What do single-action and double-action mean?
by B.B. Pelletier
This is for the call desk at Pyramyd Air, where they say they get asked this question all the time. When I use either term in print, I wonder if everyone understands it, so I'd like to explain both today.
Single-action came first
The first type of trigger was the single-action style. All the trigger does is release the hammer or striker. When the gun is cocked, the trigger can then release the hammer or striker so it can travel to the end of its path, as defined by the design of the gun. If it's a flintlock, the hammer carries the flint to strike the hardened frizzen and cause sparks. As the hammer continues to fall, it directs the sparks into the pan to ignite the gunpowder that flashes through the touchhole to set off the main charge in the barrel.
The poster boy of single-action revolvers is the Colt Single-Action Army. One of its nicknames was "Old Thumbbuster."
Colt's ever-popular M1911A1 pistol is also a single-action. The hammer must be cocked before the trigger will work. The action of the slide coming back in recoil (blowback) cocks the hammer for the next shot. But, for the first shot, the hammer is cocked manually.
If the hammer is on a percussion gun, it falls on a percussion cap with force to explode it from the shock. If the hammer is on a firearm that uses cartridges, it either impacts a primer directly and causes an explosion that ignites the main charge of gunpowder, or it strikes a firing pin that does the same thing. The single-action mechanism does just one thing: it releases a hammer or striker to do its job.
What is a striker?
A striker is a heavy firing pin that acts as both a firing pin and a hammer. It has enough mass to set off a primer, but it doesn't look like a hammer. In fact, most strikers look like fat firing pins. The firing pins on most bolt-action rifles are actually either strikers by themselves, or they are attached to extra mass and then function as strikers. So it is possible for a single-action gun not to have a hammer and still be single-action. The way to know whether it's a single-action is to understand what the trigger does. If it just releases the hammer (or striker), it's a single-action.
A striker-fired pistol
It didn't take long before the autoloading pistol did away with the hammer for the more compact striker. Hugo Borchardt invented an autoloader in 1893 that eventually became the foundation for the Luger pistol. With the hammer gone, the pistol could become smaller and more compact, though Borchardt's example is far from it!
Not a hammer in sight! The Borchardt is single-action only. It must be manually cocked to fire the first time, then the semiautomatic action takes over.
What does double-action mean?
Double-action means that the trigger cocks the gun AND also releases the hammer/striker. This type of action was perhaps invented in the early part of the 19th century, when repeating guns first began to be popular. At first it was called a "self-cocking" lock, meaning that the trigger did all the work, and it was found on pepperbox pistols as early as 1830. When the revolver became popular around 1850, the trigger also had to do one more thing - it had to advance the cylinder so a fresh cartridge would be ready to shoot each time. That's why revolvers always have a heavier trigger pull, but the striker-cocking action is also why double-action only (DAO) semiautomatic pistols also have a heavier pull.
A pepperbox is an early type of revolver that rotated the barrels instead of a cylinder. Just the top barrel fired at one time.
What does this have to do with airguns?
Everything, because airguns are also either single- or double-action. Some can be fired either way. The S&W 586 revolver, for instance, is both single and double-action. Most of the Umarex pistols are double-action only, because they have no way to cock the hammer other than the trigger. The Walther PPK/S BB pistol, however, is single-action only. You can pull the trigger with the slide forward and the hammer down and nothing will happen. Once it begins firing, however, the slide cocks the hammer for the next shot - not unlike the M1911A1 pistol we looked at earlier.
This is mostly about handguns - but not always
There are a few revolving rifles, too. The Crosman 1077 is one. It's a double-action only revolver with a 12-shot pellet clip. The Crosman Nightstalker is a semiautomatic, but the clip advances by means of the trigger, so it is the world's only DAO semiauto rifle (I think!). That's why it has a heavier trigger-pull than the Drulov DU-10 Eagle semiautomatic rifle.