Originally Posted by
SouthernBoy
First off, you must understand what the term "action" refers to and how it is used.
The "action" of a gun, in the instant context of this thread, always refers to the trigger's task, or responsibility, as it relates to the firearm. With semi-auto pistols, the common actions in use are 1) Single Action (SA), 2) Double Action (DA), sometimes called Double Action/Single Action (DA/SA), and 3) Double Action Only (DAO).
In a single action, the only task the trigger performs is to release the hammer sear or the striker. It does nothing else. Best example is the venerated 1911 pistol.
With a double action, the trigger can perform two tasks. It can cock and release the hammer or just release the hammer. A good example is the model 92 Beretta. When the hammer is at rest, a long pull of the trigger fully cocks the hammer, then releases it to fire the pistol. After this first shot, all subsequent shots can be made in single action mode since the movement of the slide cocks the hammer and the only thing the trigger does is release the hammer.
A double action only pistol always performs two tasks. I both cocks the striker, or hammer, AND releases it to fire the pistol.
The Springfield XD is a single action pistol because the striker is at full cock when the slide is exercised. There is no partial cocking with this gun. Therefore, the only thing the trigger does is release the striker to fire it.
With the Glock, when the slide is exercised, the striker is left in a half-cocked condition. When the user starts pulling the trigger to fire the pistol, the striker is moved rearward into a fully cocked condition and then released by the final pull of the trigger to fire the pistol. The Glock trigger performs two tasks and always two tasks in the firing cycle. It completes the cocking of the strike and it releases the striker to fire the pistol Therefore, it is a double action only pistol. Incidently, the BATFE defines the Glock as a DAO pistol for the reasons I have just given.