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.
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
The position, size and the general shape of the firing pin, the breech face, the
extractor, the ejector and of the chamber marks on a cartridge are the class
characteristics of the firearm.
No two firearms, even of the same make and model, produced one after the
other, will leave identical marks on the fired ammunition.
In a fired bullet, the calibre and the number, width, depth, direction, angle and
twist of the grooves and lands form the class characteristics.
The firearms of the same make and model imprint common class characteristics
on fired ammunition. The firearm of different makes or of different models of
same make, usually, imprint different class characteristics.
The finer details in the lands and grooves and individual characteristics of the
firearm. The individual features are always different. No two firearms have the
same individual characteristics.
6. 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.
7. 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.
8. 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:
1. Chambers are over-sized.
2. The chamber is fluted. It may be simple flute or may be helical.
3. The chambers are perforated.
4. 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.
9. 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
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. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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/
