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OK, I now what they mean, as in, I know what the acronyms stand for, if I dont know exactly what single action, double action actually is, or, more importantly, where the differances are in my PT145, I have never noticed any differance in the trigger regardless of how I load a round, or if it was dry-fired prior to loading, nothin.
 

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Single action (SA)
A single-action trigger, sometimes single-action only, performs the single action of releasing the hammer or striker, which discharges the firearm.[1] Almost all rifles and shotguns use this type of trigger.[2] It is also common on pistols. Single-action semi-automatic pistols require that the hammer be cocked before the first round is fired.[3] Once the first round is fired the automatic movement of the slide cocks the hammer for each subsequent shot. The pistol, once cocked, can be fired by pulling the trigger once for each shot until the magazine is empty. The M1911 is a single-action pistol that functions in this manner.[4]


Single set trigger
A set trigger allows a shooter to have a greatly reduced trigger pull (the resistance of the trigger) while maintaining a degree of safety in the field. There are two types: Single Set and Double Set. A Single Set Trigger is usually one trigger that may be fired with a conventional amount of trigger pull weight or may be 'set' by usually pushing forward on the trigger. This takes up the creep in the trigger and allows the shooter to enjoy a much lighter trigger pull.


Double set trigger
As above, a double set trigger accomplishes the same thing, but uses two triggers: one sets the trigger and the other fires the weapon. Set triggers are most likely to be seen on customized weapons and competition rifles where a light trigger pull is beneficial to accuracy.


Double action (DA)
Invented by Robert Adams, a double-action trigger performs the two functions of cocking and then releasing the hammer or striker.[5] When this term is applied to revolvers, the trigger also rotates the cylinder. Though this is technically a third action, it is correct to refer to the mechanism as double-action. More confusingly, revolvers with a double-action trigger mechanism almost always retain the single action functionality.[6] The hammer may be cocked and the trigger pulled. A typical DA revolver is the Smith and Wesson model 19 revolver.


Double action only (DAO)
A double action only is similar to a DA revolver trigger mechanism however there is no single action function.[7] For semi-automatic pistols with a traditional hammer, the hammer will return to its decocked position after each shot.[8] 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 revolver.


Double action/Single action (DA/SA)
A double action/single action firearm combines the features of both mechanisms. Often called traditional double action, these terms apply almost exclusively to semi-automatic handguns.[9] 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.[10] The Mateba Autorevolver is a hybrid revolver that functions on a DA/SA system. However, it is different in function than either a conventional revolver or semi-automatic pistol. The H&K USP is a good example of a DA/SA semi-automatic pistol. On many DA/SA pistols (including the USP) 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.
 
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