The degree of certainty in forensic ballistics

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Content THE DEGREE OE CERTAINTY IN FORENSIC BALLISTICS
A Thesis
Presented to
the Faculty of the Department of Chemistry
University of Southern California
In Partial Fulfillment
of the Requirements for the degree
Master of Science
by
Don L* Armstrong
June 1938
UMI Number: EP41490
All rights reserved
INFORMATION TO ALL USERS
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMl EP41490
Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author.
Microform Edition © ProQuest LLC.
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unauthorized copying under Title 17, United States Code
ProQuest LLC.
789 East Eisenhower Parkway
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MASTER OF SCIENCE
Secretary
Date JyNE.1938
This thesis, w ritten by
DON L^...^STRONG
under the direction of Faculty Com m ittee, /^> ^ ^
approved by a ll its members, has been
presented to and accepted by the C ouncil on
Graduate Study and Research in p a rtia l fu lfill
m ent of the requirements fo r the degree of
F a cu lty C om m ittee
C hair man
J S d A . .
5
a
3 1
TABLE OF CONTENTS '
CHAPTER PASS
I. THE PROBLEM 1
Introduction to Problem and
Justification 1
History 1
II. INTRODUCTION TO FIREARMS 3
Construction of a Gun Barrel 3
Types of Firearms Encountered 7
III. INTRODUCTION TO AMMUNITION 9
Powders 9
Composition and Types of Cartridges 10
The Stopping Power of Ammunition 13
IV. IDENTIFYING MARKS AND METHODS OF
COMPARISON 14
Marks on Bullets 14
Comparison of Marks 14
Shotgun Pellets 16
Marks on Cartridge Cases 16
Comparison of Marks 17
V. EXPERIMENTAL 19
Preparation 19
Results of Bullet Comparisons 20
Result of Cartridge Case Comparisons 22
Result of Powder Burn Tests 23
VI. SUMMARY AND CONCLUSIONS 25
Summary 25
Conclusions 25
BIBLIOGRAPHY 27
LIST OF ILLUSTRATIONS
FIGURE
I* Photograph showing construction of primer
and projectiles
II. Photograph showing "Lottie neck", rimless,
semi-rimmed, rim and rim fire cartridges
III. Photograph shov/ing construction of shotgun
cartridge
IV. Diagram of Light Rays in Comparison
Microscope
V. Photograph of Comparison Microscope
VI. Photomicrograph through Comparator
of four bullets from same gun
VII. Photomicrograph of Primer Caps of two
cases fired in the same gun
PAGE
10 A
11 A
12 A
15 A
15 B
20 A
23 A
CHAPTER I
THE PROBLEM
Introduction to Problem and Justification
"Porensic Ballistics,” is a term which, by common
usage, has developed afpeculiar meaning of its own. Its
significance here is in agreement with the now commonly
accepted meaning. It is the science of the identification
of firearms. The identification of firearms includes a
great many ramifications, but is restricted here to the
relating of three factors. This includes relating one
bullet to another, one cartridge case to another, a bullet
to a gun or a cartridge case to a gun. Relating means
showing that two bullets were fired by the same gun or that
a given bullet was fired by a given gun and so forth.
In the past, the question has often been raised as
to just how certain a legal investigator could be that an
apparent relationship was a true one and not merely a false
pretense masquerading in the name of science. It was the
purpose of this study to determine accurately that certainty
under the most unfavorable conditions.
Inasmuch as a human life has often depended directly
upon such a question, it was felt that the justification for
this study was ample.
History
The history of this, type of study has been relatively
i
brief, beginning with a most comprehensive survey by C.E.Waite
in 1915.
C.H.Goddard was among the first to become associated with
Waite in his studies and has been continuing the work with
many outstanding contributions. For example, Goddard was
directly responsible for the development of the helixometer,
an instrument for measuring the twist of the rifling in a
gun barrel.
Another name prominent in the development of this
branch of science is that of Philip O. Gravelle. He is the
first recorded user of a comparison microscope for bullet
comparison purposes.
2
More recently J.S.Hatcher has entered this field
with notable success.
1) J. of Criminal Law'and Criminology 17 no.2 254-56, (1926)
2) pirearms Investigation Identification and Evidence
Marines, H.C., Small Arms Tech. Publishing Co. (1935)
CHAPTER II
IMTRODUCTIOSF TO FIREARMS
Construction of a gun barrel
A thorough understanding of the method of construction
of both firearms and ammunition is imperative if one is to
understand the problems which arise in this work. A brief
outline of the steps in the formation of a gun barrel will
help to explain some of the theory involved. Gun barrels are
forged, either singly or in pairs, from a special alloyed
strong steel. Interspersed between each operation is an
inspection, but these will be mentioned here only once. After
forging, the barrel is roughly shaped externally and the ends
centered on a lathe. Assuming a definite final internal
diameter, a hole slightly smaller than this is drilled length
wise through the barrel. After drilling, the hole is reamed
to the final size with a series of finely ground reamers. The
diameter of the hole thus produced, when expressed in decimal
fractions of an inch, is known as the calibre. On the European
continent the metric system is used. If the barrel is for a
shotgun, no further work is done and a smooth bore gun results.
The gauge of a shotgun is the number of round lead balls, of
the same diameter as the bore, that weigh one pound.
Rifling is produced by one of two types of cutters. The
first of these is knovm as the hook cutter and derives its
name from its form. Its cutting edge is perpendicular to the
axis of the bore; consequently it is as wide as the groove it
cuts. It is ridgi&^y attached to a bar and drawn through the 3
barrel being rotated as this is done. This causes a spiral
groove to be cut in the surface of the bore. If there are to
be five grooves in the finished barrel, the cutter is returned
to its original position and rotated with no longitudinal
motion through 72°, another cut then being made. If there are
to be six grooves, a rotation of 60° is made. This is repeated
until a complete round is made and the cutter is again in its
original position. The cutting edge is now adjusted to cut
slightly deeper. It requires about seventy strokes to complete
each groove and as the total depth of the groove is about .003
inches this means .000043 inches increase in depth per stroke.
The purpose of this rifling is to impart to the bullet
a rotation which causes a stabilizing gyroscopic action. This
action increases the accuracy and distance of flight. The twist
of rifling is a measure of the rate of rotation of the cutter
with respect to the length of the barrel. An average value for
the twist in a revolver would be one complete revolution in
sixteen inches.
The second type of cutter used is the scrape cutter. This
cutter is about three times as wide as the groove it cuts and
consequently is set at e,n angle to the axis of the barrel. The
cutting edge is so arranged that it operates when the cutter
is moved in either direction. Cutters of this type occasionally
have another blade on the opposite side of the blade holder1 so
that two grooves are cut at the same time. Obviously this double
blade arrangement may be used only on guns which are to have an
even number of grooves.
When the series of grooves is completed, there remains
a like number of raised portions alternating wilth the grooves.
These raised portions are termed lands.
It is of utmost importance to understand the method by
which a metal tool cuts. Its action is atearing one rather
than a neat shearing. This results in a relatively rough
surface being left by the cutting edge. The metal is torn and
as a result, microscopic projections are found throughout the
length of the barrel. These projections mark the soft lead of
a bullet when it passes through the barrel and will put the
same marks on every bullet. Thus, a means of comparison is
afforded whereby bullets may be related to the gun from which
they were fired.
Manufacturers of highly accurate guns remove some of
these projections by a process known, as lapping. This process
consists of casting a mold of a soft metal in the barrel of
the gun and then working it back and forth in the barrel after
applying a light oil and some very fine abrasive. This, would
lead one to the conclusion that a bullet fired through such a
gun would have very few lines on it which were individual
characteristics of that gun. However, other factors @&ter
into consideration.
Some types of powder cause rusting to take place, if
the gun is not properly cleaned after use. Each*.time the gun
is fired, erosion takes place. Small bits of metal may be
sheared from a bullet and weld themselves to the barrel. All
or any of these factors would result in a change of the micro
scopic lines that would appear on a bullet. These changes
come about very gradually as will be shown later in this paper.
Primary considerations in comparing bullets are :
calibre, number, direction and pitch of the grooves. By
direction is meant right or left handed. If the cutter is
rotated in a counter clock wise direction while being drawn
toward the observer, a right handed twist will be obtained.
Calibre may be determined visually by comparison with known
standards or measured with a special type of micrometer. In
order to ascertain the bore of the gun, it is necessary to
measure the diameter between grooves on the bullet. The number
of grooves is the basis for a very simple test, for obviously
a bullet with five grooves could not have come from a barrel
having only four. In America the Colt’s Pirearms Manufacturing
Company is the only one, at present, which constructs guns
having a left handed twist. Observance of this item may save
considerable labor at a later time. The twist of the gun may
or may not be of great value depending upon the amount of
twist and upon the deformity of the bullet. Ordinarily one
cannot distinguish between two different, but similar twists.
This is due to the lack of a definite axis to base measurements
on. The bullet may be deformed by striking an object while in
flight, or it may have entered the barrel at an angle which
would allow one side of the bullet to touch before the other.
Still another possibility would be that it jumped the lands.
When a bullet first strikes the rifling, it is moving with a
high velocity, but Y/ithout rotation. Some distance is
traveled by the bullet before it takes up the full rotation
of the lands. This results in a widening of the grooves in
the front portion although this usually tapers to the true
width of the groove by the time the end of the bullet is
reached. In any event it renders comparison "by pitch very
unreliable. In an effort to avoid jumping the lands, some
rifles have been constructed with an increasing rate of pitch.
This avoids loss of speed and power by allowing the bullet to
take up the rotation gradually.
Types of Firearms Encountered
As one might expect, small hand firearms are used more
frequently in crime cases than any other type. This is due
principally to the ease of concealment and portability of a
small gun. In farming sections, one finds that the shotgun is
the most common type as this gun affords easy protection against
the encroachment of rabbits, hawks and the like. Rifles are
very seldom met unless the shooting is premeditated or due to
a hunting, ’accident1.
It might be well to differentiate between a revolver
and an automatic pistol inasmuch as these terms are often
confused. A revolver holds its supply of cartridges in a
rotating cylinder containing a number of chambers. Each time
the gun is cocked, a chamber with an unfired cartridge is
aligned with the barrel. Revolvers hold from five to nine
cartridges in a cylinder. An automatic pistol holds its supply
of cartridges in a magazine usually located in the handle. The
force generated by the discharge of one cartridge is used to
eject the empty shell and insert a new one in the barrel,
cocking the gun as this is done. An automatic, therefore, does
not fire a comiolete magazine with one pressure of the trigger,
but requires a separate pressure for each cartrdige. Some
foredgm makers do make an entirely automatic pistol which fires
8
a magzine of from nine to twenty shots in one burst. There
is great danger of over heating the barrel when this is done
and the gun is likely to jam. The third common type of hand
gun is the single shot target pistol which is loaded one shot
at a time.
Rifles vary principally in the mechanism by which a
fresh cartridge is inserted in the chamber of the barrel.
There are four common methods of loading repeating rifles.
They ares automatic, carbine action, bolt action and slide
or trombone action. A detailed description of these different
types is unnecessary as the same result is obtained by all
and the same general type of mark is made on the cartridge
case while being fired and ejected. A rifle is a very high
power gun giving double the velocity and many times the
accuracy of the hand gun. The twist of the rifle is less than
that of a revolver as the barrel is much longer, thus
giving the bullet more time to take up the rotation of the
riflings. A rifle usually fires single ball cartridges as does
a hand gun.
A shotgun differs from a rifle in that its charge
is a group of projectiles of a predetermined size which is
a fraction of the bore diameter. A shotgun with rifling
would be unnecessary as each projectile acts individually
of the others and no rotation could be imparted to such a
group as a whole. The construction of shotgun shells will
be taken up in another section.
CHAPTER III
INTRODUCTION TO AMMUNITION
Powders
Since the invention of black powder by the Chinese,
only two basically different explosives have been developed.
They are nitrocellulose and nitroglycerine.
Black powder is a mixture of potassium nitrate, sulfur
and charcoal in an approximate rationof six to one to one. The
constituents are very finely ground and then thoroughly mixed
after which they are treated with a binding agent to form small
granules. This powder is known as a bulk powder as it may be
safely measured by its volume when loading cartridges. The
nitro powders, or smokeless as they are also known, must be
weighed out to insure a safe quantity. A single base powder
contains only nitrocellulose, while a double base powder- is a
mixture of the two smokeless powders. The explosive constituent
is suspended in a gelatine mixture and after hardening is
formed into such shapes as may be required. Du Pont1s Bulls Eye
powder, for example, are round flakes which have been cut from
a long rod of the gelatine mixture. Nitro powders are much more
powerful than black powders and are sensitive to the volume in
which the explosion takes place. This is the reason for the
indented groove found around the brass cartridge cases which
contain smokeless powder. If the lead bullet were accidentally
pushed further into the case than it was originally intended
to be, it would result in a rapid and violent detonation rather
than afelatively slow explosion. Such an occurence might easily
result in the rupturing of a chamber. The ring or groove prevents
10
this from occurring* ,
The bases of bullets fired by smokeless powder are
4 1
relatively smooth and gray* If black powder was used, the
bases will be black and pitted* The blackening is due to
the large quantity of- smoke accompanying a discharge of
£
black powder and the pits are due to the hard grilles of
powder being forced against the lead before decomposition
takes place. Residue left in a gun barrel is usually greater
if black powder is used. This residue contains hygroscopic
materials and will result in a rusting of the barrel unless
it is carefully cleaned after using.
Composition and Types of Cartridges
There are four principal sections to modern ammunition/-.
These include: a container which supports or encloses the primer,
the powder charge and the projectile. This container is
commonly made of brass or copper although shotgun shells are
made of laminated paper* In the rear or base of the case is
located the primer cap, the anvil and the primer itself* The
primer is a material which is sensitive to shock. An example
is mercury fulminate. The primer cap is made of a soft mate^&i
so that when the firing pin strikes it, the primer is crushed
between that cap and the anvil, thus causing an explosion. This
explosion is communicated to the main powder charge by means
of a hole underneath the anvil. The main charge is now ignited
and the bullet is forced out of the gun. The type just described
is known as center fire ammunition as the primer is located
in the cemsfcfer of the base of the cartridge.

11
The rim-fire type differs in that there are no
separate pieces to the primer* The priming material is held
§ >
in the rim and the case acts as primer cap and anvil at the
same tipe. The firing pin obviously must strike this cartridge
on its rim in order to fire it. The rim-fire type of cartridge
is found most commonly in the .22 calibre. A special .41
calibre short is made for the Derringer, a small two barreled
pistol.
There are a great number of different types of cartridges
for every gun. A .38 calibre gun may be provided with a plain
lead ball, cuppro-nickel jacketed, armor piercing, wad cutter,
and hollow point projectiles. A .22 may fire shorts, longs,
long rifle or special cartridges. Two types of center-fire
ammunition of .22 calibre are made for special guns. They are
the, "Hornet" and, "High Pov/er". These are rifle cartridges of
extremely high pov/er. The case is known as the ’bottle neck1
type in which the powder chamber is much larger than the
projectile. They are used for hunting small, game animals such
as woodchucks, coyotes and deer.
Shotgun cartridges have a cardboard disk the size of the
bore over th^ shot group and a felt disk under the shot group.
The cardboard disk or wad keeps the shot from falling out of
the case until fired. It also tends to keep the shot group
together as long as possible to avoid spreading the shot. The
resistance of the air prevents the shot group from staying
together uore than a few feet in any case. There is another
FIGURE II
11 A
cardboard wad under the felt which keeps the grease from the
felt wad from running into the powder and thus cause a misfire*
The felt wad usually is soaked in grease and serves to lubricate
the bore when %he gun is fifed. Its principal purpose is to
prevent the shot from being spread by completely filling the
bore providing a uniform pressure surface against the shot
thus preventing leakage of the gases around and through the
shot.
In all cartridges some provision is made for the
lubrication of the barrel at the time the shot is fired. In
the shotgun shells, this is taken care of by the grease in
the felt wad. On the solid lead bullets, grooves known as
cannelures are rolled or impressed perpendicularly to the
axis of the bullet. These cannelures are filled with grease
before they are placed in the cartridge case and the case is
crimped on the bullet above these grooves. The number, size
and shape of the cannelures are of aid in determining the
make of cartridge when tracing a bullet. Some manufacturers
have developed a gilding metal which replaces the grease to
some extent. Gilding metal is an alloy of copper and tin with
a very high percent copper. A very thin film of the alloy is
applied which flakes off when passing through the barrel.
Being very soft it does not injure the barrel which is made of
a hard steel. The bullets a.re much cleaner to handle than are
the grease lubricated ones and do not pick up dirt when
carried in the pockets. They are of great disadvantage to a
criminologist in that the flaking off of the alloy destroys
S'lGUKE Ixi
12 A
■ i
h
*
13
pertinent characteristic marks of the gun* Nickel has also
been used for coating bullets.
The actual diameters of the bullets are larger than
the bore for which it is intended, but smaller than the
diameter between grooves* When a bullet passes through a
barrel, it is forced into the grooves by expansion caused
by compression due to the lands of the barrel. Thus the lateral
force on the bullet will be greatest on its grooves* The
bullet may not completely reach the bottom of the grooves
in the barrel and may receive no marks at all upon its
own lands* Therefore, when an examination of a bullet is
made, particular attention should be given to the grooves.
This will be taken up in greater detail later.
The Stopping Power of Ammunition
The stopping power of ammunition is not connected
directly with the purpose of this work and will receive
scant attention. Stopping power is not a function of any
one factor and few definite conclusions can be drawn with
regard to it. In general it may be said that velocity,
bullet weight and bullet diameter are primary factors.
However, one must also consider the bullet form. For example,
if other factors are constant, a wad-cutter would have a
greater effect than would an armor piercing bullet. A wad-
cutter is a bullet with an almost flat front, while an
armor piercing bullet has a pointed cuppro-nickel jacket.
CHAPTER IV
IDENTIFYING MARKS AND METHODS OP COMPARISON
Marks on Bullets
As has “ been mentioned "before, when a bullet passes
through a gun barrel it is compressed radially and any
projections from the interior of the barrel result in
striations on the bullet which are continuous on all parts
of the bullet which have come in contact with that projection*
If a bullet is closely observed under a twenty or thirty
power microscope, many very fine lines will be seen. Any
bullet that passes through a given barrel will receive these
same striations if none of the projections has changed and if
the bullet is the same size as the previous one. The passage
of each bullet through the barrel changes some of these
projections so that no two bullets are ever exactly alike.
Enough similarity does exist so that bullets fired consecutively,
or over one hundred apart, can be definitely shown to have
been fired from the same gun. Rusting, erosion and corrosion
all tend, to change the interior of the gun barrel and an
agreement between two bullets is much more conclusive than
a non-match.
Comparison of Marks
The inability of an observer to carry an exact picture,
of bullet striations, in his mind while looking first at one
bullet, then another under the microscope is self evident to
anyone who has ever so examined a bullet. An instrument was
devised which enables one to observe portions of two fields
15
at the same time* It is the comparison microscope* A series
of prisms provides this divided field and allows the bullet
striations to be matched directly and exactly* Photomicro
graphs may be made with this instrument as with an ordinary
microscope. The diagram following shows the path of the light
rays in the microscope.
In the past it was the custom to mount a pair of
bullets on spindles beneath the objectives by means of a
plastic material. This was found very unsatisfactory as much
time was wasted in attempting to adjust the bullets to equal
heights. The plastic material contained some organic substance
which reacted with the bullets and caused considerable
blackening. This is very undesirable as evidence must not be
disfigured in any way. Lastly, the plastic material would
suddenly lose its hold and allow the bullet to drop off in the
middle of a comparison.
This investigation devised a type of spring clip which
allowed quick changing of the bullets and practically auto
matic centering. It is perhaps best understood by the fdlowing
photograph. This holder was used throughout this investigation
and seemed to be quite satisfactory.
The method of comparison is to adjust one bullet groove
to a focus under the microscope in a fixed position. The other
objective is now focussed an a groove in the other bullet, if
no match is observed, one of the bullets is rotated one groove.
This is repeated until a complete revolution is made. The first
bullet is now rotated one more groove and the process is
FIGURE IV 15 A
r
OCULAR LRUS
PRISMS
M / / //Z
OBJECTIVE LENSES
Diagram of Light Rays in
Comparison Microscope
FIGURE V 15 B
16
repeated. This continues until all grooves have been
compared with each other.
Shotgun Pellets
Obviously no direct comparison can be made between
two shotgun pellets as there is no way of knowing which
pellet touched which side ,of the barrel if any. Otto Mezger"*’
has developed the only possible means of obtaining
information from a shotgun pellet. The method consists of
holding that portion of a pellet whose surface shows that it
was in contact with the barrel, against a series of standard
curved surfaces. By observing which surface the pellet best
fits, some indication is given as to the bore of the shotgun
which originally fired the pellet. It is also of value to
know the weight and size of pellets which are used to load
the different gauge shells.
Marks on Cartridge Cases
Snapping the firing pin upon an unloaded cartridge case
often results in an interesting phenomena. The primer cap is
forced backward almost out of the case by the force of its
own explosion. This must happen a,lso when the case is loaded,
t
but the reason it is not observed is tha.t the secondary
explosion of the main pov/der charge forces the case back alsof
jamming the case against the breech block thus causing the
cap to be forced back into its original position. Naturally
the soft primer cap receives full impressions of any
irregularities on the surface of the breech block. As these
irregularities are due to tool marks, such as filing by hand
1. Mezger, Otto, Chemiker Zeitung, (September 1930) p 775
17
or grinding, they are different for each gun. These marks
are reproduced on the primer cap and often on the "brass "base
of each cartridge fired inirthe gun. These marks are constant
and ordinarily do not change throughout the life of a gun.
They provide a means of comparison which is in many respects
very similar to the comparison of fingerprints. The firing
pin, in striking the primer cap, reproduces any projections
or depressions it may have in the same manner.
In addition to the above marks, an automatic makes two
other marks. One of these is found under the rim of the case
in the extractor groove usually on the right side. This mark
is due to the extractor which pulls the case from the chamber
when the gun is fired. The other mark is found on top of the
rim usually across from the extractor mark and is due to the
ejector. The ejector is an immovable rod which is in thd path
of the left side of the case when it is being extracted. As
the extractor is pulling underneath the rim on the other side,
the case is flung to the right, out of the mechanism through
an opening which is uncovered by the rearward motion of the
slide. As these movements are made with great force, the brass
is marked so that it is easily noticed when viewed with the
naked eye•
Comparison of Karks
Comparison is first made with the Greenough type
microscope as this allovsrs the relative depth of the marks to
be estimated. A low power is a,lYrays used so that two cases
may be viewed in the same field at once. When a comparison
is wanted at higher magnifications, a photomicrograph is made
through a rgular microscope and the prints of the pictures
are compared. A satisfactory method of doing this is to cut
identically shaped sections from two prints and interchange
them.
In making photomicrographs of cartridge cases and
bullets it is better to have light coming at a low angle to
provide shadows thus increasing contrast. This necessarily
results in uneven illumination and an unbalanced picture as
only a small amount of back lighting may be used. If great
care is taken, this may be corrected by projection control.
One must scrupiously avoid producing any lines in the print
which were not in the original.
In comparing cartridge cases, it must be remembered
that the maker’s insignia, which is on the base of every
case, may have been impressed there by the same die. If such
were true, any slight defect in the die would be reproduced
and an inexperienced observer might be lead to the false
conclusion that two cartridges had been fired from the same
CHAPTER V
EXPERIMENTAL
Preparation
As mentioned before, this study was undertaken with
the idea of securing the most unfavorable conditions for
comparison. In line with this thought, two revolvers were
especially manufactured for this investigation through the
courtesy of Major Douglas B. Wesson of the Smith and Wesson
Company. It has long been a contention in legal circles that
two guns made one after the other should produce the same
markings on bullets fired through them.
J. S. Hatcher reports an experiment in which he
compared bullets from two guns made by the same machine at the
same time. The guns were automatics and were rifled as a pair,
the barrels being cut apart later. The barrels are constructed
so that they point away from the place of separation. Two
bullets fired through two such barrels will travel in opposite
directions with respect to the original single barrel. The
revolver barrels used in this study were made separately by
the same operator on the same machine, with the same reamers
and with the same cutter. The barrels and cylinders were
numbered in the order in which they had been prepared. The
guns were of .38 calibre built on a .44 calibre frame. The
serial numbers were #1-54764 and #2-54749. It is interesting
to observe that the serial numbers are not consecutive• In
all probability this difference is due to the frames having
been made non-consecutively. This does not matter as no
part of the frame comes in contact with the bullet.
20
A series of 212 bullets were fired from these guns
into a box filled with soft cotton waste. Each bullet was
collected, numbered and set aside for later comparison. The
cotton waste stops the bullets within three feet without
deforming them in any way.
The cartridge cases from these guns were also very
carefully preserved.
Results of Bullet Comparisons
The aid of eight observers was enlisted for a
preliminary test. All had previous scientific training and
six had some experience in microscopy. Hone of them had ever
engaged in bullet or cartridge case comparison. A series of
forty eight bullets was submitted in groups of threes to
these observers. The problem was to determine whether or not
a relationship existed between any of the bullets and if so,
what that relationship was.
It should be realized that there were five possible
combinations of the three bullets.
1. All of the bullets were fired by the same gun.
2. Bullets number one and two were from the same gun.
3. Bullets number one and three were from the same gun.
4. Bullets number two and three were from the same gun.
5. All of the bullets were fired from different guns.
Thus, as there were sixteen sets of three bullets in
the forty e&ght, there were eighty possible combinations.
In the first test, all of the observers reported the
correct relation. In the second test, six observers reported
FIGUKS”VI
20 A
21
correctly the first time and the other two were correct on
the second attempt. The latter were those who had no previous
experience in microscopy. In these tests, the two bullets
from the sane gun were consecutive ones.
Attention was next directed to a comparison between
the bullets of guns one and two in order to ascertain if
any similarity existed at all. Exhaustive comparisons were
made of bullets which closely approximated one another in
number. That is, bullet number one from gun number one was
compared with bullet number one from gun number two and so
on. It was thought that if there were to be a similarity
it would be most evident by such comparisons. The highest
correspondence found between two su ch bullets was five
microscopic lines per bullet or about one per groove. The
first thirty five bullets compared had very few lines
observable. In fact there were so few, that only two lines
per groove coincided when bullets one and two from gun number
one were compared.
The number of lines observable increased with the
number of bullets fired. By the time bullets thirty five and
thirty six were reached, a fair comparison of three or four
lines per groove could be made. This increase in the number
of striations is due to the erosion of the barrel by the
bullets. In order for a definite standard to be established,
agreement of two or more lines in three separate grooves
was taken as the minimum allowable. Bullet two hundred one
agreed perfectly with bullet two hundred two and fairly well
22
with bullet thirty five.
Bullets that v/ere less than one hundred apart gave
very close agreement. These tests were all made using .38
calibre special lead bullets. The two hundred eleventh and
two hundred twelfth bullets from gun number one were Lubaloy
coated. These bullets gave very poor comparisons with one
another and did not check at all with the lead bullets. This
was due to the previously mentioned scaling or flaking off
of the gilding metal.
A most conclusive proof that the lands and grooves
of two barrels are unrelated was found by a comparison
between two consecutive bullets fired by the same gun. The
bullets had been marked before firing so that the correspondence
of grooves was already known. A comparison was then made
between different grooves. That is, groove number one on
bullet two hundred one was compared with groove number two of
bullet two hundred two, both bullets being from the same gun.
If there were any similarity among grooves at all, it would
have been most apparent, but such was not the case.
Results of Cartridge Case Comparisons
For the first series of tests, the previously mentioned
eight observers aided again. The conditions existing were the
same as the bullet trials. Five of the observers correctly
classified three cartridge cases the first trial, two the
second while one required three attempts.
The second set of twenty four cases resulted in
absolute failure by all observers. The explanation of this
23
was that only mid range loads were used in this set of
cartridges, which more than halved the pressure exerted on
the breech block. This pressure is insufficient to make
impressions on the cartridge case. This lack of pressure
also accounts for the inability to secure a comparison of
marks on .22 calibre cartridge cases. In this case, one must
rely upon the firing pin impression.
Only moderate success was made in obtaining agreement
with full range loads. The variation in hardness of the
primer cap accounts for some of this. Another important
factor was the fit of the cartridge in the chamber. If it
was loose, the case came back with great force and the primer
cap was engraved deeply. If the case fight tightly, the
expansion produced by firing caused it to stick so that very
few or no marks were visible on the base.
Comparison of the firing pin indentation was helpful
at times, though often a double impression was made. The
initial mark fired the cartridge and the second was made when
the case was forced back while the firing pin still protruded
beyond the breech block. When this occurred, the marks were
not doubled in depth, but rather obliterated. The first
time the firing pin hit, it was striking a convex surface,
the second time it was a deeply concave surface.
Results of Powder Burn Tests
These tests were not mentioned previously as they
were of a secondary nature. The test consisted of placing
paper at ai carefully measured distance in front of the
FIGURE VII
23 A
24
muzzle of the gun and firing through it. The paper was then
inspected for powder scorching, "blackening and similar
details. The purpose of this being to aid in the determination
of how far away the gun was at the time it was fired.
Many different guns were tested in this fashion at
distances of from one decimeter to one meter. In every case
v/hether nitro or black powder was used, unburned poy/der
particles were observed on the paper. Black pov/der produced
many holes in the paper, around the bullet hole, which were
due to the hard particles of pov/der. It produced a slight
blackening at distances of less than thirty centimeters with
a .38 revolver having a three and one half inch barrel. The
spread of particles was roughly proportional to the distance
from the paper for a given gun. Bor a given distance, the
spread was roughly inversely proportional to the barrel
length. One cheap revolver did not properly align the chamber
with the barrel and as a result shaved lead badly. The
ps,rticles of lead were thrown in all directions, but
*
principally forward with the bullet and sideward from the
rear end of the barrel.
CHAPTER VI
SUMMARY AMD CONCLUSIONS
Summary
The purpose of the study was to determine how accurately
one may say that a given bullet or cartridge case came from
a given gun# With the idea that it would aid in understs,nding
the problems involved, a brief resume of the construction of
gun barrels and ammunition was given# The cause of the
formation of the characteristic marks on bullets and cartridge
cases v/as discussed. The results of a study of bullets and
cases obtained from two guns manufactured consecutively were
reported together with a subsidiary test on powder burn marks.
Conlusions
Bullets from two different guns are never thought to
be from the same gun# The same is true of cartridge cases#
Bullets from the same gun may fail to be correlated if the
gun is new or if the barrel has recently been lapped. A good
correlation is usually obtained between two bullets of the
same type if fired from the same gun. The marks on cartridge
cases remain constant throughout the life of the gun unless
deliberately altered. Only a rough estimation of the distance
from which a gun was fired may be had by a study of the powder
marks on the target, and then only if the type of gun used is
known# If the gun is unknown, the conclusive assumptions are
not justified.
BIBLIOGRAPHY
27
Books
Burrard, Major Gerald, The Identification of Firearms and
Forensic Ballistics New York, Charles Scribner’s Sons,
1934 220 pp
Gunther, J.D. and C.O., The Identification of Firearms
New York, John Wiley and Sons, Inc., 1935 342 pp
Hatcher, J.S., Firearms Investigation Identification and
Evidence Marines, B.C., Small Arms Technical Publishing
Company, 1935 873 pp
Smith, Sydney and Glaister, John, Recent Advances in Forensic
Medicine Philidelphia, P. Blakiston1s Son and Company, Inc.,
1931 ppl-76
Periodicals
Goddard, C.H., "Scientific Identification of Firearms and
Bullets," Journal of Criminal Law and Criminology, XVII
(August, 1926), pp 254-6
Mezger, Otto, Chemiker Zeitung (September, 1930), p 775

Asset Metadata

Creator Armstrong, D. L (author)

Core Title The degree of certainty in forensic ballistics

Contributor Digitized by ProQuest (provenance)

Degree Master of Science

Degree Program Chemistry

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemistry, analytical,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c17-787397

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