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Weapons, Ammunition, And Basic Terminology

A firearm is a weapon designed to expel a projectile by the action of highly combustible, gas-generating gunpowder. This includes rifles, handguns, and shotguns, which are collectively referred to as small arms.8 Small arms are relatively simple mechanical devices that share the same basic anatomy, which consists of the handling portion (stock), the breech (the chamber where the bullet is seated and from which the combustive reaction is triggered), and the barrel, which acts as the projectile’s guide. The differences between small arms are largely delineated by the amount of pressure the breech can withstand and the length of the barrel.

Rifles are shoulder-fired weapons engineered to withstand the very high pressures (50,000-60,000 psi) generated from combustion of the gunpowder. They have long barrels (usually > 16 inches) with a spiraled groove cut into the inner lumen that imparts a spin on the bullet, enhancing inflight stability and accuracy. Handguns are basically small rifles with the same engineering features, but with smaller pressure tolerances (20,000-30,000 psi) and shorter barrels.9

Shotguns are similar to rifles in their basic appearance, but differ significantly in their engineering. Shotgun projectiles generate less pressure in the breech, but their barrels are thin-walled and usually contain no rifling because their ammunition is made up of numerous spherical projectiles that are propelled simultaneously.

Ammunition is a term for complete cartridges containing the projectile of a firearm, commonly referred to as a bullet. For rifles and handguns, ammunition is more accurately called a cartridge or round, which includes the casing, primer, and propellant (gunpowder), as well as the bullet.9 The bullet is the actual projectile and is typically composed primarily of lead with a rounded or pointed tip, in various sizes, or calibers. The caliber denotes the width of the bullet itself in proportion of an inch or millimeter. Bullets come in a variety of designs that affect their energy transference, such as pointed tips, and round tips, hollow points, as well as full metal jackets (FMJ), partial metal jackets, and scored or “expanding” bullets.

A useful distinction among bullets subdivides them into 2 types: expanding bullets and nonexpanding solid bullets. Expanding types are designed to maximize tissue damage; nonexpanding solid bullets are designed to have greater penetration.10 The nonexpanding solid bullet has a pointed tip and is coated with a thin metal covering, or jacket (usually copper), and is referred to as an FMJ bullet. These are the bullets used by the United States military forces. There are 3 reasons for the design of FMJ bullets. First, for a given caliber, solid and expanding bullets commonly have the same weight. Military rifles have evolved over the last 150 years to smaller calibers, with lighter, faster bullets that have better stability in flight, allowing soldiers to carry more rounds and be more accurate marksmen. Second, jacketing limits the lead residue left behind on the bore of the rifle, which can cause dangerous mechanical malfunctions. Third, the jacketing allows the bullet to maintain its shape when it hits a target rather than expanding and fragmenting. Historically, this was felt to be more “humane,” by wounding rather than killing an enemy combatant. In addition, a wounded combatant requires more resources to care for than a dead combatant.8

Expanding bullets are designed for hunting nondangerous animals and for self-defense and thus to kill or incapacitate at a lower velocity. They are typically soft-tipped (ie, lead only), which allows for nearly immediate deformation when hitting tissue and thus imparting most of their energy with less penetration of the victim. Expanding rounds vary greatly in design. They may have metal jacketing in some form; typically, the jacket covers most of the bullet except the tip, allowing for a combination of rapid expansion and increased overall integrity of the bullet. The same distinction between solid and expanding bullets applies to handguns. Handguns are limited in their velocity and range due to barrel length and pressure tolerance of the breech, but this is partly compensated for by an expanding bullet design (such as hollow point). Hollow point tip bullets have an inward depression at the tip that deforms instantly upon striking any solid object. The bullet enlarges up to 3 times its initial diameter, creating a larger permanent cavity.11

Shotgun ammunition comes in 4 basic categories: birdshot, buckshot, slugs, and sabot rounds.12 Birdshot and buckshot shells contain numerous spherical projectiles (shot) that are housed within the shotshell. They are propelled and released simultaneously when fired. The difference between birdshot and buckshot is the size and metal composition; birdshot is typically made of smaller, softer lead or similar metal shot, whereas buckshot shells contain only a few tightly packed balls of a much larger diameter, and may be made of lead or steel.

Slugs are composed of a single, large, solid projectile that may or may not have rifling built into the lead itself (thus imparting a spiral path in flight, which increases accuracy). Slugs are used primarily for hunting large game in populated areas where a heavy, relatively low-velocity projectile will have limited range. Slugs also are used by police and military personnel because of their ample stopping power.9

Weapons and their corresponding projectiles are generally categorized based on their velocity, ie, low-velocity (usually shotgun and pistol bullets, < 2000 f/s) and high-velocity (usually rifle bullets or explosive fragment missiles > 2000 f/s).13 (See Table 1.) Although conceptually useful, these categories have some important limits. First, these values are not universally applied; for instance, the British use values of 1200 f/s for “high-velocity.” This lack of agreement has the unfortunate effect of less-rigorous scientific study. Second, these terms imply the weapons’ wounding capacity. Using this logic, it would be easy to infer that low-velocity wounds are less severe, yet they can be devastating wounds. Because of the frequency of use, low-velocity weapons cause nearly twice the number of deaths of high-velocity weapons.

Ballistic Physics

Ballistic physics is divided into 3 components: internal, external, and terminal (wound).9 Of these, terminal ballistics is the primary concern of emergency clinicians, but knowledge of the other components can prove to be a valuable resource for understanding the injury pattern. Conceptually, the science of ballistics is relatively simple, but oversimplification can lead to erroneous conclusions (such as mistakenly thinking bullets are sterile due to being heated).14,15

Internal ballistics describes the path of the projectile from the breech to the muzzle (the end of the weapon barrel). Technological advances have revolutionized modern firearms into the devastating weapons they are today (smokeless powder, rifling, automated cartridge feeding, high-stress metallurgy). The differing range of pressures generated from the combustion of gunpowder is what distinguishes the various weapons.8 Rifles can withstand more pressure than a handgun, and a handgun can withstand more pressure than a shotgun. Rifling, which is a helical groove carved into the inner lumen of the barrel, imparts a spin on the bullet to prevent yawing (the deviation of a projectile’s path). Clinically speaking, internal ballistics have little relevance to treating tissue damage other than noting that the technological advances have led to faster, more accurate projectiles that can enable the projectile to impart more energy onto its target.

External ballistics describes the projectile from the point when it leaves the barrel until it hits its target (muzzle to target) or the study of the projectile in flight. While this aspect of ballistics lends itself to more traditional aspects of physics, it can be divided into the 2 components that have the most effect on a projectile - gravity and drag.

The effects of gravity can be visualized by the trajectory of any object in flight (parabolic curve). Even though all objects will drop to the ground at the same rate, faster rounds have a flatter trajectory, making them easier to aim and thus, more accurate. Drag (the resistance to a traveling object’s passage through air) affects a projectile’s velocity downrange. Drag can be limited by making a bullet more aerodynamic, with an elongated, narrow, pointed configuration.8,16 For example, birdshot and buckshot are composed of spherical projectiles and therefore have poor aerodynamic profiles with rapid loss of velocity as well as dispersion of the shot; hence, much of their energy is dissipated after about 25-50 yards.17

Terminal (or wound) ballistics is the component of ballistics that studies the interaction of penetrating projectiles with living tissue and is therefore an emergency clinician’s primary interest. Ballistic missile wounding occurs by 2 primary mechanisms, described by the cavities created by the missile: the permanent cavity and temporary cavity.3,6 (See Figure 1.) As a missile penetrates the skin, it crushes and destroys the tissue in its path (creating a permanent cavity) while simultaneously imparting a shock wave that radiates outward from this path. This shock wave causes tissue to stretch and shear outward, followed by subsequent collapse and reverberation. The size of the permanent cavity is determined by the caliber of the bullet and its corresponding fragments. The temporary cavity is transitory, lasting only milliseconds, and its size is determined by the velocity of the missile. Higher velocities create larger temporary cavities, ranging up to 10-30 times the size of the missile’s permanent cavity.8,9 There is also a measurable pressure wave that travels in front of the missile, but it has not been found to impart any significance towards the wounding mechanism.10

 

There are 3 relevant principles in appreciating the extent of the wounding mechanism: energy transference, zones of injury, and secondary wounding.18 When a bullet strikes skin, the process of energy transference starts. The ability of the missile to transfer its kinetic energy is the primary determinant of tissue damage.5,19 (See Table 2.) Although clearly important, the projectile’s velocity is neither the only nor the dominant factor in energy transference, as seen with the effectiveness of low-velocity shotgun wounds and handguns using hollow-point rounds.

The zones of injury were first described by Wang and are conceptually linked to the mechanisms of ballistic wounding.20 (See Table 3.) The first zone is the permanent cavity, which is the primary wound cavity consisting of dead, crushed tissue. The second zone is the contusion zone, consisting of tissue adjacent to and encircling the permanent cavity. The tissue here is in an inflammatory state. It is swollen and bathed in a milieu of inflammatory mediators and cellular debris. The third zone is the concussion zone, or temporary cavity. Damage here occurs by stretching, shearing, or compression and is dependent on the cavity in which the missile travels and the tissue contents. Inelastic tissues such as bone, liver, and brain are highly susceptible to injury because they have little intrinsic ability to dissipate the energy transferred even if they are remote to the primary injury path.21

Secondary wounding is caused by missiles as well as wound infections. Secondary missile wounding occurs when the projectile strikes and breaks an object and accelerates some portion of that object through adjacent tissue. Examples of secondary missiles are bone fragments or objects such as clothing or buttons. Secondary missiles tend to move at much slower velocities, but can be equally destructive if they damage vital adjacent tissues (such as an artery). Contrary to popular belief, missiles are not sterile due to the heat they retain.15 As a bullet travels through tissue, a vacuum is created when the permanent and temporary cavities are formed. This vacuum pulls in potential infective sources such as clothing, dirt, and skin flora. Other potential inoculum include components of the cartridge or shotshell and devitalized bone fragments.22

 
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Last Modified: 07/19/2018
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