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.