A handgun is designated single action or double action.
A single action handgun, like the early rifles and revolvers one has to manually cock the hammer before firing and after loading the round in the chamber or barrel. The trigger does nothing but release the hammer letting it strike the primer in the cartridge.
A double action handgun is one that the trigger pull first cocks the hammer then releases the hammer when the trigger has pulled back far enough . This extra action takes more strength as pulling the trigger has to pull against the spring of the hammer.
The early revolvers, like the Colt, Smith, Remmington and many others, were single action only. One had to pull back the hammer which rotated the cylinder and cocked the hammer. Thus the trigger pull could be very slight, sometimes called a hair trigger.
Later some revolvers were double action. The heavy trigger pull , rotated the cylinder, cocked the hammer against its spring, then released the hammer.
Modern semi automatic pistols can be single or doulble action. Or double action only. With The single/ double action modes , the first shot is double action. Since the next shot is loaded in the chamber by the recoil of the first and has the hammer cocked, it will be single action with just a slight trigger pull.
Some of the most modern pistols have no hammer at all but are fired by a striker which isa small pin hitting the primer. All shots are double action only, but the first round is cocked when the user pulls the slide back to chamber the first round. Up until he chambers that first round the gun is useless. The Glock as well as others are of this type. Some like this as every trigger pull has the same tension. Also, there is no external hammer to catch on clothing when concealed.
In early repeating rifles the user had to chamber the round then cock the hammer. I believe the Spencer was like this. The Henry and its successor, the Winchester , clambering the round also cocked the hammer.
As an aside, remember the TV show Rifleman where Chuck Conners would cock and fire his rifle blazingly fast? His rifle was equipped with a screw that intruded into the trigger guard to push the trigger back to fire automatically as he finished cocking. He did not have to pull the trigger. It was used in some scenes but not all as it would fire every time it was cocked.
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