Ballisticsthe science Bullets

The science of ballistics is the study of the physical and flight characteristics of bullets. Internal ballistics refers to those characteristics immediately following the firing of the bullet while it remains within the gunbarrel. Once the projectile exits the weapon, a new set of variables are encountered (e.g. wind drag) and impart yet another array of properties upon the bullet. How the bullet reacts to these variables is known as external ballistics. The injury patterns caused by bullets upon striking tissue are referred to as terminal ballistics and will be discussed later.

What is commonly referred to as the 'bullet' is more properly called a cartridge, and is composed of several components ( Fig 1). Impact on the firing pin, located on the base of the cartridge, ignites a small primer, which leads to an explosive burn of the gunpowder within the case. The size of this case, and therefore the amount of gunpowder loaded, at least partially determines the velocity of the bullet. For instance, a .22 long rifle and a .223 Remington (United States military bullet fired from the M-16 rifle) round are similar in size and weight (Table...,.!), but have markedly different muzzle velocities (the speed of the bullet as it exits the muzzle of the gun). The muzzle velocity of the .22 long rifle is 383 m/s, while that of the .223 Remington is over 988 m/s. This pronounced difference in velocity is due primarily to the amount of gunpowder contained within the cartridge.

Fig. 1 A cartridge and its components.

Fig. 1 A cartridge and its components.

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Table 1 Weight, muzzle velocity, and kinetic energy of some representative bullets

Although intimidating to the novice, bullet nomenclature is actually quite simple; the numbers represent the projectile diameter in either inches or millimeters. Therefore a 9 mm bullet is 9 mm in diameter and a .45 caliber bullet is 0.45 inches in diameter. The .308 Winchester bullet is the civilian version of the military 7.62 NATO cartridge (0.308 inches = 7.62 mm), and the common civilian .22 caliber is only 0.003 inches smaller than the 5.56 mm, one of the most common military rifle cartridges in the world.

The weight of the bullet and its tendency to tumble (end-over-end cartwheeling) and yaw (the degree of divergence from the straight-line path of the bullet) contribute to tissue injury and therefore are important considerations in the design of ammunition. Bullets range in weight from about 2 g for a .22 short to 15.6g for a .44 caliber.

A typical 9 mm bullet weighs about 8 g, while a .38 caliber generally weighs 10 g. The in-flight stability imparted to the bullet by the weapon's rifling deteriorates once it encounters the increased density of tissue. Tumble and yaw may be significant as the bullet travels through tissue. The cross-sectional area is increased significantly, increasing both the kinetic energy loss to the tissue and temporary and permanent cavitation.

Bullet design plays a key role in flight characteristics and wounding patterns. Bullets fired from early weapons were simply round balls of lead. Modern bullets have a variety of designs, ranging from simple round-nose lead bullets to jacketed bullets which begin to expand on contact with tissue ( Fig,2). The round-nose lead bullet is the cheapest. These bullets are frequently used for target practice, but are also often found in tissue wounds because of their availability. Since lead is irregular, these bullets have a higher friction coefficient as they travel down the barrel of the gun and during flight. Being relatively soft, lead bullets also tend to deform more on contact with tissue. Because of the addition of a thin outer layer of copper, full metal jacket bullets have a lower drag coefficient and do not usually deform or expand. However, expansion of the bullet beyond its original diameter is important when designing the missile with the intent to injure. Jacketed hollow-point bullets combine the advantages of a copper jacket with the expansion associated with a hollow-point bullet. These bullets expand by as much as two to three times their original diameter upon contact with tissue. However, the forward edge of an expanded hollow-point bullet is made of lead and is smooth and rounded. When it strikes a large blood vessel within inches of the end of its path, the vessel may be pushed aside so that injury is avoided. Bullets such as the Winchester Black Talon take this concept one step further, with a jacket whose leaves are exposed upon impact and remain attached to the expanding bullet ( Fig.S). These leaves act as six tiny knives lacerating tissue as the bullet travels through the body. Near the end of its course, these 'talons' continue their cutting action rather than pushing tissue aside. The 7.62 mm NATO SP (soft-point) is designed to fragment, sending multiple smaller missiles through the tissues. Similarly, the .223 caliber (5.56 mm) round bullet fired from the M-16 fractures at the cannelure (a circumferential groove around the bullet), again converting a single missile into several smaller ones with more diverse tissue injury.

Bullet With Body Tissue

Fig. 2 Left to right, lead round-nose (LRN), jacketed hollow-point (JHP), and Black Talon bullets.

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Fig. 3 .40 caliber Winchester Black Talon before and after firing. The leaves of the bullet's jacket are exposed upon impact and tear tissue in its path.

The barrels of most firearms contain a set of grooves, known as rifling, that impart a rotational spin to the bullet as it travels down the barrel. This spin stabilizes the bullet as it exits the gun and travels through air. The resistance encountered during flight causes some degree of angulation off the in-line trajectory of the bullet, i.e. yaw (Fig..4). Yaw angles are usually small, varying from 1.5° to almost 10°, and are known exactly only for military weapons.

Bullet Yaw

Fig. 4 During flight through air and tissue, bullets tend to yaw; this is measured as the divergence of the long axis of the bullet from the straight flight path.

Muzzle velocities are also an integral part of external and terminal ballistics. Velocities typically range from 240 m/s in handgun bullets to almost 1220 m/s in some military assault weapons. The use of the term 'high velocity' should be avoided for several reasons. First, wound damage can actually be less than that seen with lower-velocity missiles. The velocity of the bullet, even if it is known, will rarely dictate the care of the wound. Finally, the actual definition of 'high velocity' varies so much that the term is too imprecise to be used scientifically (British usage starts at 335 m/s, while American usage varies from 610 to 915 m/s). Specifying the actual velocity provides an unequivocal definition.

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