What is the difference between single action (SA), double action/single action (DA/SA) and double action only(DAO) semi-auto pistols

Question

Somewhat like revolvers, semi-automatic pistols also come in single action, double action/single action and double action only.

Also while there are similarities to how this terminology is used with revolvers, there are also differences.

What exactly are the differences between these types of actions and between the same terms being applied to both revolvers and semi-autos?

Community · Accepted Answer · 2020-06-18 08:23:47Z

Starting with simple and moving to more complex.

Single Action (SA)

A single action will only drop the hammer, it will not cock it back. If a round is chambered and then the hammer lowered, it must be manually cocked before it can fire. A good example of a single action is the M1911 pistol.
Double Action/Single Action (DA/SA)

A double action/single action will both cock and drop the hammer onto the primer. This makes it possible to carry the gun with a round in the chamber and the hammer down while not having to manually cock the hammer. However this means that the first trigger pull will be longer and heavier then the subsequent shots, which will be fired in single action mode. A good example of this would be the Beretta 92.
Double Action Only (DAO)

A double action only does not fully cock the firing mechanism (usually a striker instead of an external hammer) upon firing but rather the cocking is accomplished by pulling the trigger each time. This has the advantage of all of the trigger pulls being the same as opposed to the (DA/SA) actions. An example would be the SIG Sauer P250

As for the differences in terminology between revolvers and semi-autos,

Whether it is single of double action matters for every shot with a revolver, while after the first shot with a semi-auto the difference between single action and double action/single action no longer matters since the action of the slide after firing loads a round and cocks the hammer.
Pulling the trigger on a double action revolver will rotate the cylinder bringing a live round under the hammer. This is not true of semi-auto pistols as a round has to be chambered manually and firing that round will load the next one.

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