Rifling is a term that refers to the grooves machined into the barrel of a firearm to impart a spin on the bullet during flight. This spin helps stabilize the bullet thereby improving accuracy and increasing the distance traveled. The raised ridges traveling along the length of the barrel are called lands and the grooves are the spaces between. These lands and grooves also have a “twist” or slight rotation to the left or right (depending on make and model) which creates the spin. The number, width, direction of twist, and rate of twist can vary between makes and models. The degree of this twist is called the “twist rate” and is expressed by how many inches it takes the projectile to make one full revolution of the rifling. For example, a barrel with a 1:12 rate of twist means the bullet will make one full revolution every 12 inches of the rifling.
Rifling is found in most modern rifles and handguns (even muzzle loaders) with the exception of most shotguns which are referred to as “smoothbore”. There are different methods for machining gun barrels but generally speaking, the tools that manufacturers use to create these lands and grooves leave unique marks (scratches) on the lands and grooves. As the bullet passes down the barrel the lands and grooves will replicate these marks onto the projectile. Firearms examiners evaluate the rifling in two main ways.
First, by looking at the gross physical features (called General Rifling Characteristics; GRC) of the lands and grooves (number, width, and direction of twist) the examiner may be able to eliminate certain makes and models of firearms. The FBI maintains a database of GRC from firearms all over the world which is updated once a year and distributed to the firearms examiner community. From this an examiner can create a list of possible gun models that share the same GRC as the crime scene bullet. When a known bullet is submitted to the lab the examiner can compare the presence and location of these rifling marks on the crime scene bullet to the known standard to determine if that gun was the one that fired the bullet. I’ll have more on this process later as it is much more detailed than what I have allowed for here.
Suffice it to say that firearms examiners use these marks to include or exclude a gun as having fired a particular bullet. If you want to put something interesting into your story choose a gun that has unusual rifling marks or general rifling characteristics. This may be from a rare or antique firearm, an after-market barrel, or a custom-made one. Your local gunsmith may be able to point you in a good direction. Then your characters can spend a few scenes trying to unravel the mystery of what kind of gun was used in the shooting. Another thing you might consider is using a gun in which these rifling marks have been damaged or destroyed intentionally by the criminal (like using a drill bit to mar the barrel). Use your imagination and have some fun with it.
This is a point of confusion for a lot of inexperienced writers (i.e. ones who have not been barraged by gillions of critical e-mails). The terms “pistol” and “handgun” are basically interchangeable as they define a firearm designed to be shot with one hand, as opposed to being fired from the shoulder. Some experts further sub-classify pistol as a firearm whose chamber is integral with the barrel, (thus distinguishing it from a revolver) but for our purposes I think either would be fine. The term “revolver” describes a particular “action” wherein the chambers are part of a revolving cylinder, but it too can be called a pistol.
It bears mentioning that very few American law enforcement agencies allow their officers to carry a revolver as a primary duty weapon. Most require a semi-automatic handgun for their officers. Some agencies even require their officers to carry the same make and caliber so their ammunition is interchangeable. Almost all agencies will have an “approved” list of acceptable makes, models, and calibers.
While the above terms are interchangeable I would suggest that you pick one and stick with it. Obviously, if a character has a revolver then you need to make that clear. Pay particular attention to the ammunition capacity. Nothing bothers a reader more than a gunfight in which a character shoots nine shots from a six shot revolver without re-loading. It’s always best to select a particular model and caliber and then study the specifications. In the best case scenario you might be able to test fire one at a gun range so that you understand its functions. Remember; never shoot a firearm without proper training and or supervision by someone familiar with the weapon.
The terms single-action and double-action refer to the mechanism of ammunition cycling in a firearm. With a single-action weapon the shooter must physically cock the hammer back before pulling the trigger to initiate the cartridge. You may be most familiar with them as the cowboy “six-shooter” although the number of chambers in the cylinder can vary widely. Semi-automatic pistols can also be single action, at least for the first round.
The Colt model 1911 is one of the most popular single action semi-autos. The hammer must be cocked to fire the first round. After that the gases produced by the cartridge firing will cycle the weapon and load a new cartridge from the magazine.
With double-action firearms each pull of the trigger will either cock the hammer and rotate the cylinder (in revolvers), or release the hammer or striker (semi-automatics). With semi-automatic firearms the gasses produced by the discharge of the cartridge will force the slide rearward while extracting and ejecting the spent cartridge. When the slide moves forward it strips a new cartridge from the magazine and loads it.
As a writer it’s important to understand the distinction between the two. For instance, if you have a scene where your protagonist is in a gunfight and during that fight he/she cocks the hammer of the gun (for dramatic effect I suppose) make sure it is a single action weapon. For that matter make sure it has a hammer. Most American law enforcement agencies will not allow single action weapons to be carried by their officers. So if your character is an officer keep that in mind.