2925 14

it 12-14-62 9.0 5 1. 4x10*-3: 2991 14

I2 12-14-62 9.0 6 .5x0-,k 3 l3024 12Gun #1 (Averages) 8.7 5.5 2998 14

I in 14 twist

Gun. #3 (Averages) 9.0 6.1 2,984 12I in 12 twist

24

BULLET PENETRATION, DEFLECTION, AND BREAK-UP

Tests were performed to determine if a deflection problem existswith the caliber 0.223 bullet on impact with the limbs and twigs oftrees. Limbs of pine and oak up to 5 in. in diameter were used. Bothcaliber 0.223 bullets and caliber .30 M-2 bullets were fired at a

range of 100 yd. through the limbs and into yaw cards set at 18 ft.beyond the limbs. At this distance 900 yaws were observed with bothbullets. Bullets were deflected not more than 3 ft. in the 18 ft.from impact to the witness card even after penetrating 5 in. of oak.No bullet break-up was observed on impacts with wood.

On two occasions bullets were deflected and struck the ground.One was a caliber .30 and the other was a caliber .223. Both bulletsbroke up. It was assumed that the bullet was in a yawed condition onimpact with the ground, contributing to the break-up.

It was determined as a result of these tests that impacts uponsmall twigs and limbs do not cause bullet break-up and do not causeserious deflections.

A study performed by the Denver Research Institute (Reference 5)involving the proper design of armor grilles contributed an empiricallyderived theory of deflection. Equations were derived which predictthe angular change in direction experienced by a perforating projectile.Fig. 10 shows the expected angular deflection of steel cylinders impact-ing a mild steel plate at 450 obliquity.

Although these data are worked out for steel impacting upon steel,there is no reason to believe that the same type of curve would not beappropriate for jacketed lead bullets on wood. In fact, at higherratios of impact velocity to critical penetration velocity the deflec-tion in the latter case might be less. The limitation of time andresources did not permit further pursuit of this investigation.

Penetration tests were carried out against armor plate, mildsteel, automobile body sheet metal, oil drums, truck motors, aircraft,wood, and other materials. Some of these tests are recorded in Table3.

25

%0

-4

0

0

0 (12

0a-

0

0

0~ 0

0 0

a, 4A

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-~ 0>-4

U u0

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C ~a 4=C___M_ C___ _40

(60p) ~ ~ ~ UOO~G4

26'4-

TABLE 3. RESULTS OF PENETRATION TESTS OF THE CALIBER 0.223 BULLET

No. Target Range (yd) Result

1 1/4-in. Mill Steel 100 Complete Penetration

2 1/4-in. Armor ?late 20 Complete Penetration

3 3/8-in. Hill Steel 100 Bulged Plate Sometimes Penetrated

4 55-Gal Drum 100 Penetrated Completely through drum

5 Jeep Body 100 Penetrated 3 spaced skin thicknesses

6 Heavy Truck 100 Penetrated hood, disrupted oil lines,did not penetrate engine block

7 Heavy Truck 100 Penetrated fuel tank completely

8 F-84 Aircraft 100 Penetrated skins, destroyed hydrauliclines after penetration

Figs. 11 through 14 show the results of some of these tests. Ascan be seen, the caliber 0.223 bullet is capable of penetrating anddestroying military targets. Bullets were observed to break up;however, tnder the conditions of these tests most military bulletswould be expected to break-up.

27

Fig. 11: Penetration Through -!-in. Armor Plate (ExitHole).

Fig. 12: Penetration Through 55-Gal Drum.

28

Fig 1 3:,Petration Throu ThreeSae, SkinThivckn xesses of1 a Jelep Bodyi Entranice ThroughHood, Exit Thr'ough Rear Wheel WeIL

Fig. 4 Penetration ,.f F-uel Cell, High Angle of Ob

liquity of, Impact. Note Splatter on Mud Shield.,

49.

SECTION 4 - CONCLUSIONS

1. The Remington 0.223 caliber bullet, when fired from barrelswith one-turn-in-14-in. twist is unstable under a wide variety ofcold weather combat conditions.

2. Changing the twist in the barrel from one-turn-in-14-in.to one-turn-in-12-in. will stabilize the Remington 0.223 caliberbullet under all expected combat situations.

3. Changing the twist in the barrel does not reduce the lethalityof the bullet.

4. Substitution of a flat-base bullet for the boat-tail bulletwill result in stable projectiles from the one-turn-in-14-in. barrels,but there will be an unacceptable attendant degradation of the exteriorballistics as a result of this change.

5. The caliber 0.223 bullet is capable of penetrating and killinga wide variety of soft-material targets.

6. There are no severe deflection or break-up problems with thecaliber 0.223 bullet when firing through brush and other natural cover.

30

REFERENCES

1. Col. E. H. Harrison, USA (Ret'd), "Development in .22 MilitaryRifles" American-Rifleman, July 1958, pp 17-19.

2. NRA Staff, "the Armalite AR-15 Rifle", American Rifleman, June1959, p 24.

3. W. J. Hove and E. H. Harrison, "The Armalite Al-15 Rifle",American Rifleman, May 1962, pp 17-23.

4. Wound Ballistics, U. S. Army Medical Carps, 1962.

5. R. F. Recht and T. W. Ipson, The Dynamics of Terminal Ballistics,Denver Research Institute, 1 Feb 1962.

6. NcShans, Kelley and Reno, Exterior Ballistics, University ofDenver Press, 1953.

31

APPENDIX

BULLET STABILITY COMPUTATION TECHNIQUES

A bullet is said to be stable provided its yaw decreases, approach-ing zero angle as the time of flight increases. The conditions for sta-bility can be stated mathematically in the solutions to several equationsinvolving the aerodynamic coefficients of the projectile.

Precisely, in order for a projectile to be stable, it is necessaryand sufficient that the following inequality be satisfied:

1 4B 2 K 2 J S S

-- < 2 3s A 2 V S z

whereJA m d

5T md2

S, =J +- J -J A1 N H D A

2(SJ 2J -JA md

S = K -2 Jd +

3 H A

The J factors in the above equations are the aerodynamic coefficientsLd3

of the projectile multiplied by the density factor _d. The followingm

symbols are used:

s is stability factor

32d

32

p is the density of the medium through which the bullet is

traveling.

Il is muzzle velocity

d is the bullet diameter

W 1 is bullet angular velocity at the muzzle

m is the bullet mass

A is the axial moment of inertia of the bullet

B is the transverse moment of inertia of the bullet

2 BK - B the transverse radius of gyration of the bullet

md

3J N Pd KN Normal force coefficient

m

3D- pd K D Drag coefficient

m

J Pd 3 KH Damping coefficientH m

J pd3 K Magnus force coefficientm T

d3

J A - K Spin coefficientA m A

JM = p d 3 KM Overturning coefficientm

In the case considered herein, the condition for stability can be re-duced to the following inequality:

33

A~v2

2 v-2S >I

4B K Jm

or in a more convenient form

A 2

3 2

4B pd 1 KM

where

W=2 4nl N

Wo is angular velocity in radiaus per second

N is twist of rifling in turns per inch

1 is muzzle velocity in feet per second

Here KM is the overturning moment of the bullet and is approxi -mately of the value 1 for the range of conditions herein. A and B, theaxial and transverse moments of inertia, respectively, and the stabilityfactor may be obtained in a number of ways. These include controlledfiring on the spark range, firing through yaw cards, computations frombullet dimensions, and by dynamic balance tests. All of these methodsyield acceptable estimates.

Once the stability factor has been determined for one set of condi-tions of temperature and pressure, it can be extrapolated to a new con-dition with a high degree of confidence. Conversion from a conditioni to a condition j can be accomplished as follows:

p15. " SJ 1 p.

or

P.sj 3 4..

34

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