Single-action and Double-action Firearms

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.