Replies

Double-action

A double-action, also known as double action only (DAO) to prevent confusion with DA/SA designs, is similar to a DA revolver trigger mechanism. The trigger both cocks and releases the hammer. However there is no single action function. A good example of this action is the Sig Sauer DAK trigger. For semi-automatic pistols with a traditional hammer (that employ only the double action function of the trigger), the hammer will return to its decocked position after each shot. Subsequent shots require the double action trigger firing sequence. For striker-fired pistols such as the Taurus 24/7, the striker will remain in the rest position through the entire reloading cycle. This term applies mostly to semi-automatic handguns; however, the term can also apply to some revolvers such as the Smith & Wesson Centennial, the Type 26 Revolver, and the Enfield No. 2 Mk I revolvers, in which there is no external hammer spur. Glock and Kahr semi-automatic pistols are not DA (or DAO) pistols because the striker is “cocked” to an intermediate position by the operation of the slide and they cannot be re-activated by pulling the trigger a second time.

Double-action/single-action

A double-action/single-action (DA/SA) firearm combines the features of both mechanisms. Often called traditional double action, these terms apply almost exclusively to semi-automatic handguns. The function of this trigger mechanism is identical to a DA revolver. However, the firing mechanism automatically cocks the hammer or striker after the gun is fired. This mechanism will cock and release the hammer when the hammer is in the down position, but, on each subsequent shot, the trigger will function as a single action. The Mateba Autorevolver is a semi-automatic revolver that functions on a DA/SA system. The Beretta 92 is a good example of a DA/SA semi-automatic pistol. On many DA/SA pistols (including the Beretta), there is the option to cock the hammer before the first shot is fired. This removes the heavy pull of the double-action. Also, there is often a de-cocker to return the pistol to double-action.

A second distinct type is that used by the majority of double-action revolvers, where the weapon can be fired in either double-action mode by pulling the trigger, or single-action mode by cocking the hammer manually before firing. This is distinct from double-action only, since the weapon does not have to be fired in double-action mode, for example, the Colt Python.

(Above are Wikipedia definitions.)