Sharma, B. R.

English

Northwestern University Illinois, School of Law: Scholarly Commons

Journal of Criminal Law and CriminologyVolume 54Issue 3 September Article 16Fall 1963The Importance of Firing Pin Impressions in theIdentification of FirearmsB. R. SharmaFollow this and additional works at: https://scholarlycommons.law.northwestern.edu/jclcPart of the Criminal Law Commons, Criminology Commons, and the Criminology and CriminalJustice CommonsThis Criminology is brought to you for free and open access by Northwestern University School of Law Scholarly Commons. It has been accepted forinclusion in Journal of Criminal Law and Criminology by an authorized editor of Northwestern University School of Law Scholarly Commons.Recommended CitationB. R. Sharma, The Importance of Firing Pin Impressions in the Identification of Firearms, 54 J. Crim. L. Criminology & Police Sci. 378(1963)THE IMPORTANCE OF FIRING PIN IMPRESSIONS IN THE IDENTIFICATIONOF FIREARMSB. R. SHARMAB. R. Sharma, Ph.D., is Director, Forensic Science Laboratory, Punjab, India. He was appointedto this position in October 1957, and in addition to his administrative work specializes in the identi-fication of firearms and other ballistic problems. He received his basic training at the Police TrainingSchool of Punjab State at Phillaur, India, and has testified as an expert witness in several states inIndia. His academic training is in the field of chemistry.-EDITOR.The importance of the firing pin impressions onfired cartridges in criminal cases is immense in ourcountry. Firstly, the most common weapon ofoffence in the firearms category is shot gun. Theprojectiles fired through them do not offer muchevidence to complete the link of the crime with thecriminal, and therefore, the cartridge cases formthe only important link. Secondly, the ammunitionused by the criminals is more often than not old,which they have been able to collect from illegalsources. The pressure developed by the ammuni-tion is often low, and the breech face marks, etc.are not properly imprinted, and, therefore, wehave to depend upon the firing pin impressions foridentification purposes. Lastly, the ammunitionused by the criminals may be refilled cartridges.The cartridges normally are made by replacingthe percussion caps, the rest of the cartridgesbeing used as such. Thus a cartridge case (exclud-ing the percussion cap) may bear impressions ofmore than one gun which can prove confusing tothe expert. It is, therefore, important that thestudy of the firing pin impressions should be madethoroughly and foolproof.The fired shells are very commonly left at thescene of heinous crimes, like murders, dacoities,and attempted murders. The culprits run awaywith the weapon, and the firearm sometimesbecomes the only connecting link with the sceneof occurrence. The fired shells are identified byfiring test shells from the suspected weapon andcomparison of the markings on the incriminatingshells with the test shells. The various portions ofthe weapon which come into contact with theshells are firing pin or the striker; the breech face,the portion which surrounds the firing pin; theextractor; the ejector; and the chamber. Theseparts leave their imprints on the fired shells dueto the immense pressure developed when thecartridge is fired. The pressure is transmitted inall directions and to the extent of 3 to 22 tons peisquare inch, and brings the cartridge cases incontact with the various parts of the gun. In thisway striation details from the surfaces are perma-nently left on the fired cartridges and form thebasis of identification.It is inherent in the process of manufacture thatthese surface irregularities on the various partsare never alike in any two weapons though theymay be the same make, kind, and batch. Even inthe case of double barrel weapons, there will benon-identity of the markings of the two barrels.This is due to the fact that in the way the surfacesare processed, it is impossible to obtain twosurfaces alike. The surfaces are cut with cutterswhich get blunted after each stroke. The surfacesmay be levelled with files/machines. Again thefiling process is so irregular that no two surfacesare obtained with like striation. The surfaces mayfurther be finished with sand and/or emery papers,etc. However, all these processes are again irregu-lar, and the microscopic details left on the surfacesis never found identical. It is non-identity of thesesurfaces which individualize the weapon fromwhich a particular cartridge case is fired, becauseit is the impression of the surfaces which forms thethumb print of the gun on a particular cartridgecase.It may be argued that the surfaces should alsochange when they come into contact with thecartridge cases at such a high pressure as mentionedabove. Some changes do take place due to thiscontact, but weapons are normally made fromsteel while the cartridge cases are made either frombrass or copper. The latter metals are much softerthan steel. So the changes made on the surfacesof the weapon are, even after quite a good numberof shots almost unnoticeable. However, if thou-FIRING PIN IMPRESSIONSsands of shots have been fired from the sameweapon, or it is ill-kept or misused, it may becomedifficult, if not impossible, to establish the identityof the markings on the shells of the first and lastshot.The most important constituent in the "thumbprint" of the weapon is the firing pin impression.The cartridge is fired only if the firing pin strikeswith a certain amount of force. Therefore, it hasto come into contact with the cartridge withoutfail and leave its impressions on its percussion cap.The clarity of the markings of the firing pin will ofcourse depend on the pressure which is developedby the cartridge. Only in a few cases will the mark-ings be indistinct so as not to permit identification.As regards the breech face, it also comes into con-tact with the cartridge. However, its impressionwill be imprinted on the cartridge only if the casestrikes back the breech face with a certain mini-mum velocity which may be prevented in most ofthe cases either by the low pressure created in thecartridge or by the chamber which may hold thecartridge case if it is slightly tight. The markingsleft by the ejector and extractor become prominentonly if the firing develops sufficient pressure whichis not a fact in most of the cases. Chamber marksare still less common due to the fact that thediameter of the chambers of the weapons are madeslightly larger than the diameter of the cartridgeswhich are intended to be fired from the weapon,and unless there is some defect in the chamber itsmarkings are seldom encountered.The author has made a critical study of the firedcartridge cases in about 600 firearm cases wherethe weapons used were mostly 12 bore guns, afew rifles, and a few revolvers and pistols. Thebreech face markings on most of these cartridgecases were practically non-existent. The extractor,ejector, or chamber markings were found only inrare cases. Therefore, only the firing pin impres-sions could be examined and compared with thoseimpressions on fired test cartridges. They werecharacteristic and individualistic in, at least, 98%icases while in less than 2% cases the impressionswere feeble and could not permit identification.This excludes those cases where the firing pinimpressions were mutilated by several hits or byattempts to destroy the cartridge cases by hammer-ing, filing, or burning. Further, it also does notinclude cases where the firing pins of the suspectedweapons were tampered with or destroyed alto-gether. The experience has convinced the writerthat the firing pin impressions are alone sufficientto permit positive identifications. These findingsare consistent with those reported by Lucas.'The cartridge cases are normally examinedunder the stereo microscope for preliminary checkup where three dimensional view is obtained withmagnified image up to about 20 diameter. Largermagnification up to 100 diameter can also be easilyachieved with the stereo microscope, but the sameis seldom needed. After the preliminary examina-tion the firing pin impressions are examined undercomparison microscope, and the various points ofsimilarity of the firing pin impressions are observedside by side and also adjudged for their relativepositions. The combined result of the examina-tions, i.e. that of stereo microscope and of thecomparison microscope does lead to foolproofresults and can be illustrated by photomicrographsof the firing pin impressions taken side by sideand also by composite photomicrographs to thelayman.From the above, it must not be construed thatthe identification of the fired shells from the firingpin impressions is a smooth-sailing process. In theabsence of breech face, the extractor, and chambermarks, the difficulties of identification are increasedmanifolds. The main difficulties in identification offired shells from firing pin impressions result fromthe smallness of the impressions, sometimes dueto the shattering of the percussion cap and quitefrequently due to tampering of the firing pin.The firing pin impression is formed at somedepth. The depth of the impression may vary fromone cartridge to another depending upon the pres-sure created. If abnormal pressure is developedthe cartridge case is shattered. Even with highpressure, the percussion cap bulges out and makesthe study of the striation details more difficult.It is, therefore, desirable that the test cartridgeshould be of the same make and batch wheneverpossible. Or at least, the test cartridges fired shoulddevelop approximately the same amount of pres-sure which had been developed in the crimecartridge. This pressure can be adjudged by depthof the impression itself. When the pressures arealmost alike the difficulty of the examination ofthe striation details is overcome to a great extent.The important factor in the study of the stria-IA. LucAs, FoRENsIc CHEmSTRY AND ScENTIcINVESTIGATION, pages 210, 240 and 242. Lucas reportson an investigation of 250 cartridge cases that 97.8%contained firing pin impressions which were distinctiveand characteristic.19631B. R. SHARMAtion details is proper type of illumination. Thesmallness of the firing pin impression and slightdifferences in the pressure developed in the cart-ridge cases make it imperative that the lights strikethe cartridge cases at the same angle and with alikeintensities. The striation details are studied by thebrightness and the shadows created by them andunless the illuminations are at the same angle thedetails revealed by the brightness and the shadowswill not be alike and can confuse the examiner.This is especially true, when the percussion capsin the fired cartridge cases had bulged out to differ-ent extents. However, if all the factors are takeninto consideration and proper illuminations utilizedthe difficulties are not so great as are consideredby some authorities.The shattering of the percussion cap introduces adifferent type of difficulty in the study of thestriation details. If it is so destructive that theportions of the percussion caps have blown off,nothing much can be done. If the firing pin hasonly pierced the cap at a particular point, thestudy can still be made. The portion bearing theimpression is straightened and is properly cleanedto bring it to its original brightness. Great care istaken to avoid introduction of additional scratchesetc. on the surface bearing the impression. Thecartridges with pellet charge and some of the wadsremoved are fired for comparison purposes. Re-moval of pellet charge etc. prevents shattering.If however, the firing pin still shatters the cartridgecase, the powder content of the cartridge case isalso reduced by a suitable amount. The study of thestriation details is then carried out in the usualway. It may be mentioned that the study can alsobe made with a shattered test cartridge, though itis not desirable.The other type of difficulty in the study of thefiring pin impressions is the tampering of theimpression on the crime cartridges which is rarebut frequently can prevent identification.The last source of difficulty may be the relativepositions of the firing pin impression and thebreech face markings. They may not be identicalin the crime and test cartridges. This happenssometimes due to either the looseness of the per-cussion cap or due to the revolving of the firing pinor due to the displacement of the base where thebase is separate from the cartridge tube. It canconfuse the inexperienced workers. In such casesthe firing pin impressions and the breech facemarkings are compared independently.It may be pointed out that in the course of myevidence, certain learned Sessions Judges anddefence counsels remarked that the diameter ofthe firing pin is so small that the striation detailscontained on the firing pins and hence imprintedon the cartridge cases could yield erroneous resultsat least sometimes. However, it was pointed out tothem that the small diameter of the firing pinwhich is about 2 millimeter or less may be insuffi-cient to permit the examination of the details bythe naked eye, but this examination should not beattempted with eyes alone. The cartridge cases areexamined for the firing pin impressions undermicroscope where the magnification of a thousandtimes can easily be achieved and hence the smalldiameter of 2 mm can be enlarged to 200 cm butmagnification of this order is never required and isnever attempted.Thus, we see that with proper examinationsfiring pin impressions can be identified and manyshootings which otherwise cannot be cleared upare solved.[Vol. 54

The Importance of Firing Pin Impressions in the Identification of Firearms

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The Importance of Firing Pin Impressions in the Identification of Firearms

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