Waghmare - Forensic Science Communications - January 2004
January 2004 - Volume 6 - Number 1
Research and Technology
Wounding Power of .315/8mm Bullets Fired Through Glass Windowpanes
Narayan P. Waghmare
Senior Scientific Officer
Central Forensic Science Laboratory
Bureau of Police Research and Development
Department of Physics
Mottamari S. Rao
Chief Forensic Scientist
Bureau of Police Research and Development
Ministry of Home Affairs
New Delhi, India
In India, a .315 caliber sporting rifle is a very common licensed weapon. It, in conjunction with the soft-nose bullet, is often the weapon of crime in the northeastern region of India. Glass windowpanes are a common intermediate target for bullets or pellets and may become a secondary missile. Frequently, the glass serves as a clue in a criminal shooting investigation. This paper addresses the wounding potential and power of a soft-nose bullet at the striking velocity and the remaining velocity when fired through glass of various thicknesses.
The physical characteristics of glass, such as its refractive index and density, are well known. Also well known is the behavior of a glass windowpane fracture under the stress of a bullet impact. Frequently, it is crucial to know the wounding capability of a bullet after it has perforated glass. In forensic ballistics, there is interest in investigating the relationship among the extreme ranges of various firearms and their wounding capability.
In the investigation of a crime when a person is alleged to have been shot through a glass door or window, it could be necessary to determine if the injuries were caused by the bullet that was fired through the glass. Since velocity is an important factor in the wounding power of a bullet, an answer could be found if the remaining velocity of the components of the soft-nose bullet, like the lead core or the jacket fragments after perforating the glass, is known. An investigation of forensic ballistics literature reveals that there is a lack of data on the velocity and wounding capacity of bullets that have perforated a glass window or door. Data about variables commonly encountered in shooting incidents should be collected, processed, and analyzed to provide accurate information about what occurred at the crime scene.
Sellier and Kneubuehl (1994) and Oven-Smith (1981) studied wounding power by experimenting with cloth, wood, and cadavers and comparing their findings with experiences on the battlefield. Many countries used full-lead round-nose bullets weighing between 20g and 25g with initial velocities of approximately 425m/sec. They concluded that in cases of physical injuries and skull wounds, the hydraulic pressure caused by the projectile is important and that some of the lead from the projectile might melt on impact with the bone but not during penetration into soft tissue.
Sellier and Kneubuehl (1994) experimented with the properties of small-arms projectiles. They also studied the effectiveness of jacketed, nonjacketed, and semijacketed bullets that were propelled from the rifle into the human body.
McLaughlin and Beardsley (1956) experimented with what takes place when a bullet passes through a glass intermediary target. They studied the distribution of gunpowder residue on the glass and on cloth targets after test firings. They found that the glass was intact except for the hole caused by the bullet. Although it has been discussed, there is a paucity of information in forensic science literature concerning the effect of intermediate targets on bullets.
Fackler (1994) studied energy expenditure and suggested that much of the energy is dissipated as heat during the creation of the hole. Some of the heat could be from chemical energy, often transformed directly into heat. Some of the heat could be a final product of the expenditure of kinetic energy. In short, the heat resulting from the projectile's mechanical disruption of matter (e.g., air, flesh, or a glass target) gives further injury to the human body.
Depending on its shape, a moving object loses velocity, thus energy, because of air drag. The efficiency of the shape depends upon the flow of air across the object; efficient shapes disturb the airflow very little, whereas inefficient shapes disturb the airflow more (Harrison 1971). A high-speed projectile fired from a great distance may cause an irregular wound entrance as it enters the skin over a bony surface (Hather 1987). Around a gunshot wound, there may be evidence of abrasion, scorching, or burning. The skin may be singed, or the underlying tissue may contain blackened gunpowder residue or glass residue from an intermediary target. There may be metal chips at the entrance and in the track of the wound.
Finck (1965) determined that the casualty criteria, or the amount of kinetic energy required to put a man out of combat, was 4kg-m (39.223 joules) from a French weapon, 6.3kg-m (61.771 joules) from a Swiss weapon, 8kg-m (78.453 joules) from a German weapon, 24kg-m (235.356 joules) from a Russian weapon, and 8.2kg-m (79.993 joules) from an English weapon.
Sellier and Kneubuehl (1994), experimenting with projectiles of various shapes, demonstrated the amount of kinetic energy per surface area required to produce contusions and large-bone fractures in men and horses. They stated that a spherical lead bullet measuring 11.25mm in diameter and weighing 8.5g at a velocity of 46m/sec produces only a contusion on the bare human skin. Its kinetic energy is approximately 1kg-m (9.490 joules). The same type of bullet at 70m/sec penetrates into the target with a kinetic energy of approximately 2kg-m.
For this paper, measurements of the initial striking and remaining velocities of soft-nose bullets fired through glass windowpanes of various thicknesses were recorded. A .315 caliber sporting rifle manufactured in India in 1973, bearing serial number 93 AB 3228, was used to fire the bullets. A numerical analysis was done to correlate wound formation by a soft-nose bullet fired through windowpanes of various thicknesses to its resulting loss of energy. These mathematical models support laboratory results of injuries caused by glass fragments as well as bullets on persons standing near windowpanes or glass doors. The range of firing can be calculated from the muzzle velocity, impact velocity, remaining velocity, and the ballistic coefficient of the bullet. To estimate the wounding criteria (i.e., 79.993 joules), the firing distance is included with any remaining velocity and energy that would be dangerous to human beings (Warren Commission 1964; Sellier and Kneubuehl 1994).
In general, a .315/8mm cartridge manufactured in India consists of a cartridge case and a soft-nose bullet. The bullets are made up of a lead core surrounded by a jacket that fragments when fired through a windowpane. If d is the diameter of any lead core/jacketed component and l the thickness, the overall exposed surface area S can be given as:
|Equation 1||S = π d [d/2 + l]|
The lead core and jacket fragments do not behave as a stabilized projectile during their flight after penetrating glass. They tumble in an irregular trajectory. Because ballistic coefficients for the ejected fragments vary with each shot, it becomes necessary to take the average for approximate values.
Cummings (1953) found that by ignoring the complication from residual spin, he could show that the ballistic coefficient of a tumbling lead core and jacket fragments approximates that of a sphere of the same weight W and surface area S. With this estimation, an expression for the diameter (ds) of the equalization sphere can be derived as:
Several experts in the wound ballistics field have suggested formulae for the resistance offered by the bullet. Burrard (1955) suggests the following equation for calculating the ballistic coefficient C of a spherical projectile as:
|Equation 3||C = W / (2.3 ds2).l|
If W is in grams, then Equation 3 can be written as:
|Equation 4||c = 0.00096 W / (d [l + d/2])|
Hence, Equations 1 through 4 can be used to evaluate the trajectory of the lead core and jacketed portions, where W is the weight of projectile in pounds, and ds is in inches.
|If|| R = resistance to the motion of bullet
v = remaining velocity after passage through the glass target
r = radius of the cross section of the bullet
v 0 = velocity at the moment of impact
a and b = constants that depend on the nature of the glass that the projectile penetrates
k = empirical constant
Cranz (1943) suggests that:
| R = a ∏ r2
R = ∏ r2 (a + b.v2)
R = ∏ r2 (a + b.v2) . (1 + k. vo)2
Computation software developed by Central Forensic Science Laboratory in Kolkata, India, was applied to the above formulae to establish the relationship between the thickness of glass and the loss of energy.
If penetration and the effect of acceleration is considered as a straight line, and the effect of acceleration from gravity is negligible, the equation of motion of the projectile fired through the glass target is given as:
|Equation 5||mv dv/dx = −∏ r2 . a.i|
where i = shape factor for the bullet. The integration of Equation 5 becomes:
|∫dv/dx = -δ1 ∏ r2 . a.i.mv
dv = m (vo - v)2/2 −∏ r2.a.i
|Equation 6||x = m (vo - v)2 / ∏ r2 .1/2.a.i|
In a given experiment, v = remaining velocity after passage through the target. Because a steel projectile may inflict more injury, the distance of perforation in the target becomes:
|Equation 7||x = m (vo - v)2 / ∏ r2 .1/2.a.i|
Perforation of glass target (i.e., depth of penetration), x is proportional to:
|m (v0-v)2 / ∏ r2|
The value of a.i for the soft-nose bullet can be found experimentally with targets simulated to represent the abdomen, thighs, or other areas of the body.
The result can be calculated on the strong assumption that the projectile strikes head on, although the projectile in flight moves with a slight yaw, varying with its trajectory. Hence, the area of cross section ∏ r2 may not be absolutely correct. This could be replaced by (∏ r2cos∂ + 2 l.r. sin∂ ). Therefore, Equation 7 could be written as:
|Equation 8||x = mvo2 / (∏ r2cos∂ + 2 l.r. sin∂) x .1/2.a.i|
When the irregularly shaped bullet fragments do not follow aerodynamic rules in a dense medium, the projectiles quickly try to seek a state of stabilization. Once this stabilization is achieved, the fragments continue to penetrate in a fixed orientation. The loss of velocity of each fragmented bullet has been computed by the Central Forensic Science Laboratory in Kolkata, India. Hence, under ideal conditions the multiple projectiles can be very damaging. Prefragmented bullets may penetrate deeper into the human body to reach vital organs more easily.
Materials and Method
Commercial windowpanes measuring 30.4 by 30.4cm in size, available at 2.6, 3.8, 5.2, 8.5, and 10mm thicknesses, were selected for the experiments. A .315/8mm caliber sporting gun was used to conduct experimental soft-nose bullet firings in the laboratory (Table 1).
Two electronic timers (ET 425A) manufactured by Electronics Corporation of India, Limited (Hyderabad, India), were used to measure the initial velocity and the corresponding remaining velocity of soft-nose bullets fired through the various windowpanes. The timer, with an accurate and stable crystal-controlled oscillator as the standard timing source, is a versatile transistorized unit capable of measuring time intervals as low as 10F/sec. Indicator tubes display the units of measurement with the decimal point automatically indicated.
The 30.4cm square windowpanes were held in a vertical plane by a specially fabricated iron frame. The firing arrangement is shown in Figure 1. The rifle was held at a right angle to the target fixed on a heavy wooden table. To measure the initial striking velocity, the bullet was fired through two aluminum foil screens placed 60cm apart that were connected to one of the timers. These screens started and stopped the timer.
The screen nearer the glass plate was at a distance of 70cm from the glass. In this way, the timer gave the mean velocity of the bullet at various distances in front of the windowpane (i.e., initial striking velocity). During the experiments, care was taken to ensure that the screen nearer the firearm was well beyond the range of muzzle blast so that the timer was actually started by the bullets and not the muzzle blast.
To measure the remaining velocity, two more aluminum foil screens, connected to the second timer, were placed behind the windowpane. To be sure the bullet did not fail to break the screen from possible deviation as it passed through the windowpane, the distance of the aluminum foil screen that started the second timer was 30cm from the windowpane. The distance between the two aluminum foil screens behind the windowpane was 40cm. Thus, the second electronic timer gave the mean instrumental velocity of the bullet at a distance of 50cm behind the windowpane (i.e., remaining velocity).
Measurements and calculations on initial striking and remaining velocity for the .315/8mm firearm, the cartridge, and the target combinations were calculated to acquire the energy. The loss of velocity/energy and their percentages are shown in Table 2.
Results and Discussion
When a soft-nose bullet perforates a glass windowpane, the resistance of the glass causes it to lose some of its velocity, hence the remaining velocity of a bullet becomes less than its initial striking velocity. This can be observed from the striking and remaining velocities given in Table 1. Remaining bullet velocity becomes a function of its initial striking velocity as well as the thickness of the glass. It also depends on the ballistic coefficient of the bullet as well as on the physical properties of the glass. In the test-firing arrangement, the maximum distance between the muzzle of the firearm and the windowpane was approximately 2.5m. At this distance, the bullets could be considered to have hit the glass with muzzle velocities. The remaining velocity measurements in Table 1 show that for windowpanes and glass doors of 2.6, 3.8, 5.2, 8.5, and 10mm thicknesses, the striking velocities are much higher than the minimums prescribed for the penetration of human bone and skin (60m/sec, 37.5–51m/sec) shown by Coates and Beyer (1962). Table 2 reveals that the percentage loss of velocity/energy for the soft-nose bullet is not maximum for higher thicknesses of windowpanes.
The soft-nose bullets generally were found unstable, and the lead core with jacketed portions separated after penetrating the glass target. This instability, coupled with loss of velocity/energy suffered during the penetration of the glass plates, makes a bullet lose its velocity/energy rapidly as it continues to travel. Therefore, it is concluded that the lethal range of a bullet is likely to decrease rapidly after perforating a windowpane.
The factors that affect a wound are shape, weight, and velocity of the projectile; density and characteristics of tissue; direction; and the amount of transmission of energy. In a contact wound (i.e., when the muzzle of the weapon touches the body of the victim), the velocity of the projectile at impact equals its muzzle velocity. In close-range firing, the velocity of the projectile is near muzzle velocity. In distance shots, however, comparative velocity measurements should be carried out at various distances. Two similar bullets within the same range of striking velocity and kinetic energy may produce wounds of different patterns and dimension.
It is difficult and unrealistic to set and use a standard casualty criterion. There are numerous physical, anatomical, and physiological factors that may modify any predetermined casualty criterion. In addition to the striking kinetic energy, the shape of the projectile and the anatomic structure of the injured tissue influence the magnitude of a human casualty. Also, the more the projectiles penetrate, the greater the chances of hitting vital tissue.
To evaluate the wounding power (calculated remaining energy) of a bullet according to its velocity/energy, the relationship between the length, diameter, and weight must be calculated. Table 3 and Table 4 show the calculations for a lead bullet fired at close range through a windowpane serving as an intermediary target.
The difference in the condition of the freely supported glass plates and the built-in supported glass plates is apparent. In the freely supported position, the glass plates are held by a frame in the lower edge of the glass plate, but in the built-in supported position the glass plates are held by an iron frame surrounding the glass plate, although it is not evident in the low-velocity region. All the cracks may be considered to have formed as a result of the dissipation of the stored elastic strain energy at fracture. Glass fracture patterns were observed on the fired windowpane (i.e., radial, concentric). It can be concluded that the total crack length represents the magnitude of elastic strain energy released during fracture.
Assuming that approximately 79.993 joules of energy are required for skin penetration and approximately 27.889 joules for the penetration of bone, the striking and remaining velocity/energy would have to be at least 48.5m/sec, 1.4kg-m. Jauhari and Bandyopadhayy (1975) determined that the striking energy of 7.9kg-m is the minimum to cause a disabling wound. The bullet would produce a wound more easily in the softer tissues. In all cases, the striking velocities and energies were higher than the threshold velocity prescribed for the penetration of human skin and bone or to cause a disabling wound. The remaining energy (12.8kg-m) was also higher than the minimum required to cause a disabling wound. Table 5 and Table 6 show that the mean remaining energy of a lead core and its jacketed fragments after perforating said thickness of windowpane are 534.862 joules and 191.428 joules respectively, which are also more than the minimum prescribed for causing a disabling wound.
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Coates, J. B. and Beyer, J. C., eds. Wound Ballistics. U. S. Office of the Surgeon General, Department of the Army, Washington, DC, 1962, pp.143-233.
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|Test No.||Thickness of Glass (mm)||Distance from Muzzle to Glass Target (mm)||Velocity of Bullet Before Hitting Target (m/sec)||Remaining Velocity (m/sec)||Diameter of Hole After Firing (mm)||Position of Glass on Target|
Firearm—.315 caliber, Rifle Factory, Ichhapur, West Bengal, India
Ammunition—.315/8mm, Kirkee Factory, Pune, India, 1993
Ballistic coefficient of .315/8mm projectile before hitting glass target—.03353
|Test No.||Initial Velocity (m/sec)||Remaining Velocity (m/sec)||Loss of Velocity (m/sec)||Loss of Velocity (%)||Initial Energy (joules)||Remaining Energy (joules)||Loss of Energy (joules)||Loss of Energy (%)|
|Test No.||Weight of Fired Bullet (grams)||Weight of Fired Lead Core (grams)||Diameter of Fired Lead Core (mm)||Length of Fired Lead Core (mm)||Average Ballistic Coefficient of Fired Lead Core|
|Test No.||Weight of Fired Bullet (grams)||Weight of Fired Jacketed Portion (grams)||Diameter of Fired Jacketed Portion (mm)||Length of Fired Jacketed Portion (mm)||Average Ballistic Coefficient of Fired Jacketed Portion|
|Test No.||Distance Lead Core Traveled [X] (meter)||Minimum Energy Required for Lethal (joules)||Remaining Velocity [VR] (m/sec)||Remaining Energy [Calculated] (joules)||Time of Flight [t] (seconds)|
|Test No.||Distance That Jacketed Portions Traveled [X] (meter)||Minimum Energy Required for Lethal (joules)||Remaining Velocity [VR] (m/sec)||Remaining Energy [Calculated] (joules)||Time of Flight [t] (seconds)|
Figure 1: Firing Arrangement (Click to see larger image)