Gunshot Wound Ballistics
M. Bradley Evans, M.D.
February 12, 2004

The patient is a 23-year-old Asian male who presented to the Ben Taub General Hospital Emergency Department after sustaining a gunshot wound to the left neck. On arrival, the patient was noted to have stridor and hoarseness, as well as the inability to move his extremities. Radiographic examination in the shock room confirmed a cervical spine fracture of C6 and C7 with bullet fragments tracking from anterosuperiorly to posteroinferiorly. The bullet was visualized in the subcutaneous tissues of the left shoulder. The patient was taken emergently to the operating room where fiberoptic intubation was performed. The Otolaryngology Service was consulted intraoperatively to perform direct laryngoscopy and esophagoscopy, and there was noted to be a large amount of edema of the upper aerodigestive tract as well as a laceration in the hypopharynx. After insuring that the patient was stable, a CT scan was obtained to assess the damage to the cervical spine. A comminuted fracture of C6 and C7 was confirmed, and it was determined that no acute intervention was necessary for the spine injury by neurosurgery. A decision was made to take the patient back to the operating room for a neck exploration. On adequate exposure, there was noted to be perforating wounds to the thyrohyoid membrane and the posterior cervical esophagus. These injuries were repaired primarily, a tracheotomy was performed, and drains were placed and the wound closed. The postoperative course was complicated by the formation of a pharyngocutaneous fistula on post operative day #5, which eventually required operative irrigation and debridement. The remainder of the hospital course was unremarkable. The patient remained quadriplegic and was subsequently discharged to a rehabilitation facility and decannulated a month later.

The objectives for today's talk are to describe an array of weapons and ammunition which are commonly seen in gunshot wound injuries, as well as to describe the ballistics of projectiles from the time the trigger is pulled to the time the target is struck. I will also describe typical characteristics of entry and exit wounds, as well as describing the tissue effects of various penetrating projectiles.

The term "ballistics" actually refers to the science of the travel of a projectile in flight; and the flight path of a bullet includes the travel down the barrel, the path through the air, and the path through the target.

Gunpowder is the force behind the bullet which accelerates it forward, and the quantity and type of gunpowder affects the initial energy of the projectile. Gunpowder was formally described in the eleventh century by Roger Bacon, but its recorded use was in China centuries before that. It was known as black powder and was composed of mostly potassium nitrate with smaller amounts of charcoal and sulfur. The powder which is used in modern firearms has been modified greatly since its original formulation. The formula used today, which is termed smokeless powder because it provides much less smoke, provides for the smallest amount of powder with the greatest volatility once it is ignited. Thus it is much more potent than its predecessor.

When firearms were first introduced, they were known as muzzleloaders, which meant that a charge of powder was poured into the barrel, followed by wadding and then a bullet which was stuffed into the barrel. The mechanism for igniting the powder originally consisted of a piece of flint as can be seen in the top picture which, once the trigger was pulled, was struck onto a metal plate called the frizzen which ignited the primer in the pan and sent a flame into the barrel, igniting the larger powder charge and sending the bullet forward. This graduated on to percussion ignition which consisted of a hammer striking a cap containing the primer which then ignited and sent a flame into the barrel again igniting the powder. Modern weapons are termed breech loaders; and it is a much more compact mechanism where the powder charge, the primer, and the bullet are all encased into a metal cartridge and introduced directly into the chamber. Magnum cartridges have a heavier than standard powder charge, increasing the projectile energy by 20 to 60%.

The bullets themselves are composed primarily of lead; but varying amounts of other metals make up the final formulation of a bullet depending on the desired final hardness of the projectile. As can be seen in the picture, there is a wide array of different bullet sizes and shapes which affect their energy transfer to the target, as well as different amounts of powder packed behind this bullet to provide differing velocities.

There are several modifications which can be made to bullets which become important when transferring energy to a target. Some of these include full or partial metal jacketing, partial metal jacketing with a cavity at the tip also known as hollow point ammunition, aluminum bullets which have a controlled expansion, bullets which are either scored or made from bonded metal, those which combine multiple projectiles into one cartridge, and bullets which also add an explosive charge within them. Fully jacketed bullets, or more commonly known as the full metal jacket, are used primarily in assault rifles. It was at The Hague Peace Conference of 1899 in which a treaty was signed which forbade the use of bullets which deformed once they struck a target. This was to decrease the amount of gross morbidity and mortality during wartime; however, in reality, changing to full metal jacket bullets probably had less to do with compliance with the Treaty than the fact than an unjacketed lead bullet cannot travel greater than 2,000 feet per second or it will simply melt. So, coating the lead or steel core with metal such as copper nickel or a gilded metal such as copper or zinc actually minimizes the deformation at very high velocities.

Soft-nose and hollow point bullets actually flatten out or mushroom on impact, which greatly increases the amount of kinetic energy delivered to the tissue that we will see in subsequent slides; and they can have an expansion of 2 to 2½ times their original diameter at velocities in excess of 2,000 feet per second.

Bonded bullets, also known as frangible ammunition, are composed of bonded fragments of iron, lead, or copper; and they are designed to disintegrate on striking a hard surface, thus increasing their kinetic energy delivered to the tissue. As can be seen in the bottom picture, that is a byproduct of the bullet striking the target.

There are a couple of pellet-containing cartridges, the first of which is called a shot shell, in which shot is immediately released upon leaving the barrel and it spreads like a mini shotgun. The second is called the Glazer safety slug, which is actually a copper jacketed tip closed with a hard plastic or Teflon plug. The pellets are contained within the bullet until it strikes the target, at which time they are released causing more tissue damage.

Firearms themselves are classified as either small arms or artillery, depending on the size of the projectile which they fire. This talk today will obviously be focusing on the small arms which include handguns, rifles, and machine guns. These weapons, as well as the projectiles which they fire, are measured in terms of caliber which is defined as the diameter of the bore of the barrel. It is measured in either decimal fractions of an inch or by utilizing the metric system using millimeters such as in common examples of the .45 and .38 caliber handguns as well as the 9 mm pistol.

Handguns are typically low-energy weapons with muzzle velocities less than 1400 feet per second. They are the most frequently used firearms in gunshot injuries, and there are three basic types: single-shot pistols, revolvers, or semiautomatics. Examples include the .38 caliber, 9 mm, and the .45 caliber semiautomatic pistols with the muzzle velocities shown in parentheses.

Rifles are named for their rifle barrel which consists of a series of helical grooves within the bore of the firearm which impart spin to the bullet providing more stability, as will be seen shortly. These weapons are grouped as single-barrel sporting, double-barrel sporting, or high-powered military assault-type rifles; and common types include the single-shot automatic and the lever, bolt and pump action. Assault rifles typically shoot a higher velocity projectile and are mainly used in the military. These bullets retain over two-thirds of their original muzzle velocity at distances up to 300 yards. Common examples include the M16, shown in the top picture, the AK47, which is shown in the bottom picture, and also the newer AK74.

Shotguns are similar in appearance to rifles; however, as their name suggests, they lack rifling inside the barrel with a smooth bore only. They fire a missile which consists of a fuse of hundreds of pellets with muzzle velocities of 1,000 to 1,500 feet per second. So, even though they are technically considered low-velocity weapons, at close range they are definitely the most destructive of all small arms. Common types include the single-shot, double-barrel, and also the automatic and pump action. There are several terms used when talking about shotguns. The first is choke, which refers to a partial constriction of the bore at the muzzle that condenses and controls the shot pattern. So a tighter choke would make a smaller spread of the pellets and a greater length of the shot column. The term gauge is actually an archaic term which is still used to describe shotguns today. It refers to the number of lead balls of a given bore diameter required to weigh one pound. Common examples include .12, .16, and .20 gauge. The load of a shotgun is the actual pellet contained within a plastic shell which is thrown forward out of the barrel and, as stated before, can consist of several hundred pellets known as bird shot, to just a few pellets known as buckshot. The wadding is the material which fills up the dead space in the shell, protecting the powder from the shot. It also seals the bore during firing to keep the gas behind the pellets and accelerating forward. It is produced using either paper, felt, cardboard, plastic, or composite materials.

The study of ballistics is divided into three areas: internal or initial ballistics are the actions of the projectile within the gun, external ballistics are the actions of the projectile in the air, and terminal ballistics are the actions of the projectile on the target. What happens when you pull a trigger? First of all, the hammer is released, and the firing pin strikes the cartridge which ignites the primer. An intense flame is produced which goes on to ignite the larger powder charge in the cartridge. This produces a large amount of gas and heat up to 5200 degrees which subsequently produces intense pressures. These intense pressures are what is used to eject the bullet from the barrel. As can be seen in this schematic, when the trigger is pulled the hammer cocks back, striking the primer in the cartridge and causing a mini explosion inside the cartridge sending the bullet down the barrel. This is a common mechanism of a revolving handgun.

A more advanced weapon is the M16 fully automatic assault rifle. Fully automatic mean that as the trigger is held down the bullets are fired in succession. As can be seen, there is a bolt which pushes the firing pen forward, striking the cartridge and causing an explosion, sending the bullet forward. What causes the automatic weapon to work is that the gas which is produced by pushing the bullet down the barrel is harnessed into a gas cylinder and pushes the piston backwards which is attached to the bolt, recocking the weapon. This can be done up to 600 times a minute, firing multiple rounds. In general, bullets from rifles have much more energy than bullets fired from handguns, and there are several reasons for this. The first is that there is more powder in rifle cartridges. This is because rifle chambers have been designed to withstand more pressure than handgun chambers. It can also be seen that the energy transmitted to the bullet depends upon the mass of the bullet times the force which is applied to that bullet times the time interval over which the force is applied. The time interval over which the force is applied is a function of the length of the barrel. So as the bullet accelerates down the barrel as the expanding gases push it, up to a certain point the longer the barrel the greater the acceleration of the bullet. Once the bullet leaves the barrel, the inertia of the projectile acts through its center of mass, which lies along its line of sight. However, there are several retarding forces including the wind and drag which act as the center of pressure. The center of pressure lies in front of the center of mass at the tip of the bullet. Optimal stability is achieved with the spin of the bullet along its central axis, and this is provided by the helical grooves in the bore of the firearm or the barrel rifling as shown in the previous slides. A non-spinning bullet is inherently unstable and has a tendency to tumble through air.

There are several motions which are well described when describing a bullet in flight. The first of these is yaw which is defined as any degree of deviation of the longitudinal axis from the line of flight. The second motion is procession, or the action of spin on a yawing bullet. This motion is analogous to that of a spinning top. Mutation is a second motion of higher frequency and lower amplitude which imparts a rosette pattern of motion to the bullet. Once the bullet strikes its target, its wounding capacity is directly related to the kinetic energy at impact. For those of you who do not remember the formula for kinetic energy, it is one-half mass times the square of velocity [KE=(1/2 mv 2)]. So in looking at this formula, it can be seen that increasing a mass only results in a linear increase in energy; however, increasing the velocity results in an exponential increase in energy to the second power. Hence, throughout history greater emphasis has been given to lighter, spin-stabilized projectiles at high velocities rather than large slower projectiles.

In general, bullet wounds can be classified as either low velocity or high velocity wounds with 2,000 feet per second being the cutoff between these two in the American literature. Low velocity wounds are generally considered less severe. They are more common in the civilian population and are usually imparted by handguns. High velocity wounds tend to have more substantial tissue damage and are caused by military and hunting weapons; however, these terms can be misleading, as will be seen in the next few slides. Low velocity handguns can have just as much or more tissue destruction as a high velocity military assault rife. The reason for this is because the projectile's mass and striking velocity only determine the potential for tissue disruption. The actual amount, type, and location of the disruption depend on the efficiency of the energy transfer by the bullet. This is affected by several factors. First of all, the farther the projectile is away from the target, the lower the velocity at impact, and hence the lower the kinetic energy it will contain. Also, in regards to the projectile's stability and entrance profile, the more yaw that a bullet has when it strikes its target the more energy it will transfer to its target; and this is maximal at 90 degrees yaw. You can think about if a bullet is traveling at the same velocity one which is pointed nose forward striking a target will cause much less tissue damage than one which is turned sideways.

Very important in affecting energy transfer is the caliber, construction, configuration, and shape of the projectile. Some of these factors predict the deformation or the fragmentation of the bullet. As mentioned earlier, the full metal jacket bullet will tend not to deform once it strikes tissue and may pass through the target without imparting much of its kinetic energy. In contrast, a hollow point or soft point bullet will flatten out or mushroom on impact, greatly increasing the kinetic energy delivered to the tissues. Also affecting energy transfer is the distance traveled within the body. Bullets which do not exit deliver their total kinetic energy to the target; however, those which do exit transfer significantly less kinetic energy. It is by this thinking that military assault bullets can pass through the target and still contain much kinetic energy which they did not deliver to their target and thus have less tissue damage. Also, the biological characteristics of tissues are important when affecting energy transfer of the bullet. Those tissues which have more density will have greater damage. Those which have greater elasticity will have less damage. This is the reason why organs such as the brain and the liver, which are very dense, inelastic organs, will have much more damage than with a similar bullet which strikes a lung which is very elastic and not dense.

In describing bullet wounds, it only takes a speed of about 125 to 230 feet per second to penetrate the skin. Bullet wounds are typically oval to circular in shape and have a punched-out, clean appearance with a surrounding zone of reddish, damaged skin which is termed the abrasion ring. There is variable amount of powder tattooing depending on the range of the bullet. As can be seen in the top picture, it was a contact wound with the imprint of the muzzle on the skin; and the bottom one is a very close-range gunshot wound with a very small diameter powder tattooing around the wound. The histology slide is that of an entrance wound with coagulant and necrosis at the edges and gunshot residue within the deeper layers of skin.

Contrary to popular opinion, exit wounds are not always larger than entry wounds. This can be thought of in terms of kinetic energy again. If a bullet enters its target and delivers most of its kinetic energy before it exits, then it will have a cone-shaped zone of injury based toward the injury site and will not provide a very large wound on exit, if it exits at all. Bone is penetrated at about 200 feet per second. Entrance wounds are typically punched out, round to oval in shape, with a sharp beveled edge. The exit wounds are excavated with a cone-line appearance and a variable amount of comminution and, generally speaking, the greater the velocity the greater the comminution at entry and exit.

The damage which is created by a projectile is caused by three different mechanisms. The first is laceration and crushing which is the sole method by which low-velocity handguns cause damage to tissue. Higher velocity weapons will stretch the tissue in the wake of the bullet, forming what is called a temporary cavity as well as the smaller permanent cavity as can be seen in the next couple of slides. A controversial subject is that of shock waves which are present and travel ahead of the bullet which last a few microseconds. It was once theorized that these shock waves could cause damage to the tissues; however, this has been refuted in recent studies. The temporary cavity is created by stretching forces in a vacuum in the wake of a bullet, and the volume of this cavity is proportional to the energy which is transferred, with a maximum diameter being measured at 10 to 40 times the bullet diameter. This temporary cavity will actually collapse and reform repeatedly with a diminishing amplitude until it settles down to what will be the permanent cavity. This entire process only lasts one to five milliseconds and, as can be seen in this picture, these are the exact same weight and caliber bullets—a .30-caliber bullet fired into a block of gelatin. Picture A, on the left, was fired at 1,000 feet per second; and picture B was fired at 2,800 feet per second. You can see the substantial increase in the diameter of the temporary cavity and hence more tissue destruction by higher velocity projectiles. The permanent cavity is the visible track of the bullet through the tissue. It is primarily made up of tissue which is crushed by direct contact with the bullet, and the diameter of the permanent cavity is variable depending on the behavior of the bullet as well as the anatomic characteristics of the tissues traversed. Whether the bullet yaws once it enters the tissue will increase the diameter of the permanent cavity.

These are a few wound profiles which have been studied by firing bullets into blocks of gelatin. The top picture is that of a 7.62 mm NATO military assault rife with a full metal jacket bullet. As you can see, the bullet begins to yaw in the tissue at approximately 16 cm, greatly increasing the size of the temporary and permanent cavities. The bullet finally comes to rest about 64 cm within the block. What is interesting about the bottom picture is that this is an example of how the different kinds of bullets can cause different tissue damage. It is the exact same size, shape, and weight bullet fired from the same cartridge, the difference being that it is a soft-point bullet which deforms and fragments once it hits the tissue. You can see the greater diameter of the permanent and temporary cavities, causing much more tissue damage. The top picture here is that of a .38 special hollow point bullet which was carried by the FBI for many years when revolvers were the issue handgun, and you can see that the bullet flattens out on impact and comes to rest with a mushroom-type shape at about 32 cm. The bottom picture is termed a Vetterli 10.4 mm bullet which was a bullet that was used in the latter half of the 19 th century by the Swiss and Italian armies. What is interesting about this bullet is that it is analogous to a .44 magnum hollow point rifle bullet and creates a similar type of wound profile. The top picture is an AK47, which is the most commonly used assault rife in the world. It is fired at a velocity of 2340 feet per second. As you can see, it penetrates approximately 26 cm before the bullet begins to yaw in tissue. You can see the differing diameters of the temporary and permanent cavities as the bullet tumbles through the tissue, and the bullet finally comes to rest face forward at approximately 74 cm. The bottom picture is a very small diameter bullet. It is a 6.5 mm Mannlicher-Carcano bullet. It is not a very well known bullet by that name, but it is notable for being the same projectile which was used to assassinate John F. Kennedy. As you can see, it does not begin to yaw until it penetrates very deeply into the tissues at approximately 60 cm and finally comes to rest at 104 cm. According to the author who wrote this paper, Dr. Fackler, if these wound profiles would have been available during the time of the murder investigation, it would easily have been apparent that one bullet could have actually passed through Kennedy, through Connally's wrist, striking his radius and into his thigh. This bullet could have done it.

A few words about the shotgun wound. The range is estimated by the diameter of spread, and the wounds caused by shotguns are graded by the range of injury. The type three wounds are point blank, which is a tightly packed group of pellets destroying everything in its path. Wadding is usually embedded in these wounds. Type two are close-range wounds which are almost as severe but are less likely to have wadding, as this falls away after about two yards. Type one are longer range wounds which consist of widely scattered bird shot and rarely causes significant soft tissue injury, whereas type zero wounds involve skin penetration only. It has been shown that negligible damage is created by ranges over 20 to 50 yards. One exception to this is buckshot pellets or shotgun slugs which actually have the same mass as a .22 caliber bullet and produce significant damage up to 150 yards.

In conclusion, there is a wide variety of weapons and ammunition which is available for inflicting injury upon victims. There is a predictable behavior of traveling projectiles from within the firearm to the time they strike the target; however, once they strike the target, there are several factors which make this a very complex process depending on the mass, velocity, and the type of ammunition used. The degree of injury is estimated by the amount of kinetic energy which is released by the projectile to the surrounding tissue.

Case Presentation

T.T. is a 23-year-old Asian male who presented to the Ben Taub General Hospital emergency department after sustaining a gunshot wound to the left neck. On arrival, the patient was noted to have stridor and hoarseness as well as the inability to move his extremities. Radiographic examination confirmed a cervical spine fracture of C6 and C7 with bullet fragments tracking from anterosuperiorly to posteroinferiorly.

The bullet was visualized in the subcutaneous tissue of the left shoulder. The patient was taken emergently to the operating room where a fiberoptic intubation was performed. The otolaryngology service was consulted intraoperatively to perform direct laryngoscopy and esophagoscopy, and there was noted to be a large amount of edema of the upper aerodigestive tract as well as a laceration in the hypopharynx. After ensuring that the patient was stable, a CT scan was obtained to assess the damage to the cervical spine. A comminuted fracture of C6 and C7 was confirmed. It was determined that no acute intervention was necessary for the spine injury per neurosurgery, and the decision was made to take the patient back to the operating room for neck exploration.

Upon adequate exposure, there was noted to be perforating wounds of the thyrohyoid membrane and posterior cervical esophagus. These injuries were repaired primarily, a tracheotomy was performed, and drains were placed. The postoperative course was complicated by the formation of a pharyngocutaneous fistula on postoperative day 5 which eventually required operative irrigation and debridement. The remainder of the hospital course was unremarkable. The patient remained quadriplegic and was subsequently discharged to a rehabilitation facility and decannulated months later.

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