If we go by movies and TV, murders are most frequently committed with semi-automatic pistols in either 9mm or .45 caliber. Whether it’s a crime of passion, a gangbanger spray-down, or a calculated assassination, the camera prefers the sleek, sexy lines of a semi-automatic over the bulkier revolver. Novels too seem to contain an overwhelming penchant for the semi-auto pistol. This is natural since the variety of semi-auto pistols has mushroomed in the past 30 years. A semi-auto pistol generally holds more rounds than a revolver and is easier to reload in a high-stress situation. Increased firepower is an attractive feature.

For writers, another beauty of the semi-automatic pistol is that it leaves evidence behind: shell casings. Even readers who have never owned or handled a gun now assume that at the scene of a crime there will be one or more empty casings thrown from fired weapons. To be sure, a shell casing offers a great deal of information that can be used as evidence to both solve a crime and to convict the perpetrator.

Most cartridge casings are made from brass. Brass is softer than the steel which is used to make the business components of a pistol: barrel, firing pin, ejector, firing chamber. The steel of these components will leave scratches and indentations on brass. For instance, the firing pin in a specific gun hits a bullet primer in a specific place every time. This spot may be centered or off center to the left or right or up or down. The indentation will have a specific depth. The surface of the firing pin will leave microscopic scratches on the primer that correspond to the wear marks on the firing pin. Those specific characteristics of that specific firing pin in that specific gun will match the empty shell casing to the gun and are hard evidence for prosecutors.

Likewise, the marks made from the ejector claw that pulls the empty casing out of the firing chamber are also specific, measurable, and effective as evidence. And if the criminal didn’t use gloves while loading the gun, the brass may also contain his fingerprints.

Writers make great hay with these pieces of evidence. That’s why they like to use semi-automatic guns in crimes. Without them, solving the crime would be more difficult.

But here’s the problem. Other than for a sudden crime of passion, why would the perpetrator of any planned crime choose to leave evidence behind? Any professional would want to minimize the data points that police could use to link a crime to him. Writers sometimes throw in “forensic countermeasures” like having the criminal pick up his shell casings. It makes for good storytelling, but in the real world, does this make sense? Why drop evidence and then have to pick it up and run the risk that one of those casings lands under a sofa or pops out a window? How long would you hang around looking for your shell casings after you’ve committed a nasty crime?

In any crime a hundred things can go wrong and if the criminal thinks of even half of them, he’s a genius. The best forensic countermeasure would be to use a revolver. The fired shell casings stay in the gun’s cylinder.

What kind of revolver should the criminal use? This depends on the situation. If a robbery is involved or if the criminal can’t get very close to his victim, something like a .357 magnum or .44 magnum would probably be the weapons of choice. They are accurate and have plenty of penetrating power to shoot through a car door or a wall or some other protection the criminal might encounter. Their use also would depend on the criminal’s concerns about noise. They’re loud guns.

For a quieter crime, such as a close-up mob hit, a .22 caliber revolver is frequently used. When the killer can get close to his target, even though this is a fairly weak cartridge, it is very effective. A .22 using a sub-sonic round could be shot in a home and the sound would not be heard down the block. Up close, a head shot is the assassin’s choice because the .22 bullet will penetrate the skull and usually won’t come out. Instead, the bullet usually spins around the curved interior of the skull, creating massive damage and assuring almost instant death.

A criminal’s gun selection can provide the writer with interesting plot points and can be used to show the criminal’s expertise or lack of it.

Matthew Bayan is a writer and a firearms expert. Learn more at www.matthewbayan.com.