I - DEFINITION

Wound ballistics (W.B.) is the study of the interaction between a projectile and living tissue. If the term "wound ballistics" is recent, the observation of bullet wounds and the attempts to understand the mechanisms that generated them are old.In Europe, the first descriptions can be traced back to Ambroise PARÉ (c. 1510 – 20 December 1590) who was a French barber surgeon. For more information on the history of wound ballistics, see the wound ballistics history page. The role of wound ballistics has become more important as the protection of man against firearms and the mastery of therapeutic gestures to treat his wounds have become more urgent. Modern wound ballistics is an observational and experimental discipline that draws on a variety of scientific and technical disciplines, including physical measurement and computer science.

II - THE MULTIDISCIPLINARY NATURE OF THE W.B.

The mastery of wound ballistics requires knowledge in several scientific branches. Their interconnections can be schematized as follows:

BIOLOGY - BIOMECHANICS The biomechanics of living tissue is well advanced. The field of fast shocks specific to projectile impacts is still under development. Histology is of interest for the study of lesions that cannot be observed macroscopically.

EMERGENCY MEDICINE Emergency medicine is a discipline that benefits from W.B. The assessment of the severity of injuries due to conventional weapons or less lethal weapons cannot be achieved without joint work.		PHYSICS & BALLISTICS Physics, which includes mechanics and therefore ballistics, is essential to the understanding of the phenomena that generate projectile injuries. Any attempt to interpret the injuries observed without relying on the laws of physics leads us down a hazardous path.

It is impossible to be a specialist in each of these scientific fields. Experiments in this discipline require teamwork in which each specialist brings his or her own skills.

III - THE FIELDS OF APPLICATION OF WOUND BALLISTICS

The fields of application of the B.L. are multiple. The main ones can be summarized as follows:

AGGRESSION				EMERGENCIES AND THERAPEUTIC ATTITUDES - Treatment of wounds
				PROTECTIONS - Ballistic protections - Rear Trauma Effects - Blunt shocks - Blow and knife protection
				EXPERTISES - Forensic science - Injury potential of ammunition - Protections

The study of a discipline logically begins with an analysis of the causes that led to its birth and the reasons for its development. This is what we will do in a synthetic way in the following chapter.

IV - IN THE BEGINNING WERE THE MYTHS

If there is one area where myths are rampant, it is the area of weapons and their effect on humans. These beliefs insinuate themselves with disconcerting ease into all minds, even the most cultured, as soon as they refuse, by principle or ignorance, to analyze them and then eventually to fight them with the weapons of science.

The sight of the lesions caused by the projectiles naturally led the observers to give an opinion on the underlying phenomena. Numerous theories, at least false and sometimes even fanciful, have emerged, reminding us that, in this field as in others, there is no salvation outside science.

• The first attempts to explain the phenomena underlying the lesions

The importance and severity of the wounds caused by certain projectiles, generally fired by long guns in wartime, seemed difficult to explain by the simple direct interaction between projectile and living tissue. The first observers on the battlefields tried to explain the causes by mechanisms borrowed from physics but, however, badly understood. Each of these explanations was, in turn, all the more easily accepted as it was issued with aplomb by a recognized authority of a learned society but, nevertheless, closed to the fields of science. The new theory circulated and spread with all the more ease, as its name seemed mysterious, and therefore disturbing, and as the absence of a scientific analysis gave it free rein.

It was indeed difficult to conceive that a few grams of metal could, by themselves, produce the effects observed on a man. It was necessary to look for others responsible for such injuries. The ignorance of the laws of physics led the observers to assume inadequate mechanical phenomena : perhaps a zone of high pressure at the front of the projectile relaxed at the moment of impact, creating a large cavity that dilated the tissue. Or perhaps, and why not surely, the devastating effects of the shock wave, the projectiles being supersonic. In many cases, fragments of the projectile were found. If it was admitted that a metal projectile could damage tissues, it was hardly imaginable that these same tissues could damage a projectile. Immediately, the use of explosive projectiles was imagined. Some had in mind the invention of a certain Mr. Diesel.

Of all these false theories, it is certainly that of the shock wave associated with the supposedly devastating effects of high-speed projectiles that has been the most persistent and, unfortunately, the most deleterious. Even today, although it does not stand up to scientific analysis, it is still sometimes invoked. A short retrospective of therapeutic attitudes to bullet wounds is instructive in this regard.

• The evolution of therapeutic methods for bullet wounds

Without going far back in the history of wound ballistics, almost forty years ago, there was a significant evolution in the therapeutic attitudes towards bullet injuries.The use of high-velocity projectiles, particularly during the Vietnam War, was the source of the most inaccurate theories that led to surgical procedures that had the most harmful consequences on the war wounded, to the point where "surgical procedures were more disabling than the action of the projectile itself". It must be recognized that the media and even advertising actions of the manufacturers also have their share of responsibility. Below is a short review of the history of wound ballistics devoted to this subject.

t was not useless to approach, even very briefly, this theme. It reveals to what extent beliefs, without scientific basis, can be born even in our time and, sometimes, persist.

V - THE BREAKTHROUGH OF THE RATIONAL

The need for a scientific and therefore multidisciplinary approach to the W.B. has proven to be indispensable. In the following, we present the experimental methods that have allowed the discipline to evolve.

The most rational way today to approach the lesional effects resulting from the projectile/human body interaction is to study the two protagonists in order of increasing complexity: the projectile and the human body.

V - 1 - THE TWO PROTAGONISTS OF THE W.B.

1 - The bullet. Its analysis allows to know its constitution (material(s) used), its shape, its mass, how it is distributed and its velocity. These last two values give information on its kinetic energy at the moment of impact, thus on the mechanical work that it is potentially able to provide in target.

2 - The human body. Its study in the mechanical sense is a difficult task, but not impossible. The human body can be considered as a set of inhomogeneous materials. Practically, there is only the bones of the skeleton from which we can derive laws of behavior by the methods of the physics of materials. On the other hand, tests on biological reactors make it possible to highlight the rupture limits of the various tissues, in particular those constituting the highly vascularized organs whose significant hemorrhagic lesions are likely to engage the vital prognosis. This knowledge is particularly useful when evaluating kinetic energy weapons that create blunt injuries, such as defensive bullet launchers (rubber bullets), or when testing ballistic or impact protection. Nevertheless, we can immediately see the gap that exists between an intact organ and the same organ that is heavily damaged, cracked, subject to hemorrhage, as can be the case for example with the liver or the spleen. The choice is usually made to focus on the acute phase. The long-term evolution of an injury is not taken into account because it is generally poorly documented, statistically unpredictable and unlikely to be legally demonstrable.

To understand the phenomena at the origin of the lesions observed in wound ballistics requires to rely on the laws of physics and in particular of mechanics. It is therefore natural to explore the scientific background of the W.B.

V - 2 - THE SCIENTIFIC BACKGROUND OF WOUND BALLISTICS

• The action of the forces

For a body, in the general sense of the term, to deform it must be subjected to at least one force. When a projectile interacts with a medium, as long as it is not a vacuum, it is slowed down, undergoes a deceleration or, in other words, a negative acceleration.

According to the laws of mechanics, the projectile is subjected to forces that slow it down and the medium that slows it down is subjected to forces that are equal in intensity but in opposite directions. It is worth repeating that the interaction of the projectile with the target generates forces and it is these forces that deform the tissues until they eventually rupture, thus creating lesions. Depending on their intensity, these forces are even capable of deforming or breaking the projectile.

The wound potential of a bullet depends on the energy it is capable of diffusing into the target. It must therefore have some. For that one gives him some in the form of a kinetic energy Ec which depends on its mass 'm' and its velocity 'V'. The expression of the kinetic energy 'Ec' is given below.

The mass is due to the material(s) that constitute(s) the projectile. It is given a velocity by using a launcher, a weapon. The expression above allows us to see that if we double the speed we multiply the kinetic energy by 4. To achieve the same result, the mass would have to be multiplied by 4. In firearms, the kinetics of gases and various constraints impose a limit speed. Hence the interest to also act on the mass by using the heaviest materials to have the most kinetic energy possible. Having a lot of kinetic energy is good; using it well on target is better. The art of the munitions manufacturer is to create projectiles that leave the weapon with a lot of kinetic energy and are able to diffuse it as much as possible and as well as possible on target.

A question remains : why refer to kinetic energy ? The answer is given a little further down.

• The equivalence between kinetic energy and mechanical work

It is never useless to repeat things: the kinetic energy Ec of a projectile is the half product of its mass m by the square of its velocity V :

The mechanical work 'W‘ is the product of a force F and the distance ‘d' over which it is applied.

Starting from the second law of dynamics, we demonstrate quite simply the equivalence between mechanical work and kinetic energy. We arrive at a relationship which is fundamental for the understanding of the mechanisms of creation of lesions : for a given kinetic energy, the greater the braking, therefore 'd' small, the greater the force F as well as the constraints imposed on the tissues. This relationship deserves to be framed since it is in some way the key to understanding wound ballistics.

• The interaction surface, a factor favouring braking

We have seen that having a lot of kinetic energy is good but using it well is better. o use it well means for the munitions manufacturer to spend as much kinetic energy as possible by seeking strong braking in order to generate the greatest forces in the target. Let us note in passing that very strong braking can cause the fragmentation of the projectile and thus, economically, an energy loss. Indeed, the energy lost in the fragmentation of the projectile is not used on the target. However, the multiple wounds resulting from this fragmentation usually more than compensates for the energy loss in terms of lesions. The same reasoning can be applied to the energy "loss" during the expansion of the projectile. However, it is also a useful loss since it produces stronger braking and therefore a better interaction, in the lesion sense, between the projectile and the target.

There are several parameters that favor the braking of a bullet. Among these, the interaction surface 'S', also called master torque, is preponderant. If, for reasons of aerodynamics, it is in the interest of the bullet to have the smallest possible surface area ’S' during the flight path, it is to be found advantageous to increase it during the interaction with the target. In the synoptic diagram below, equation 2 shows the interaction force between the bullet and the target. If this equation is relatively complex, it appears that, all the other parameters being fixed, when the surface ’S' increases the braking force ‘F' also increases, thus the constraints on the target. It is in the regions of the target where braking is maximum that the energy expended is also maximum and that the lesions are the most significant. The subtlety for munitions designers is to ensure that the projectile gives up the maximum, if not all, of its kinetic energy in the target and, above all, at the right place by adjusting the ’S' parameter.

We have just obtained an interesting information. To increase the wound potential of a projectile, all other parameters being fixed, it will be necessary to increase its master torque, its surface of interaction with the target. There are three ways of achieving this result and they are the consequences of the behaviour classically observed for bullets in a target. These are expansion, tilting or overturning and, to a lesser extent because it is difficult to control, fragmentation. We will study these phenomena in more detail in chapter VI. But before that, let's see if kinetic energy is the only criterion to be taken into account when evaluating the effectiveness of a projectile.

• Kinetic energy, the only criterion ?

To characterize the wound potential of a bullet, we used its kinetic energy and in particular the equivalence between the kinetic energy and the mechanical work applied to the target. One may wonder if other relations could be used. The answer is that if we want to approach the bullet/target interaction in the most rational way, we must take into account the kinetic energy. It is this energy which gives information on the capacity of the projectile to produce mechanical work and therefore lesions. Nevertheless, some people have tried to model the stopping power of projectiles using other approaches. In order to be exhaustive, we present below, through the article by Serge LOPEZ (currently in French), a synthesis of the results of research on stopping power.

• Stopping power

In view of certain observations, particularly in the field of hunting, a few theories have emerged which have attempted to explain the stopping power of ammunition, and in particular certain sideration effects more particularly attributed to high-velocity bullets.These researches have generally led to formulas in which we find a mixture of different percentages of kinetic energy and momentum. Within the framework of this presentation, we will not go any further in this field and advise the interested reader to consult the synthesis article (currently in French) of Serge LOPEZ, safari guide and holder of the university diploma of wound ballistics.

• Kinetic energy versus momentum

In wound ballistics, one may have to make a choice between kinetic energy and quantity of movement.

The momentum Q represents an impulse, a mechanical shock. It is given by the product of the mass and the velocity V of the projectile:

The fundamental difference between momentum and energy in general and kinetic energy in particular is that the latter can be transformed into any other type of energy. The momentum, on the other hand, is created by motion and can only give motion (see the diagram below).

• Making the right choice

The equivalence between mechanical work and kinetic energy naturally leads us to choose the latter to predict the effectiveness of a bulletin a target. We observe, for example, a direct link between the volume of the temporary cavity and the kinetic energy lost by the bullet.

Nevertheless, in certain cases of blunt impact, the link between injury and momentum appears more obvious. This is particularly the case when we measure the formation of the dynamic cone of deformation, which generates lesions, behind the flexible ballistic protections.

VI - EXPERIMENTAL TECHNIQUES FOR WOUND BALLISTICS

VI - 1 - THE CONCEPT OF A MODEL

Wound ballistics is concerned with the lesions that bullets can create on humans. In this field, the methods of experimentation are necessarily ethically limited and we are led to use models that can be of various natures.

• Constraints when choosing a model

Whatever the model chosen, it must be faithful. That is to say that from what we observe during the bullet/model interaction, we must be able to predict the consequences of the bullet/human interaction.

Regardless of the model used, it is not possible to predict injury to the body directly from raw experimental observations. The results of the bullet/model interaction must be processed through a "transfer matrix" that will transform the consequences of the stresses experienced by the model into a probability of injury on humans. The transfer matrix is fed by various data, including feedback from the field, biomechanical data, autopsy findings...

- Feedback from the field

This is the most important information because it allows us to know the immediate consequences of the bullets attacks. All the phases of the scene are present. In terms of initial conditions, if the velocity of the bullet, and therefore its kinetic energy, is not known, the engagement distance is known and the remaining velocity on impact can be deduced. We also know if it is a direct hit or if the projectile has previously passed through a screen, thus altering its integrity. The wounds observed may be directly related to the nature and attitude of the projectile at the time of impact.

 

- Biomechanical data

These data are of crucial importance if we wish to model and, ultimately, predict the consequences of the projectile/biological tissue interaction.

 

Blunt impact

 

- Autopsy findings

Forensic autopsies are not the most appropriate exploratory methods to provide relevant information in wound ballistics research. The forensic doctor intervenes well after the facts and has the task of answering the magistrate's questions: direction of the shot, chronology of the shots if there were several, lethality of the shot. The conditions under which the shooting(s) took place are not known, except perhaps to the investigators. It is therefore generally difficult to derive any useful data for experimenters from the findings of forensic autopsies, except that, apart from injuries to the brain, haemorrhage is the main cause of death of a shooting victim. However, this information is important.

TEST LABORATORY		ADAPTATION OF EXPERIMENTAL RESULTS		PROBABILITY OF INJURY TO HUMANS
EXPERIMENTAL RESULTS - Biological reactors - Simulant materials - Computerized model		TRANSFER MATRIX

VI - 2 - THE REFERENCE MATERIALS

Experimentally, one method of studying the lesion potential of bullets consists of firing on reference materials (gelatin, ballistic gels) called, somewhat abusively but by simplification, "simulants".

nalyses of feedback from the field concerning the lesionary effects of bullets, associated with the study of their behaviour on "simulants", make it possible to generate transfer matrices allowing the extrapolation of observations on "simulants" to the lesionary effects likely to be observed on living tissue.

• The gelatin model

Gelatin is an organic material (one or a set of proteins) made from the collagen of bones and animal skin. It is an elastic material that gives good indications on the behavior of projectiles in certain organic tissues. Gelatin at 10% concentration and 4 degrees C shows, in this case, a good similarity with muscle tissue at rest in terms of penetration distances. Its main advantage is its relative transparency, which makes it possible to take pictures or videos at high velocity.

Its use requires certain precautions in the manufacturing process and in particular of storage considering that it is a biological material.

• Ballistic gels

This is an evolution, mainly in terms of ease of use, compared to the gelatin model. The gels are more transparent and do not require special care for storage. They can be recast with a few precautions.

Nevertheless, the adoption of a new reference material is not without difficulty, especially if its mechanical behavior differs from the old one. In this case, the calibration process must be repeated in order to obtain new transfer matrices.

• Standardization of tests

Regardless of the reference material chosen, the exchange of results between laboratories requires standardization of tests. For example, in the case of the use of a ballistic gelatin :

- Hardness of the material in degrees Bloom, usually 250 Bloom ;

- Dimensions of the ballistic gelatin blocks: 250 x 250 x 500 mm (mass effect) ;

- Concentration and temperature: 10% at 4° C or 20% at 20° C, for example.

• Tissues with different mechanical characteristics

By modifying the concentration and/or the temperature of the gelatin, it is possible to approximate the mechanical behavior of many organic tissues, from the tonic one of the muscle in contraction to the very aeric one of the lung, passing by the more friable ones of the liver or the spleen.

For bones, it is possible to make inclusions, at the time of manufacturing the test block, with fresh bones in order to evaluate the projectile/bone interaction. This allows us to observe the fragmentation of the bone and generally of the bullet during a direct impact or possibly a fracture due to a remote effect. If the absence of fixation by tendons or ligaments can have an influence in quasi-static mode (low interaction velocity), it has almost no influence during the impact of a bullet given the interaction velocity and the inertia of the tested material. The video* below shows the direct hit of a pig femur, embedded in a block of gelatin at 10% and 4° C, by a .223 Remington projectile.

VI - 2 - THE WOUND PROFILE

Observations of lesions on animals or humans have led to the adoption of a general scheme representing the interaction between bullets and living tissue. It has been given the name of wound profile. This name was certainly not chosen by chance.Indeed, although all profiles have the same constituent elements, their appearance differs according to the modes of interaction of the different bullets.

• Presentation of the wound profile

ntroduced by J. BRETEAU and M. FACKLER, the wound profile is in a way the cornerstone of wound ballistics. It characterizes the interaction of the bullet with the material through which it passes, and the nature and extent of the lesions on the biological tissues.

The shape and dimensions of the wound profile depend on the mechanical characteristics of the medium through which it passes and on those of the bullet, in particular the material(s) of which it is made, its shape, its obliquity (yaw + pitch) and its integrity on impact, its kinetic energy and the behavior it will adopt in the target.

Below is a diagram of a typical wound profile, the volume of which can be linked to the mechanical work required to form it, i.e. to the kinetic energy of the bullet diffused during its path in the target.

A : proximal path (neck)
B : Tumbling zone, mushrooming zone etc. (according to the type of bullet)
C : distal path
1 : permanent cavity
2 : temporary cavity

• Interpretation of the wound profile

The proximal path is the region where the interaction between the target and the bullet begins. The projectile is always stable or only slightly destabilized. If it is an expanding bullet, expansion begins. The faster the projectile expands or tilts, the shorter the proximal path or "neck".

The tilting or mushrooming zone is the region where the bullet has the greatest deceleration, where it loses the most velocity and therefore kinetic energy, and where the injuries observed are the most significant.

The distal path is the end of the bullet's journey. It has lost velocity, possibly mass, and therefore kinetic energy. The mechanical work that it provides is weak.

If we observe the general shape of a wound profile, we can see that the dimensions of the exit orifice depend on the thickness of the target and therefore on the attitude of the projectile at the exit.

The permanent cavity is a region of destroyed tissue, crushed by the passage of the projectile. After healing, the replacement fibrous tissue will no longer have the functions of the original parenchyma.

The temporary cavity is a region of tissue that is violently accelerated. As its name indicates, it exists only for a short time. High pressures are measured there, decreasing with distance. Tissue damage depends on the ability of the tissue involved to withstand these high mechanical stresses.

The wound profile provides information on the mechanics that caused the tissue damage. In addition to visual findings, the use of accelerometers and/or pressure sensors can highlight the mechanisms that generate lesions that cannot be directly observed.

VI - 3 - THE MECHANISMS UNDERLYING THE LESIONS

The mechanisms underlying the injuries are compression and stretching. These two phenomena occur in varying proportions and intensities depending on the distance from the projectile path. They are of maximum intensity along the path of the projectile and diminish towards the periphery.

- Compression is most intense at the point of interaction of the bullet with the tissue, creating a crushing of the biological material. Adjacent tissues are violently pushed back and high pressures are measured within them, the levels of which decrease with distance from the bullet's path.

- The stretching is due to the action of the bullet which pushes the tissues apart in its path. The latter are stretched and, depending on their degree of elasticity, may reach their breaking point: they tear. As with compression, the intensity of this phenomenon diminishes with distance.

Compression and stretching are two concomitant phenomena. Depending on the power of the bullet, the volume of tissue damage may exceed that of the bullet's path, and necrosis may be observed at a distance even though there was no direct interaction between these tissues and the bullet. Fractures may be observed if the bullet passes close enough to a bone without hitting it. The extent and severity of the injury will depend on the ability of the affected tissue to withstand the mechanical stresses of compression and stretching.

VI - 4 - THE MODES OF ACTION OF A PROJECTILE IN A SOFT TARGET

When a bullet interacts with a target, it can exhibit three kinds of behavior :

1 - The tumbling or turnaround. The projectile was stable in the air, but the forces of interaction with the target overcame this stability ;

2 - The expansion. There is deformation of the bullet. Nowadays, it is generally foreseen and controlled. These are expansive projectiles ;

3 - Fragmentation. It is not always desired, although, depending on the hardness chosen for the materials making up a bullet, it can be favored or even caused. Whether desired or not, it increases the effectiveness of the bullet by the multiple wounds of which it is at the origin.

Below is an illustration of these three modes of action.

• The tumbling or tendency of the bullet to overturning

On modern bullets with generally oblong shape, the distribution of the masses means that the resultant FR of the aerodynamic forces is located, by reason of symmetry, on the longitudinal axis and in front of the center of gravity G. The combined action of the weight P of the bullet translated to the center of gravity and of this resultant creates a torque tending to cause the bullet to overturn. This phenomenon is schematically described below.

The tendency to overturn into a target increases the obliquity (yaw + pitch) of the bullet and thus the interaction surface. The image below shows, in red, the increase in the interaction surface with an obliquity varying from 0 to 90 degrees. We can deduce that the control of the phenomenon of overturning leads to the control of the wound potential of the bullet without using expansion or fragmentation. However, on the human body, which is heterogeneous by nature, fragmentation can occur in certain cases when interacting with bone tissue. Fragmentation can also occur in soft tissue, when the forces experienced by the bullet exceed its mechanical resistance limits. It can be considered "accidental" because it was not necessarily intended during the design of the projectile. When this is the case, it appears at the moment of maximum obliquity, when braking is most intense.

Increase of the interaction surface with the obliquity

Some bullets, by design, have a high propensity to turn over. The photo below of a section of a 5.45 mm x 39 projectile intended for the AK 74 assault rifle gives an example. This projectile, due to its constituent elements and the distribution of its masses, has a natural tendency to turn over on target, while at the same time having an excellent capacity to perforate the ballistic protections classically worn by combatants (excluding ceramic plates). It should be noted that this projectile was the "ordinary" bullet of the Warsaw Pact armed forces. Its effectiveness is far superior to the ordinary bullets (lead core and brass lining) of NATO forces, while at the same time conforming to the Hague Conventions because it does not fragment, at least in soft tissue, even at short firing distances.

The video below shows the overturning of the 5.45 mm x 39 AK 74 bullet which comes out of the gelatin block with the base forward.

The video below shows the overturning of a 7.62mm x 39 AK 47 projectile.

The graphs below show the results of experiments on a pig leg, i.e. in muscle tissue. They illustrate the correlation between the attitude of the bullet in target, its deceleration and the importance of the lesions.

• Expansion

Some bullets have a natural tendency to expand. These are usually bullet made of one or more relatively ductile materials such as, for example, certain lead or brass alloys. The expansion of these bullets depends mainly on the intensity of their interaction with the target. Since this tendency to expand is neither constant nor reproducible, it cannot be considered a reliable parameter of effectiveness. It is therefore necessary to create bullets whose expansion is almost certain, except for very low impact velocities, and, as far as possible, controlled. The photos below show two types of bullets whose design predisposes them to expansion. The 9 mm Parabellum bullet also has break-off points at the ogive to facilitate a symmetrical mushrooming.

.38 Spécial soft hollow point	.38 Spécial soft hollow point	9 mm parabellum hollow point	9 mm parabellum hollow point

The video below shows the expansion of a 9 mm Parabellum Gold Dot bullet. It can be seen that the expansion occurs very quickly, making the proximal path barely visible. We can also observe that the well-controlled expansion leads to an excellent stability of the bullet in the material, it is true, homogeneous and isotropic.

• Fragmentation

Bullet fragmentation occurs when the forces generated by the bullet/target interaction exceed its breaking point. In the human body, this often occurs when the bullet strikes bone. However, some bullets can break up in soft tissue. In this case, fragmentation occurs in the phase of the tumbling where they present the maximum interaction surface, thus when braking forces are most intense. Although part of the kinetic energy is "lost" in the fragmentation phenomenon, the mechanical work is still likely to create important injuries. The loss of energy during fragmentation is to some extent compensated for by the potentiation of the wounding effect following the multiple wounds created by the fragments, each creating its own wound profile and thus increasing the risk of haemorrhage.

The video below and the two images that follow show the fragmentation of an ordinary 5.56 mm x 45 (.223 Remington) bullet (lead core and brass jacket) in a block of gelatin at 10% and 4° C.

Below, the details of the fragmentation (main fragments).

Fragmentation of a .223 Remington bullet - Ordinary bullet Firing distance : 10 m

Below, the image of the results of a fragmentation. On the left, the bullet in its initial state. On the right, the remains of the fragmented bullet. In this case, only a part of the jacket and lead particles disseminated in the gelatin were found. The wounds found on the human, during short distance shooting, show the same type of fragmentation.

Résultats d'une fragmentation

Photo : GRICOURT, HIERNAUX, TOUSSIGNANT Practical work of the university diploma of wound ballistics

• Different bullets, different effects

While respecting the laws of aerodynamics, the bullets are given different shapes and structures according to the desired effect on the target. Good stability and absence of deformation are the main characteristics of bullets with a high penetration capacity (good perforation of ballistic protection even with low-power weapons). If an efficient use of kinetic energy is desired, expansion should be chosen. If regulations prohibit it, destabilization should be chosen.

VI - 5 - EXPERIMENTAL INTERPRETATION OF THE WOUND PROFILE

The examples of interpretation of wound profiles presented below are shots made on gelatin blocks at 10% and 4° C. The dimensions of the blocks are: 25 x 25x 50 cm.

The interpretation of the wound profile in a reference material can be done in two modes: dynamic or static.

- The dynamic mode requires the use of heavy and expensive shooting equipment. In fast video, the frame rate must be around 30,000 fps which requires powerful lighting. Heat problems with test materials are easier to manage since the advent of LED lighting. The dynamic mode is the only one that allows to visualize the formation of the temporary cavity.

 

Fragmentation of a 7.62 mm NATO projectile Firing distance 10 m

 

- The static mode makes it possible to measure the effect of the passage of the projectile afterwards. In the gelatin, cracks are observed that can be measured and a probability of tissue volume damaged or, at least, exposed to the mechanical constraints produced by the passage of the bullet can be deduced. Once the shot has been fired, measurements are made on the trace left by the passage of the bullet in the gelatin block. This trace is also called, by habit and certainly a little abusively, "wound profile".

Trace of the passage of a Brenneke bullet of 12 gauge Shooting distance: 10 m

Photo : GRICOURT, HIERNAUX, TOUSSIGNANT Practical work of the university diploma of wound ballistics

A transverse cut of the block with a heating wire allows to sample the "wound profile" and to obtain a data matrix.

7 x 64 mm RWS bullet Section slice made at 10 cm from the entrance hole

Photo : GRICOURT, HIERNAUX, TOUSSIGNANT Practical work of the university diploma of wound ballistics

The recorded data will be used to numerically process the bullet/reference material interaction.

12 gauge Sauvestre bullet Digital reconstruction of the wound profile

Photo : GRICOURT, HIERNAUX, TOUSSIGNANT Practical work of the university diploma of wound ballistics

• Cracking of the gelatin and temporary cavity

The first experimenters (BRETEAU, FACKLER and others) had established a correspondence between the lesions observed on the animals or humans and the volume delimited by the zone of cracking in the gelatin. At that time, the rapid visualization means were heavy, cumbersome, complex to implement and the interpretation of the images took a relatively long time (typically, on a 35 mm film with a length of 300 meters, only the central third was exploitable for technical reasons and the analysis was done image by image). As for the flash radiography systems, they were reserved for very specialized laboratories. As these rapid imaging systems were generally lacking, the experimenters of the time did not have the possibility of observing very fleeting phenomena which nowadays, in order to be well studied, require velocities of about 30,000 images per second. The only way to exploit the results of the shooting of gelatin blocks was by static analysis. The experimenters had therefore quite naturally assimilated the volume delimited by the cracks to that of the temporary cavity. The fast digital video shots show that this is not the case. The volume of the temporary cavity, at the maximum of its expansion, is much larger than the one delimited by the cracks that can be measured in static mode. Below, the two images of the shooting of the same bullet show this perfectly.

It can be seen in dynamic mode that the mechanical effects of the passage of the bullet go far beyond what the analysis of the cracking zone would suggest.

• Beware of hasty interpretations

The temporary cavities created on the gelatin blocks can be, depending on the power of the weapon, impressive. However, one must be careful not to extrapolate directly the observations made on the reference material to the human body. Indeed, the block is made of gelatin, which is a homogeneous and isotropic material. Since it consists mainly of water, the transmission of forces and pressures by hydrodynamic effect is at its maximum.

This is certainly not the case on the human body, which is inhomogeneous by nature, where the hydrodynamic effect, by damping and filtering, is probably less, variable according to the anatomical regions and inconsistent (a stomach full of liquid will react differently to an impact than if it is empty). However, some highly vascularized organs, with a denser and more homogeneous structure such as the liver or the spleen, or even the kidneys, may show behaviors quite close to those observed on reference material. In any case, we come back to the importance of the transfer matrix which is an essential interface between the reference material and the human body.

If certain lesions macroscopically observed during forensic autopsies can give the observer the impression of a similarity between a bullet lesion, generally due to handguns, and that due to a bladed weapon, the point of a sword for example, it is necessary that the wise experimenter, especially if he is working for the protection of the living man, adopts a more prudent behavior. The passage of a sword blade near a bone has never broken it. Sometimes there seems to be a debate between minimalists and maximalists. Without doubt the truth lies between the two, moving with the power of the weapons.

As far as we are concerned, within the framework of experiments for the Faculty of Medicine of Lyon I (university diploma of wound ballistics), we were able to provoke bone fractures at the level of the diaphysis and the epiphysis of pig femurs when 5.56 mm projectiles were fired close to the bone. The shots were fired either with the bone in situ, in the thigh (alone), or with the bone embedded in a block of gelatin at 10% and 4° C. During our joint work, J. BRETEAU informed us of nerve concussions observed when the same projectiles were fired near the cervical spine in pigs. These concussions, without macroscopic signs of trauma, resulted in temporary tetraplegia. At the end, recovery was complete.

he dynamic visualization of the formation of the temporary cavity on reference material as well as the observation of lesions at a distance allows us to put forward the idea that the action of the projectile is certainly felt beyond the area of macroscopically observable lesions. The volume of the temporary cavity and especially its velocity of formation are certainly important factors in the action at a distance, the effect of sideration, and the immediate disabling of combat, probably due to causes other than massive hemorrhage. Some researchers invoke a remote action, by pressure waves, on the central nervous system during impacts, notably in the thoracic region*. Some areas of research are still open.

* Links between traumatic brain injury and ballistic pressure waves originating in the thoracic cavity and extremities. Amy Courtney, PhD - Department of Physics, United States Military Academy, West Point, NY 10996 & Michael Courtney, PhD - Ballistics Testing Group, P.O. Box 24, West Point, NY 10996.

• Kinetic energy versus momentum. Making the right choice (bis)

Experiments show that while the volume of the temporary cavity is related to the kinetic energy expended, the depth of the dynamic cone behind the flexible ballistic guards is correlated to the momentum.

Kinetic energy versus momentum

VII - NUMERICAL SIMULATION

Numerical simulation has become an important part of the study of the interaction between bullets and living tissues. The calculation engines are powerful.

The method consists in using finite element calculation. We take a small element of a material and we study its behavior law, that is to say the way it reacts to a stress. From this small element we can predict how this stress will be transmitted from one to another. This assumes that the material studied is homogeneous. Generally, biological tissues are not homogeneous, but often the approximation that they are gives a very good approach especially if we are interested in the limits of rupture.

• On the usefulness of simulation in general

One can wonder about the usefulness of simulation, taken in its general sense, whether on reference material or through numerical calculation. A first answer is that if simulation was born and has taken such a boom, it is because it is useful, not to say essential, in the field of research.

Some negative opinions can sometimes be given by people who adopt a rejectionist stance, often irrational, towards a method they have difficulty to understand. These people fear that a technique which is increasingly tending towards accuracy, given its scientific basis, is intruding into their own area. The classic criticism is the lack of direct similarity with the human body. This argument does not hold because it has never been a question of establishing a direct correspondence between a reference material and the human body. We are here in the pure domain of psychology.

Others, on the contrary, with an open mind to different techniques, welcome simulation, which allows them to obtain quick answers, to advance their research by freeing themselves from the ethical problems specific to this field, while being economical in time and budget.

In conclusion, the best way to know if the simulation is useful is to make an analogy with topography. A topographic map or a GPS screen does not show all the details of the place we are in. But they allow us to find our way, to follow our route and to arrive at our destination.