This question is frequently asked or seen being incorrectly defined. I thought that it might be of use to some to explain these terms and provide examples.
Traditionally, the term “single-action” refers to a specific handgun action in which the trigger performs but a single function; it drops the hammer which fires the gun. Similarly, “double-action” describes handgun actions in which pressing the trigger performs two roles; it moves the hammer rearward and then releases it.
This STI 1911pattern pistol (left) is a single-action autoloading handgun. Its hammer must be cocked before the pistol can be fired. All the trigger does on this single-action is release the hammer so that it can strike the firing pin which detonates the chambered cartridge’s primer to fire the pistol. The Walther P-38 (right) is a traditional DA/SA automatic. In other words, it can be fired either double-action or single-action. It is pictured with a round chambered and the hammer down. Pressing the trigger for the first shot will both retract and then release the hammer to fire the pistol. The rearward movement of the slide will cock the hammer and subsequent shots will be fired in single-action mode, i.e.; the trigger will perform only one function: dropping the hammer. When you hear the term, “double-action semiautomatic”, this type setup is usually what is meant: double-action capable for the first shot with subsequent shots being fired single-action. These pistols frequently have slide-mounted thumb safeties that also act as decocking levers.
What about “DAO”? That term is frequently seen when describing some semiautomatic handguns. It means “double-action-only”. In other words, not just the first shot is fired double-action. All shots are. The hammer is designed not to remain cocked. On DAO semiautomatics, the hammer safely follows the slide forward as it chambers another round. The hammer on a DAO pistol cannot be manually cocked.
This SIG-Sauer P229 is an example of a DAO semiautomatic. Notice the lack of hammer spur on this pistol. It is not needed; this version of the P229 cannot be cocked. The pistol is designed so that the hammer safely follows the slide forward during the firing sequence. DAO is thought to be less likely to be inadvertently or negligently fired. It also provides a uniform trigger-pull for each shot as does the single-action autoloader, albeit both longer and harder. There is no “cocked-and-locked” carry with the DAO pistol, which is desired by some individuals and mandated by many police agencies and administrators. Some opine that it is safer while others suggest that it is a mechanical approach to a training issue.
We usually do not see the “single-action-only” designation except in models that have traditionally been double-action. The SAO offers the same exact operation as the traditional SA. An example would be some of the SIG-Sauer P220 handguns that are now available in single-action. CZ also offers some SAO variants of the CZ-75 line of handguns. One would be the Tactical Sport model.
Speaking of CZ-75’s, here is one of the Pre-B versions. It is sometimes referred to as “selective double-action”. What this means is that it can be fired double-action for the first shot and single-action for all shots thereafter like the Walther P-38 or it can be carried “cocked and locked” like the 1911 or Hi Power. Engaging the safety does not automatically drop the hammer.
Another example of a “selective double-action” is this early Taurus PT-92. It can be carried with a chambered round and the hammer down for a double-action first shot with single-action ones following or it can be carried in Condition One (cocked-and-locked) and fire all shots single-action. Current versions of this pistol still allow for Condition One carry but the safety also serves as a decocking lever so that the hammer can be lowered without risk of inadvertently firing the pistol. In other words, there is no need to press the trigger other than to fire the pistol on any type autoloader having a manual decocking lever.
The Springfield Armory XD line of polymer frame handguns do not have hammers that strike a firing pin. Like the Glock, Luger P08 and some Browning autoloaders, they have a spring-loaded firing pin or striker. With the grip safety depressed and the Glock-like trigger-safety engaged, pressing the trigger performs one function: releasing the spring-loaded striker. It would therefore be a more modern version of a single-action autoloader.
Like the XD, Glock pistols do not have a hammer, but rather a spring-loaded striker. Unlike the XD, the Glock’s is not fully compressed and pressing the trigger does not just release it. The striker is but partially retracted when the gun has a chambered round. Pressing the trigger finishes retracting it completely and then releases it. Combined with the Glock’s on-the-trigger-safety-lever, Glock refers to this as “Safe Action”. Actually, pressing the trigger is simply performing two functions: cocking and releasing the striker to fire the pistol. It is a newer example of a double-action semiautomatic. Whether it is “safer” than the XD or not can be debated, but the fact that it is designated “double-action” has helped its adoption by more than a few law enforcement agencies.
Revolvers are also described as single or double-action and the previous definition for each holds true.
Shown is the Uberti copy of the 1873 Colt Peacemaker. This revolver’s hammer must be manually cocked for each shot. The trigger performs only one function and as most know, this is a single-action revolver.
More modern revolvers are traditional double-action. This means that the trigger both cocks and releases the hammer to fire the cartridge in-line with the barrel. It does not mean that the gun is capable of both double and single-action fire. The same definitions previously cited hold true here too. DA-revolvers not capable of being cocked for single-action fire get the “DAO” mantle just like the semiautomatics.
Here we see an S&W Model 64 (top). It is a double-action revolver. This is not because it is capable of both double and single-action firing but because the hammer can be both retracted and released by merely pressing the trigger. The Model 642 (bottom) has an internal hammer and is double-action-only. Whether or not a revolver has two modes of fire or just one, it belongs in the double-action family if the trigger can perform two functions.
The traditional double-action S&W Model 14 was designed for use on the range at distances out to 50 yards and beyond. The one pictured is double-action and can also be cocked for single-action shooting. At one time, these revolvers could also be ordered in SAO for match shooters only competing in bullseye matches were single-action was allowed.
With the Glock 19 and the S&W Model 10 (that has been made unable to shoot single-action), we see examples of both the DAO semiautomatic and revolver. (Just because the hammer spur is missing on either the auto or wheelgun does not in itself dictate that the gun is DAO. Full-cock capability must have been removed.)
Hopefully, this explanation has been of use to some readers.
Best to you and yours.