Revolver

A revolver is a type of gun. Revolvers are repeating firearms with a cylinder that spins. Revolvers have several chambers each holding one cartridge. Most are handguns, but other types of weapons also use revolving cylinders. They are also called revolving guns, though revolver is used far more often. The first revolvers date back to late 1500s in Europe. It was not until the 1700s or 1800s, however, they became affordable. Revolvers are sometimes used by law enforcement agencies. On some occasions, these guns can be reset by one hand, then loaded with the other.

 Action 
 Single-action 
In a single-action revolver, the hammer is manually cocked. This is usually done with the thumb. This action turns the cylinder to the next cartridge. It also locks the cylinder in place with the chamber aligned with the barrel. The trigger, when pulled, releases the hammer, which fires the round in the chamber. To fire again, the hammer must be manually cocked again. This is called "single-action" because the trigger only performs a single action—that of releasing the hammer. Because only a single action is performed and trigger pull is very light, firing a revolver in this way allows most shooters to achieve greater accuracy. Additionally, the need to cock the hammer manually acts as a safety. The gun will not fire unless the hammer is cocked back. The Colt Paterson Revolver, the Walker Colt, the Colt's Dragoon and the Colt Single Action Army pistol of the American Frontier era are all good examples of this system.William J Tilstone; Kathleen A Savage; Leigh A Clark, Forensic Science: An Encyclopedia of History, Methods, and Techniques (Santa Barbara, CA: ABC-CLIO, 2006), pp. 158–159

 Double-action 
In double-action, the stroke of the trigger pull generates three actions: 
The hammer is pulled back to the cocked position.
At the same time, the cylinder turns to the next round.
The hammer is released to strike the firing pin. 
Double-action means that a cocking action separate from the trigger pull is not needed. Every trigger pull causes a complete cycle and a bullet is fired. This allows the gun to be carried uncocked. It also allows the gun to be drawn from the holster and fired using only the trigger. However, a double-action revolver has a longer and harder trigger pull. However, this drawback can also be viewed as a safety feature, as the gun is safer against accidental discharges from being dropped.

Most double-action revolvers may be fired in two ways.
 The first way is single-action. It can be used the same a single-action revolver. The hammer is cocked with the thumb, which indexes the cylinder and when the trigger is pulled the hammer fires the round. In this mode the trigger pull is lighter.
 The second way is double-action, or from a hammer-down position. In this case, the trigger first cocks the hammer and revolves the cylinder. It then trips the hammer at the rear of the trigger stroke, firing the round in the chamber. In this mode the trigger pull is much heavier.

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