A semi-automatic pistol (Figure 2; Figure 4) is a handgun that contains only a single chamber, but has an automatic mechanism to remove an empty casing from the chamber and insert a new cartridge from the magazine. Semi-automatic pistols are manufactured in both single action and double action styles; however, this general classification of the action applies only to the first shot fired. On both single and double action pistols, the recoil of the slide automatically recocks the hammer for subsequent shots (Table 1). Therefore, after the initial shot is fired, the trigger pull only releases the hammer. This is different from double action-only pistols, where the hammer is recocked each time the trigger is pulled, rather than by the recoil of the slide. Not all pistols have hammers. Hammerless pistols operate in a similar manner to those with hammers, except that the mainspring is connected directly to the firing pin.

To fire a single action semi-automatic pistol, one must first cock the hammer manually by pulling back the slide (Figure 2, number 10). Pulling the trigger then discharges the cartridge. This discharge generates kinetic energy, causing the slide to recoil backward. As the slide recoils, it opens the breech, ejecting the casing. At the same time, the motion of the slide recocks the hammer for the next shot. The slide mechanism is spring-loaded, so that when it reaches its point of maximum recoil, it automatically returns to its resting position with the chamber closed. During the return of the slide, a new cartridge is automatically fed from the magazine into the chamber. This entire process takes only fractions of a second to complete. Then the pistol is ready to fire again. This process can be repeated in rapid succession until all of the available cartridges have been discharged.

Double action pistols operate exactly the same as single action pistols, except on the initial shot in the round. Rather than cocking the hammer manually before the first shot, as on single action pistols, the hammer on double action pistols is cocked by pulling the trigger. After the initial shot, double action pistols are automatically recocked for subsequent shots in the same way as single action pistols.

Double action-only pistols eject spent casings and reload new cartridges in the same way as single action pistols. Double action-only pistols, however, do not use the motion of the slide to recock the hammer. Rather, for both the initial shot and all subsequent shots, the hammer is cocked and released by pulling the trigger.

A revolver (Figure 1; Figure 5) is a multi-shot handgun that contains multiple chambers in a rotating cylinder. A cartridge can be stored in each chamber, waiting to be fired. When the trigger is pulled, the cartridge in

the chamber aligned with the barrel is discharged. Then, as the hammer is recocked, the next chamber rotates into place. In this way, several shots can be fired in rapid succession until all of the cartridges have been discharged. At that point, the chambers must be manually emptied and reloaded. The number of chambers in the cylinder can vary between models, ranging from 5 to 10. The direction of cylinder rotation is also variable, being either clockwise or counter-clockwise, depending on the design. A revolver can be purchased in single action, double action, and double action-only styles. Unlike pistols, the type of action for a revolver defines how it will be cocked on every shot fired, not just the initial shot.

A Derringer (Figure 6) is a small handgun with no magazine and no cylinder. It contains one chamber for each barrel, and each chamber must be manually reloaded after discharge. These guns, however, often contain multiple barrels, allowing for more than one shot to be fired before reloading. Typically, Derringers are small handguns, often lacking trigger guards. Most derringer models are single action. A few are double action, but none are double action-only.