Shotgun shells are similar to center-fire cartridges with one important distinction; the wad or shot cup. Shotgun shells come in a variety of calibers (such as .410, 20 gauge, 12 gauge which are the most common) and typically fire pellets instead of a single projectile. Having said that, there are shells designed to fire a single projectile called a “slug”. The size and number of pellets varies as well and are designed for specific uses. Size 8 or 9 shot are small BB sized pellets used for target practice and small game hunting while larger shot such as 00Buck are used for home defense and predator hunting.
When a typical cartridge is fired at a crime scene the CSI might look for both the bullet and the cartridge case. The other component (powder) is generally burned up but can be found in a residue. But with a shotgun discharge the CSI may also be looking for the wad or shot cup . Most modern ammunition uses a shotcup which is a plastic “cup” with separate ”petals” which fold away after leaving the muzzle and slow its forward movement (see photo). Older shotgun shells commonly used a wad which was a small fibrous disc of the same diameter as the shell. It’s kind of like a little padded sponge but without the holes and denser. Without a wad or shotcup the pellets would exit the muzzle with much les energy because the pellets can’t hold back the pressure as well as a solid projectile. Shotguns are smooth-bore weapons and generally do not have rifling which means the examiner can not do a traditional rifling comparison like they would with an expended bullet.
Badguys will sometimes pick up their expended cases or shells (often referred to as “policing your brass”) but finding the shotcup would be very difficult as it can travel quite some distance from the muzzle (I have found them over 100 feet away). Sometimes, if the muzzle is in contact or close contact with the victim the shotcup or wad may even be recovered from inside the victim at autopsy. Now a firearms examiner can not link a shotcup to a particular weapon but examining the shotcup might reveal what type and size of shot was used and even the manufacturer of the ammunition. This might be important circumstantial evidence to consider when searching a suspect’s home, vehicle, or financial records.
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.