Double action
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The term double action was originally applied to revolvers. In the 19th century revolvers were often classified as to how their hammers were cocked. They were either "thumb-cocked", in which the hammer had to be cocked for each shot by the thumb, or "trigger-cocked", in which a long pull on the trigger first cocked the hammer and then released it to fall and fire the weapon.
Before the American Civil War most revolver designs were limited to one of these two ways of cocking the hammer. For example, the early Colt revolvers were "thumb-cocked" and early Webley revolvers were "trigger-cocked". As the development of revolver mechanisms progressed, new designs were created in which the hammer could be cocked either way, that is, the user could first cock the hammer with the thumb and then use the trigger to fire, or he could simply pull the trigger far enough so that the hammer was first drawn rearward and then released to fire. As time passed the terms "thumb-cocked" and "trigger-cocked" fell into disuse. Thumb-cocked actions came to be called "single actions", meaning actions in which the hammer always had to be cocked by the thumb for each shot. During the latter 19th century the "trigger-cocked" actions fell out of general favor and were steadily replaced by newer actions in which the hammer could be either thumb-cocked or trigger-cocked. This arrangement is now probably the most common for revolvers and is increasingly used for semiautomatic pistols. n the U.S., the original term for the newer type of hammer cocking arrangement was "double action" because the shooter now had a choice of two (double) methods of cocking the hammer.
Eventually this reason for the use of the term "double action" was generally forgotten and the term "double action" firing is now commonly used only to mean using a single motion of the trigger to both cock the hammer and fire in the "trigger-cocked" mode. Ironically, a few types of revolvers and pistols which can only be trigger-cocked (lacking a way to cock the hammer manually in the single action manner) are now usually (and misleadingly) termed "double action only" in spite of the fact that the term "double action" originally meant that the shooter had two choices of how to cock the hammer.