Different types of marks produce during firing process

When a firearm is discharged, a cartridge case comes in clase contact with the firing pin, the breech face, the extractor, the ejector and the chamber. The fired projectiles come in close

contact with the lead and the barrel.

These parts undergo processes like cutting, hammering, reaming, grinding, filing and polishing. Various tools are used for the purpose. The surface profiles of the tools used continue to change due to wear and tear as the tools work on steel.

1. Different types of marks produced during firing process

2. Principle When a firearm is discharged, a cartridge case comes in close contact with the firing pin, the breech face, the extractor, the ejector and the chamber. The fired projectiles come in close contact with the lead and the barrel.  These parts undergo processes like cutting, hammering, reaming, grinding, filing and polishing. Various tools are used for the purpose. The surface profiles of the tools used continue to change due to wear and tear as the tools work on steel.  The finished surface profiles of the relevant parts of the firearms after undergoing such operations are, therefore, different pattern, even when the two surfaces are made with the same set of tools, one after the other.  Consequently, all the firearms, even the two barrels of the same firearm, are individualistic, unique. The peculiarities of the various parts of a firearm are enhanced with the use of the firearm due to wear and tear and accidental damage. When a cartridge is fired, the cartridge case and the bullet come in close contact with the relevant parts of the firearm. A negative profile of the surface (of the firearm) is imprinted on surfaces of the fired cartridge case/projectile when the two surfaces come in close contact with force. The negative profile is called the thumbprint of the firearm. It permits its linkage with the used firearm only.

3. Class and Individual Characteristics

4. Identification Basis of the Fired Cartridge:

5. Firing Pin The firing pin strikes the cartridge on the percussion cap and imprints its mark. The impact explodes the cap composition. The flame and the heat produced, rapidly convert the propellant charge into gases. The high pressure developed presses the case against the firing pin. The clarity of the firing pin marks is thus, enhanced.

6. Breech Face  The surface of the action block surrounding the firing pin is  breech face. On firing, the gases push the cartridge case violently against the breech face, which imprints its negative profile on the cartridge base.  The breech face profile is highly characteristic. It, alone, is sufficient for the identification of the firearm used if the profile on the base of the cartridge is clear. The breech face marks are often absent on the cartridges which develop low pressures. When the ammunition develops high pressure, breach face marks are printed. 

7. Chamber The standard (made to specifications) factory made firearms seldom leave chamber marks on the fired ammunition. When the chamber has some defect or some deposit it imprints its negative profile on the tubular portion of the cartridge case. An over-sized cartridge (case) may also register chamber mark Improvised firearms often create chamber marks on the cartridges fired from them, because there is no adequate space for the normal expansion of the cartridge case, on firing. The chamber marks permit the linkage of the firearm with the fired cartridge. In India, conventional structure of chambers continues. Western countries and China has modified the internal structure of chambers, in some firearms:  Chambers are over-sized. The chamber is fluted. It may be simple flute or may be helical.  The chambers are perforated. Annular rings are created. The changes in the chambers are intended to accelerate/retard the cartridge case. Cartridges fired from such modified chambers, carry characteristic marks: They are useful to link the cartridge case with the firearm.  The clarity of the marks varies and depends upon the nature of the charges, the pressure developed in the firing process, the nature of the tubular surface of the cartridge and the size of the chamber.

8. Extractor Extractor marks, like chamber marks, are rarely significant. They may permit the identification of the firearm, in some cases, whenever the prints are clear. Extractor marks help in fixing the orientation of the cartridge in the chamber. The orientation is often helpful in locating the part of the breechblock (or other parts) which may have imprinted the marks on the cartridge case. Ejector I n automatic and semi-automatic firearms, the ejector stamps its marks on the cartridge case when it strikes against it, in its backward motion. The ejector is so positioned that the fired shell, after striking the device, is pushed out of the firearm. The ejector marks may link the fired shells with the used firearm. The clarity of the marks, however, varies tremendously from one firearm to another and even from one shot to another in the same firearm. The marks are, therefore, more useful as class characteristics to fix or eliminate the brand of the firearms.

9. Ejector In automatic and semi-automatic firearms, the ejector stamps its marks on the cartridge case when it strikes against it, in its backward motion. The ejector is so positioned that the fired shell, after striking the device, is pushed out of the firearm. The ejector marks may link the fired shells with the used firearm. The clarity of the marks, however, varies tremendously from one firearm to another and even from one shot to another in the same firearm. The marks are, therefore, more useful as class characteristics to fix or eliminate the brand of the firearms.

10. Methodology for analysis and comparison Visual Examination The visual examination is carried out with ordinary light, allowed to fall at various angles. Hand magnifier is useful and improves the visual examination. Visual examination of a cartridge case gives an idea about the positions and the sizes of the firing pin, the breech face, the extractor, the ejector and the chamber marks. Stereomicroscopy The stereomicroscopic examination of the bullets is useful for the determination of their class characteristics. They facilitate the elimination of the weapons of the other makes and models. The examination also facilitates the comparison work. It gives the idea as to what to look and where, in the comparison microscope.

11. Comparison Microscopy It gives quick, efficient and presentable link/de-linking evidence present of crime/test fired ammunition. The principle on which the instrument works is simple. The comparison of the two sets of marks is facilitated if they are magnified to the same extent and brought in juxtaposition in the same plane, in an eyepiece or on a screen, simultaneously. A comparison microscope does the same. Side by Side (juxtapose) Match
Here the relevant corresponding portions of the crime and the test exhibits are viewed side by side. The prominent marks are compared and the comparison establishes the match or the non-match. The marks in the matching positions are photographed.
The side-by-side match is useful for the non-striated impact marks from firing pin indentations, the breech face marks, the chamber marks etc. on the fired cartridges.

12. Composite Match
This technique is applied, almost invariably, in the study of all the striated marks:
The firing pin scrapes on the bases of cartridges, the barrel marks on the bullets and buckshots and the scratches, if any, produced by the breech faces, the chambers, the extractors and the ejectors. The striated marks in the firing pin indentations, if present, are studied/compared/photographed by this process like other striated marks.
The corresponding positions of the crime and the test exhibits bearing the striated marks are brought together at the dividing line. The test or the crime exhibit is so manipulated that the striations on one side of the dividing line, show continuity across the dividing line, if the striated marks are from the same firearm.

13. Superimposition Technique
Not all comparison microscopes permit superimpositions of the test and crime marks. The Leitz and the Projectina comparison microscopes are handy for this technique.
The images of the crime and the test exhibits are recorded side by side in half of the film (‘C’ and ‘T’). On the lower half of the film, the superimposed photograph is obtained. The images here are so adjusted that the dividing line bisects the two images exactly, as in composite photograph. Once this is achieved, the dividing line (prism) is moved to one side slowly until the image in the field of view (on the screen) is only from one of the two exhibits (say, crime exhibit). The image is now photographed by exposing it for three-fourth the normal time of the exposure required. The exposure is stopped but the film is kept in the same position. The image of the other exhibit (test exhibit) is obtained on the screen by moving the dividing line (the prism) to the other side. This image is recorded on the same film over the first image, exposing it for three-fourth the normal exposure time.
This second image is found superimposed on the first one, if the adjustment of the exhibits and their images are proper and are not disturbed during the exposure process.

14. REFERENCES
Module 4 – Forensic Examination and Firearms Evidence (introduction-to-forensic-science-and-criminology) – https://wilber-learndev.com/courses firearms-and- toolmarks - overview.pdf from https://www.crime-scene-investigator.net https://ncdoj.gov/crime-lab/firearms-and-tool-mark/
F I R E A R M I D E N T I F I C AT I O N I N T H E F O R E N S I C S C I E N C E L A B O R AT O R Y By Robert M. Thompson – https://www.nist.gov/system/files/documents/forensics/Firearms_identity_NDAAsm.pdf
Different Types Of Marks Produced During Firing Process On Cartridge ppt – LAVANYA PS ASSISTANT PROFESSOR SRINIVAS UNIVERSITY
Technical Procedure for Fired Cartridge Case/ Shotshell Examination North Carolina State Crime Laboratory – Firearms Section Issued by Firearms Technical Leader
How can a bullet be traced to a particular gun? (March 21, 2005) – https://www.scientificamerican.com/article/how-can-a-bullet-be-trace/ EXAMINATION OF FIRED AMMUNITION COMPONENTS.pdf - https://cfsmigrationdocs.blob.core.windows.net/

15. Thank you