The terms we use most often to describe the various designs of today’s semi-auto pistols are derived from the terms originally used with revolvers.
In the vernacular of the revolver, Single Action means that in order for the gun to fire, the hammer has to be manually cocked (which also rotates the cylinder) prior to pulling the trigger, When pulled, the trigger’s only job was to simply releases the hammer.( I use the word “was” here because today many Single Action revolvers actually have a firing pin block of some sort that is deactivated when the trigger is pulled)
Double Action revolvers cock the hammer, rotate the cylinder and fire the cartridge all with the pull of the trigger. In addition most Double Action revolvers can function like a Single Action as if the shooter so chooses, they can the hammer can be cocked manually prior to pulling the trigger providing for a lighter, shorter trigger pull to fire the cartridge. The exception to this is in the DAO revolver which is typically seen in small revolvers with bobbed or shrouded hammers that do not have the ability to be manually cocked.
Note that there are and historically speaking were many other variations on the revolver including early revolvers like the multi barreled pepperbox that had to be cocked, rotated, and fired manually to more advanced designs like the Webley Fosbery that actually combined the revolver platform with the recoil harnessing principles of the semi-auto.
Looking at the various action types that apply to the subcategory of handguns we paint with the word revolver, it’s no wonder that there is so much misinformation, misunderstanding and downright confusion in gun shops across America today when it comes to understanding the differences between the multitude of semi-auto handgun choices available.
For the sake of this discussion we will only looking at the impacts different designs have on the necessary steps the shooter must complete to safely operate the firearm, not necessarily the internal differences in action types such as the various passive safety features (Series 80, Grip Safety, Firing Pin Block, etc) blowback (CZ 83) or locking designs like short recoil (CZ 75, Dan Wesson 1911.)
A semi-auto handgun with a true double action trigger system has the ability to fully cock the action through the trigger pull. Only a handgun with a true double action has second strike capability. The Second Strike is useful when a cartridge primer fails to fire the first time the trigger is pulled. In this situation, if the pistol has second strike capability, the trigger can be pulled a second time giving the shooter the chance to send that bullet down range towards the intended target.
Subcategories of the double action include:
Double Action Only (DAO) that would include the discontinued CZ 100 and CZ 75 DAO,
DA/SA like most of the original CZ 75, most other 75 variations (excepting the SA and TS variations), CZ 83, CZ 85, CZ 97, CZ P-01, CZ SP-01. DA/SA can further be broken down into those with a manual safety and those models that have a mechanical decocker integrated into their design.
Modern striker fired designs found on many polymer framed models are commonly referred to as double action, though it would be more accurate to describe them as “a little more action than single”. These designs and many other similar trigger systems use the energy from the recoil to partially cock the action, but your finger cocks it the rest of the way when pulling through the trigger stroke. Note that most of these designs do not have “second strike” capability.
The trigger does not compress the hammer spring or striker spring of the Single Action semi-auto handgun at all, but the trigger’s primary purpose is to trip the sear, eventually powering the firing pin forward (via a striker or hammer) towards the cartridge primer subsequently firing the cartridge. As the recoil cycles the slide backwards, the action is automatically cocked and readied for the trigger to trip the sear and start the cycle over again. Single action pistols typically have a lighter, shorter trigger pull. Most such as the 1911 or CZ 75 SA incorporate a manual safety lever that when activated prevents the trigger from moving and/or the firing mechanism from operating.
For Single Acton (only) models, once a round has been chambered and the safety lever is deactivated, the first and all subsequent shots enjoy a lighter trigger pull and shorter trigger travel and reset.
Examples of Single Action pistols include the line of Dan Wesson 1911s, CZ 75 SA, and CZ 75 Tactical Sports.
Double Action/Single Action (DA/SA)
The first shot is taken with the hammer down (or at half cock) by manually or mechanically decocking the pistol after loading and before holstering. As the trigger is pulled for the first shot, the action is cocked fully (or partially if holstered at half cock) by the trigger pull. At the end of the trigger pull, the sear is tripped powering the firing pin forward towards the cartridge primer firing the cartridge. As the recoil cycles the slide backwards, the action is automatically cocked and readied for the trigger to trip the sear allowing the second and subsequent shots to be fired from the single action mode. When the shooting is done, a DA/SA gun should be “put on safe” or decocked. When shooting a gun that starts from the hammer down (or first shot in double action), there is no safety lever to deactivate. Much like the double action revolver, the long and relatively heavier trigger pull of the double action is a safety mechanism in and of it’s self.
Double Action models equipped with a manual safety, like the 75 B are versatile in that they can be carried in SA mode (hammer back, safety on) or manually decocked and carried as a DA/SA with the first shot double action and each subsequent shot single action.