The ??? Action and [fill in the blank] Fired Guns...
There can be a confusing set of terms used, often by writers, gun reviewers, and experienced gun hobbyists. But all of the terms are there for a reason, mean something specific, and give important insight into not only how a gun works, but also the history and evolution of firearms themselves. This article will focus on the terms Single Action, Double Action, Hammer Fired, and Striker Fired.
The action, for the purposes of our discussion, is the combination of the trigger, the firing pin, the kinetic driver which slams the firing pin into the cartridge primer, and the mechanism, sometimes called the "linkage," which connects the trigger and the firing pin & kinetic driver.
The point of pressing the trigger is to release the firing pin into the cartridge primer, thereby firing the cartridge. But there's a lot of stuff which may have to go on "behind the scenes." Primarily to this discussion is the fact that some sort of kinetic driver has to be set, primed, or engaged to provide the energy for the firing pin to slam home into the cartridge primer. This is almost always a spring of some sort. In the early days of firearms, it was often a leaf spring. Now, it is more often a coil type compression spring. Energy has to be added somehow, from somewhere, to compress this spring, "filling" it with potential energy and setting it to be ready to release that potential energy and drive the firing pin.
[Leaf Spring as the Hammer Spring]
[Compression Coil Spring as the Hammer Spring]
When discussing Single Action or Double Action, there are two words and both of them are important. They describe what is happening; an "Action." A Single Action means that only one thing is happening, a "Single" action, when the trigger is pressed. It releases the mechanism restraining the kinetic driver, freeing it to release potential energy into kinetic energy and drive the firing pin. If there is no stored potential energy in the spring, then nothing happens. In order for a Single Action event to have any meaning, the hammer or spring must already be compressed, or in common firearm parlance, "cocked." That is all it does. One, "Single" action; releasing the mechanism to drive the firing pin.
It is important here to understand a bit of the history of firearms and trigger systems. We'll skip over hand-cannons and simple Match-Locks and head straight to Wheel-Locks. Wheel-Locks were probably the earliest form of personal firearm which connected a trigger to a spring in order to use the energy stored in the compressed spring to create kinetic energy for use in firing the gun. A "wheel" and spring similar to that of a wind up clock of the day was manually wound with a gear and ratchet restraining system connected to it. The ratchet was released by the trigger, freeing the wheel to spin, under spring power, which drove a piece of flint against steel, grinding off a shower of white-hot steel bits as sparks, which would ignite the flash-pan powder. Pressing the trigger accomplished one single action, disengaging the ratchet latch. We'll move forward a few iterations to Flint-Locks. This was a technological improvement because, instead of a wheel which must be laboriously wound, a "hammer" which we would today recognize, replaced it. The hammer, called a "cock," was locked back manually, but comparatively quickly, compressing the spring, without any accompanying requirements of the trigger to set the spring. Pressing the trigger on a Flint-Lock, again, accomplished one single action: releasing the hammer to strike the flint down on the frizzen, initiating the ignition sequence. Moving forward again, the next major improvement we should look at is the Cap-Lock. This was, again, a major technological improvement. The gun powder was completely enclosed and ignition access to the firing chamber was through a steel nipple. A soft metallic percussion cap was placed over the nipple which, when struck, would send a small jet of flame into the chamber, igniting the gunpowder. This percussion cap was struck, and ignited, with a hammer, sometimes still called a "cock." Still, the hammer must be manually pulled back, compressing the spring. This basic requirement followed all the way up through the introduction of modern self contained metallic cartridges. Compressing the hammer-spring was a separate action, apart from pressing the trigger. Pressing the trigger performed only one action, releasing the restraint on the spring and allowing the hammer and firing pin to strike the primer.
[Double Barreled Cap-Lock Pistol]
This series is what is commonly known as Single Action (SA). It means that the spring must be compressed by some other activity than the movement of the trigger. With a Revolver, a Single Action requires the user to manually move, "cock," the hammer (compressing the spring) with each shot. With a Semi-Automatic pistol, a Single Action requires that the user must "cock" the hammer for the first shot and the act of the slide reciprocating cocks the hammer for each successive shot. In neither case is the hammer cocked or the spring compressed by the user pulling the trigger. Triggers operating in Single Action typically require much less force to pull ("lighter") and usually have a much shorter "travel" because all that is required is just to release the sear and let the compressed spring release its energy.
It doesn't take Einstein to realize that requiring two separate and distinct actions to compress the driving spring and then release it is not the most efficient method and an alternative must exist which would be a time saving measure, sure to increase combat effectiveness of the firearm: Doing both at once. This innovation was applied to Revolvers soon after the U.S. Civil War. Thus, Double Action is newer but still "old." In a Double Action firearm, pulling the trigger performs double the number of actions: i.e., two. Pulling the trigger will compress the spring and "cock" the hammer to the point of usefulness and then, all in the same action sequence, release the mechanism, allowing the hammer to fall, strike the firing pin, and complete the ignition process. But the piper must be paid. The extra energy required to compress the spring must be provided by the user's finger. This means that it is usually harder for a user to pull a trigger through a Double Action sequence, which can mean it is harder for the user to shoot a Double Action gun with accuracy.
We'll get a bit more into it below, but most Double Action pistols with an exposed hammer are also capable of a Single Action firing sequence. The user would manually "cock" the hammer as in a Single Action, providing a lighter, easier trigger pull. Alternately, the user could pull the trigger all the way through in Double Action. This capability to do both is called "Double Action/Single Action" (DA/SA), or, alternately, Traditional Double Action.
But designing and building a gun that has both Double Action and Single Action capability requires added complexity, which means added expense, additional ways in which the gun (a machine) could break, and requires extra training for the user which could mean mistakes during use. This lead to the birth of Double Action Only (DAO). A Double Action Only gun has no option for Single Action and the user is required to press the trigger through the longer, heavier, trigger pull. Double Action Only guns with a hammer often have an enclosed ("shrouded") hammer or have the hammer spur removed ("bobbed"). Double Action Only designs are common for military, law enforcement, and self defense guns which have no intention of bullseye pinpoint accuracy. They are still plenty accurate, but guns which can fire in Single Action are generally easier to shoot accurately.
[British Military Issue revolver, Double Action Only, with "bobbed" Hammer]
For most of firearms history, guns had an exposed "Cock" or hammer. Knowing how these machines evolved it is easy to understand why. It worked. If it isn't broken, don't "fix" it, as the old saying goes. This worked perfectly all the way up through Cap-Lock guns, which required a flat hammer face to strike the percussion cap. When the percussion cap was included as a component of the modern self-contained metallic cartridge, another way to strike and ignite the primer had to be developed. There were competing systems, such as the Pin-Fire, but eventually a primer cap set in the rear of the cartridge became the standard. Because the cartridge, and therefore also the primer, were fully encapsulated by the gun, a way to strike the primer which was being held inside the gun was developed in the form of a pin which could protrude into the gun. Early Hammer Fired metallic cartridge guns had the firing pin directly attached to the hammer. This system is still used by some Revolvers today. However, as technology progressed, the firing pin was separated from the actual hammer, would be enclosed in the firearm's housing, and would be struck, then driven forward, by the hammer. This is common in many Hammer Fired Semi-Automatic guns, such as the venerable 1911. In any case, however, the hammer was driven by the spring causing the firing pin to impact the primer, initiating the firing sequence.
[Revolver with Firing Pin directly connected to the Hammer]
[The Firing Pin removed removed from the slide of a Hammer Fired semi-automatic pistol]
[Exposed, flat faced Hammer on a 1911 - The falling Hammer strikes the end of the Firing Pin enclosed by the slide]
At some point, some genius recognized that a hammer may be redundant. The spring could be connected directly to the firing pin without the requirement of a hammer "middle man." The firing pin, now directly driven by the compressed spring, is called a Striker. Striker Fired guns are generally considered to have the advantages of a simpler design, less complexity, and fewer moving parts. However, it is difficult to design Striker Fired Revolvers and so the concept is almost exclusively applied to Semi-Automatic guns.
[The Striker (Firing Pin) and Compression Coil Spring from a Striker Fired pistol]
[Mix and Match]
It is possible to match either type of firing mechanism, Hammer Fired or Striker Fired, with any type of trigger action, Single Action, Double Action, or Double Action Only. Due to issues of design and manufacture complexity, most Striker Fired guns are Single Action. They use the action of the slide reciprocating to reset and compress the spring, often still called a "Hammer Spring" even in the absence of an actual hammer. But this is not an exclusive rule. There are many Semi-Automatic, Striker Fired guns which have Double Action/Single Action, or even Double Action Only designs. There is even one oddball design, the Glock. Glock handguns will partially, but not fully, reset and compress the spring when the slide reciprocates. This leaves a trigger pull weight somewhere between that of Single Action and Double Action. The design is intended as both a safety and ergonomic feature, reducing somewhat how heavy the trigger pull is but still being heavier than a very light Single Action trigger, which might be so light as to allow for unintended trigger pulls (sometimes called, "Unintended Discharge," "Accidental Discharge," or "Negligent Discharge," but that's an entirely different argument).
[The Taurus PT-111 is a popular Double Action/Single Action, Striker Fired pistol]
[There is no exposed hammer to be seen on the butt of the Taurus PT-111 because it is Striker Fired]
So now when some gun reviewer pops off jargon about Single Action or Striker Fired, you'll not only know what he's talking about but you'll know why the terminology is used and how it came about.