Details
R C Jones
In case some of you unpluggers might someday wish to write about a situation involving a shooting, some information about firearm identification might come in handy. First, a bit of basic background. For the sake of brevity, I will limit the discussion to handguns, that is, to revolvers and semi-automatic pistols. Both fire cartridges each of which comprises a casing, within which is held an explosive powder, a bullet held in the forward end of the casing, and a primer disposed at the base, or head, of the casing.
Each type of handgun has at least one chamber within which a cartridge is fired when a spring-driven hammer is released by a trigger and slams a firing pin against the primer. When struck, the primer ignites the powder; and rapidly expanding gases from the burning powder accelerate the bullet through a barrel. Helically shaped riflings within the barrel spin a passing bullet, giving it an improved trajectory.
A major difference between the two types of handguns is that a revolver has a cylinder in which there are usually five or six chambers, and a pistol has but one chamber located at the rear end of its barrel. The cylinder in a revolver serves as a storage area for cartridges and is rotatable so that each chamber can be brought to a position behind and in line with the barrel. One cartridge can be stored in the chamber of a pistol, and others are stored in a magazine, or clip. After a revolver is fired, rotating the cylinder brings a successive cartridge into alignment with the barrel.
Revolvers come in two basic flavors: single action and double action. In a single-action revolver, the hammer is cocked to rotate the cylinder; and a trigger is then pulled to release the hammer and fire a cartridge. In a double-action revolver, pulling the trigger cocks the hammer, rotates the cylinder and releases the hammer to fire a cartridge. In a semi-automatic pistol, when a cartridge is fired, recoil forces are used to recock the hammer, to cause an extractor to yank the empty casing from the chamber and to cause an ejector to expel the casing from the gun. Spring forces are then used to inject a fresh cartridge into the chamber.
Most of us have probably seen movies where crime-scene evidence is analyzed by experts. Among the items popularly eyeballed are, of course, recovered bullets. These are often shown clamped in a pair of optically bridged macroscopes and rotated to compare striations carved into one bullet by barrel rifling with striations carved into another bullet. A crime-scene bullet and sample bullets fired by a suspect gun can be so compared to link the former bullet to the suspect gun. Fired bullets retrieved from different crime sites can be compared to link separate shootings to each other.
I have heard TV and movie experts mention fingerprints left on a gun or cartridges, powder residue, the caliber of bullets and striated marks carved by rifling. They have even mentioned whether the rifling had a right-handed or left-handed twist. I missed it, however, if they ever mentioned the rifling twist rate – the number of inches of rifled barrel needed to twist a bullet completely around once.
In addition to rifling striations, which reflect imperfections in rifling associated with a particular gun, the caliber, weight and shape of a bullet are all clues that help limit the number of guns that could have been involved in a shooting to those made by a certain manufacturer during a certain period. In fact, there are many more things to consider than one might think.
Recovered cartridge casings, colloquially referred to as brass even though they are made from a number of different metals, can be a repository of many additional clues. A chamber itself can have rough portions that scratch a cartridge casing as the casing is loaded and unloaded, even if the cartridge is not fired. Since casings expand within a chamber when fired, they are then even more susceptible to being scratched while being unloaded.
After a firing pin strikes a cartridge primer, the pin might remain slightly imbedded in the primer. If the pistol is one where a barrel drops slightly when a recoil forces an action open, the firing pin might be dragged across the primer as the pin is retracted from the impression, leaving what is known as a firing pin drag mark. As a fired cartridge recoils, its base and primer are forced against a breech face having a hole through which the firing pin struck the primer. A portion of the primer might be forced into the hole. If the barrel drops, the primer can be scratched by an edge of the hole to create what is known as a shear mark. When a recoiling cartridge is forced against a breech face, any imperfections in the face will be imbedded in the primer. Such marks are known as breech marks.
A casing of most cartridges has a circular extractor groove near its base. As previously mentioned, recoil forces cause an extractor to hook into the extractor groove and extract an empty casing from a chamber; and an ejector ejects the casing from the gun. The action of the extractor can leave extractor marks, and that of the ejector, ejector marks, even if the cartridge is not fired. Ejector marks can be in the form of both striations and impressions.
A firing pin, in addition to leaving drag marks, also leaves impressions in primers. The impression size, shape, depth, surface imperfections and strike position are all clues to compare with cartridges fired by a suspect gun. They can also be helpful in determining the type and make of the gun that fired them.
One thing I would like to know is why, on law enforcement shows, do they handle evidence with rubber gloves as if that would not smear any existing fingerprints.
— R. C. Jones
