Ballistics

Ballistics is the study of the motion or travel of projectiles and their flight performance. Internal or initial[Page 44]ballistics studies the projectile from within the weapon, including forces within the gun barrel and the velocity at which a bullet exits the chamber, also known as muzzle velocity. The speed at which a bullet travels through a given weapon's barrel is based on factors such as bullet composition and shape, length of the barrel, how much energy is transferred to the bullet from the expansion of burning gunpowder, and the amount of gunpowder available for efficient burning within a given cartridge. External ballistics studies a bullet's flight pattern, including the energy with which a bullet leaves the weapon's chamber, the mass and shape of the bullet, air resistance/drag, gravitational forces, and distance of travel.

Terminal ballistics refers to the injury pattern of a bullet on the target. A long, heavy bullet composed of an alloy of lead and antimony jacketed in copper offers the least dispersion of energy from the bullet to the target, thus allowing the bullet to travel deeper, if not through, the intended target. At the other extreme, spherical-shaped bullets offer the greatest release of energy to the target and in fact may not even make contact with the intended target. Different types of bullet shapes are designed to give up various amounts of energy to their intended targets depending on the type of weapon used, range from the weapon to the target, and purpose of use.

Terminal ballistics also encompasses the wounding potential of a bullet. Tissue damage is primarily produced in two ways: (a) laceration and crushing, as seen with low-velocity bullets and (b) cavitation, wherein a permanent cavity is caused by the path of the bullet, as seen in high-powered rifles. Shock waves, which compress the medium and travel ahead of the bullet, may cause damage to a target, but serious effects seen from cavitation are extremely rare. Bullet shape plays an important role in wounding potential. A bullet having a soft lead point or “hollow point” is designed specifically to deform on impact, allowing the bullet to impart all its energy to the target.

The examination of bullets (partial or whole) in crime laboratories occurs through the comparison of class and individual characteristics. Recovered bullets from a crime scene may be compared with bullets obtained from test-firing a suspected weapon. The bullets are then compared by juxtaposition using a comparison microscope. Class comparisons specifically refer to the type of caliber and rifling patterns on a bullet made as the bullet was fired from a weapon. Caliber and rifling marks can be readily seen in whole bullets, whereas in partial bullets or deformed bullets, this comparison may be next to impossible. Once a positive class comparison is obtained, individual characteristics are then examined to determine whether a specific weapon was used. Individual characteristics are based on bullet striae arising from imperfections within a weapon's barrel. Weapon barrel imperfections cause a striae pattern as specific and unique as a fingerprint.

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