Rifled Firearms William A. Cox, M.D. FCAP www.forensicjournals.com

July 1, 2017

INTRODUCTION

This blog article is a synopsis of our last Forensic Science Newsletter ‘Rifled Weapons.’

Rifled weapons include handguns, pistols and revolvers, and rifles. These are weapons in which spiral grooves have been cut into the length of the interior or bore of the barrel (Fig. 1). Rifling consist of ‘lands’ and ‘grooves’ (Fig. 2). The ‘lands’ grip the bullet as it passes down the barrel, giving the bullet a rotation, which has a gyroscopic effect that increases the stability of the bullet’s trajectory and thus its accuracy (Fig. 3). In contradistinction to rifled weapons, shotguns are smooth bored. This is because rifling in a shotgun barrel would cause the shot to disperse to quickly in a 360 degree arc.

GENERAL INFORMATION

Handguns and rifles are breech-loading firearms in which the cartridge is inserted or loaded into a chamber, which is integral to the rear portion of the barrel. The breech or chamber has a slightly greater diameter than the rest of the barrel. The missile fits snugly into the breech-chamber, but is too large to pass through the barrel being squeezed. The barrel is grooved in a spiral fashion from the breech-chamber to the muzzle, with the friction between the squeezed missile and the ridges between the grooves (lands) imparting to the missile a spinning motion.

Handguns include the single-shot pistols, Derringers, revolvers and auto-loading pistols (automatics) (Figs. 4-7). There are two types of revolvers, single-action and double-action. Automatic pistols (semiautomatic handguns) are divided into three categories: single-action, traditional double-action or double-action only (Figs. 8-10).

Rifles are firearms designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves (“rifling”) cut into the barrel walls. The word “rifle” originally referred to this grooving, and a rifle was called a “rifled gun.” This rifling, as discussed above, implants a spin to the fired missile, impacting a spin around an axis corresponding to the orientation of the weapon. Once the missile leaves the barrel, this spin lends gyroscopic stability to the projectile and prevents tumbling (Fig. 3). This allows the use of aerodynamically efficient bullets, as opposed to the spherical balls used in muskets, which improves range and accuracy. This spin also contributes to the total kinetic energy released by the missile on its penetration of the target. Rifles are divided into single-shot, lever-shot, bolt-action, pump-action, and autoloading or semiautomatic rifles and automatic rifles (Figs. 11-21).

CALIBER

The caliber of a rifle or handgun, is either the internal diameter of the bore of the barrel before the rifling grooves were cut, measuring from ‘land’ to ‘land’ (Fig. 2) or the diameter of the bullet. Common caliber designations are given in either the English system in hundredths of an inch, I.e., .22, .38, .45 or in the metric system in millimeters, I.e., 7.42 or 9 mm. Thus, a .22-caliber gun fires a .22-caliber bullet.

In the autopsy reports of forensic pathologists, they often describe bullets as small (.22, .25), medium (.32, .38, and 9 mm), or large (.40, .45, .50) caliber, which is based on the measurement of the bullet’s diameter.

It is important to understand, the caliber designation as used in the United States can be confusing for it is neither accurate or consistent. For example, the .303 Savage fires a .308-inch-diameter bullet. The .303 British cartridge has a 0.312- inch-diameter bullet. Both the .30-06 and the .308 Winchester cartridges are loaded with bullets having a diameter of 0.308 inches.

The European system of cartridge designation, which uses the metric system, is more thorough and logical than the US system. It identifies a cartridge by giving the bullet diameter and the case length in millimeters, as well as designating the type of cartridge case. Thus, the Russian rimmed service round becomes the 7.62 x 54 mmR. The 7.62 refers to the diameter of the bullet; 54 mm indicates the length of the cartridge case and R indicates the round is rimmed (Fig. 22).

ANOTHER FORM OF IDENTIFICATION OF BULLETS

Bullets can also be identified by whether they are jacketed (Figs. 23-25).

WOUND BALLISTICS

The total kinetic energy of a bullet is due to two factors: Rotational or angular kinetic energy and the energy due to linear or translational motion.

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