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Anatomy of Firearms

Modern firearms are manufactured in a variety of shapes and sizes to fit multiple purposes. There was a time when the forensic pathologist was faced with a less complex situation, with fewer types, models, and mechanisms available for use. Unfortunately, the proliferation of firearms in this country includes not only sheer numbers but also a staggering array of models. Included in this array are semi-automatic and even automatic weapons built primarily for military usage. However, virtually any type of gun can be found on the streets in use by youth gangs, persons involved in drug trafficking, paramilitary "survivalists" and even what we would consider "ordinary" citizens.


From the very start, a handgun was conceived as a compact weapon for self defense. Even though today there are handguns made specifically for target competition or hunting, most are still designed with defense in mind. Thus, handguns are compact for concealability and ease of carrying. This becomes a part of the legal definition of a handgun, as they are considered "concealable" and therefore deemed dangerous and are controlled by law in most states. Of course, a handgun should be capable of firing a projectile accurately at a target. The energy delivered must be sufficient to quell any attack, yet be light enough so that the recoil generated does not wrest the gun from the shooter's hand; this is difficult in practice and there is no perfect choice, so many types of handguns are manufactured for different situations. Finally, since no one can be guaranteed a perfect shot or a single attacker, a handgun must fire multiple shots. The two most common defensive handguns are the double action revolver and the semiautomatic pistol.


The revolver has several advantages and unique features. Importantly, they are less expensive to manufacture, simpler in design, and more reliable in operation than semiautomatics. A revolver is easy to master, even for novices. Revolvers with fewer moving parts may be more accurate than semiautomatics. Rrevolvers are limited to six shots, are relatively slow to reload, the gap between barrel and cylinder makes them less efficient, and the trigger pull is greater. The anatomy of a representative double action revolver is shown below:

Barrel length is smaller for concealability and longer for accuracy or energy. The ejector rod under the barrel is used to eject fired cartridges before reloading. Sights on a revolver are usually a blade in the front and a notch on the rear. The frame is the largest part, and all other pieces attach to it. Frames are usually made of blued or plated steel, stainless steel, or lightweight alloys. A revolver may weigh less than 1 lb to more than 4 lbs. The cylinder contains five or six holes for the cartridges and can be swung out for easy reloading. This must be a conscious act, so that no empty cartridge cases will be found at a crime scene unless the assailant stopped to reload.

There is a gap between cylinder and barrel to allow the cylinder to turn freely, but this also allows gases to escape laterally, which at close range may deposit gunshot residue on surrounding structures and allow the forensic pathologist to reconstruct the scene. The lockwork translates the trigger pull to rotation of the cylinder, cocking and fall of the hammer. If this is done in one motion of pulling the trigger, it is termed "double-action." Single-action revolvers (old Colts of "cowboys") require manual cocking of the hammer before the trigger is pulled. Different types of grips are employed; larger grips allow more accuracy, smaller grips provide.

Semiautomatic pistol

This is a more recent development than the revolver, originating late in the 19th century, mostly through the efforts of John Browning. In fact, almost every semiautomatic handgun available today is a copy of his two most famous designs: the Colt model 1911A government 45 and the Browning Hi Power 9 mm. The anatomy of a semiautomatic pistol is given below:

The advantage of semiautomatics is the use of recoil generated by the fired cartridge to eject the empty cartridge case, load the next cartridge, and cock the hammer. This is more conducive to firing multiple shots, so many are designed to carry 15 to 19 rounds. Disadvantages include a more complicated mechanism, require more practice to use, and cartridge cases must be short to work well. Revolver cartridges are more powerful than semiautomatic cartridges for this last reason.

The barrel is normally hidden by the slide. Choices of barrel length are limited. The slide is a key part to the operation of a semiautomatic:

The slide is able to move back along the axis of the barrel under tension from a spring. Since the cartridge base rests on the slide, the slide does just that under the force of recoil generated by the firing of the cartridge. As the slide and empty cartridge case are accelerating backwards, the case is struck by a stationary piece of metal that bumps it to the side. This is conveniently located next to a hole in the slide, so that the empty cartridge case continues its acceleration in a direction perpendicular to the pistol and into the air, landing from 2 to 20 feet from the fired gun. The rearward- moving slide also cocks the hammer. After the case is clear the slide hits a stop and the spring tension starts it forward. The magazine spring is pushing on a column of rounds tight up against the bottom of the slide. As the slide comes back by the column of cartridges, it grabs the top one and pushes it forward and up a short ramp into the chamber where the slide locks it in place. The radiograph below demonstrates the location of the spring in relation to the slide and the barrel.

The handle, or butt, is more important here because it contains the magazine holding the cartridges. Safety mechanisms prevent accidental firing. Some lock the hammer, while other designs lock the trigger. In the radiograph below, a loaded magazine is present in the handle.

Even on open ground ejected cases may be difficult to find, as they typically roll into a hiding place such as grass or small depressions in the ground. Thus, ejected cases will virtually always be left behind at the scene, but must be searched for diligently.


Rifles differ from handguns in the length of the barrel and the presence of a butt stock. They are harder to carry, are poorly concealable, and more loosely regulated than handguns. However, they are much more accurate and shoot more powerful cartridges than handguns. Rifles may be manufactured as single shot, but most commonly are bolt action, used for large caliber hunting rifles. Military rifles are seimiautomatic or automatic, having a detachable magazine holding 5 to 50 rounds. Pump action and lever action rifles, usually of lower caliber, have magazines below the barrel.


Shotguns have a similar external appearance to rifles, but differ in the lack of rifling inside the barrel, which is the basis for their legal definition. A shotgun shell may contain one large projectile (called a slug), a few pellets of large shot, or many tiny pellets. Shotguns are available in single shot (break action), double barrel, pump action, and semiautomatic.

Other types of firearms

The single action revolver has remained popular for its historic appeal, reliable design, and uncanny balance. For some reason a single action Colt 45 is easier to shoot from the hip than a modern revolver, and is used almost exclusively in trick shooting.

Semiautomatic versions of submachine guns (such as the Uzi) are classed as pistols for legal reasons. These often have the ability to hold 20 to 30 rounds, but are otherwise identical to conventional handguns in similar caliber. The expense of such weapons precludes their use by most criminals, but they may be used by persons involved in organized crime, drug-dealing, and gangs.

Air guns which use pneumatic pressure to fire a projectile are generally known as "BB guns" and have been around for over 200 years. Three mechanisms are employed:

  1. Air is pumped into a pressure chamber reservoir and released by trigger pull
  2. A spring compression system is used to drive a piston to compress air (most "toys" are of this variety)
  3. A pressurized, carbon dioxide filled cartridge is attached. (Harris et al, 1983)

Even though cheap "Saturday night specials" are readily available, youths and youth gang members may attempt to build their own firearms. Typically, they are crude, and adapted to fire available ammunition. In one study, such guns caused unusual muzzle imprints, intensive soot deposits at the entrance wounds and on the hands, intensive CO-effects, burns, and in one case a skin laceration of the hand holding the weapon. The bullets showed a reduced penetration depth, and characteristic firing marks were missing. (Karger et al, 1995)

Lastly, there has arisen a new group of handguns for hunting big game and long range target competition that are nothing but single shot rifles with shortened barrels and no buttstock. These shoot rifle or hybrid rifle cartridges and deliver rifle energies.


A muzzle brake, or compensator, is positioned at the end of the barrel and redirects gases outward and forward. This aids in reduces the recoil and rise of the barrel. However, gases and particulates, as well as sound, are partly directed back to the shooter so that eye and ear protection are needed. M/P>

A flash suppressor, or flash hider, is attached to the muzzle of a rifle to reduce visible light from burning powder. The burning gases are more greatly dispersed as they exit the muzzle. This diminishes distracting, blinding light experienced by the shooter in darker surroundings.

A ported barrel has slots cut at the top near the muzzle to release gas behind the bullet in order to reduce muzzle lift. Reduced lift would be advantageous in keeping the gun more level on target when multiple rounds are fired in succession. However, the hot gas released is closer to the shooter and may direct particles of unburned powder, lead, or jacket materials back toward the shooter, or toward a bystander if the gun is not perpendicular. Also, bullet muzzle velocity will be reduced.

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