A trigger is a mechanism that actuates the firing of firearms. Triggers almost universally consist of levers or buttons actuated by the index finger. Rare variations use the thumb or weak fingers to actuate the trigger. Examples are the M2 Browning machine gun and the Springfield Armory M6 Scout respectively.
Firearms use triggers to initiate the firing of a cartridge in the firing chamber of the weapon. This is accomplished by actuating a striking device through a combination of spring and kinetic energy operating through a firing pin to strike and ignite the primer. There are two primary types of striking mechanisms, hammers and strikers. Hammers are spring-tensioned masses of metal that pivot on a pin when released and strike a firing pin to discharge a cartridge. Strikers are, essentially, spring-loaded firing pins that travel on an axis in-line with the cartridge eliminating the need for a separate hammer. The connection between the trigger and the hammer is generally referred to as the sear surface. Variable mechanisms will have this surface directly on the trigger and hammer or have separate sears or other connecting parts.
There are numerous types of trigger mechanisms. They are categorized according to which functions the trigger is to perform. In addition to releasing the hammer or the striker, a trigger may cock the hammer or striker, rotate a revolver's cylinder, deactivate passive safeties, select between semi-automatic and full-automatic fire such as the Steyr AUG, or pre-set a 'set trigger.' Most modern firearms use the trigger to deactivate passive safeties but this does not change how they are identified.
 Single action (SA)
A single-action trigger, sometimes single-action only, performs the single action of releasing the hammer or striker, which discharges the firearm. Almost all rifles and shotguns use this type of trigger. It is also common on pistols. Single-action semi-automatic pistols require that the hammer be cocked before the first round is fired. Once the first round is fired the automatic movement of the slide cocks the hammer for each subsequent shot. The pistol, once cocked, can be fired by pulling the trigger once for each shot until the magazine is empty. The M1911 is a single-action pistol that functions in this manner.
 Single set trigger
A set trigger allows a shooter to have a greatly reduced trigger pull (the resistance of the trigger) while maintaining a degree of safety in the field. There are two types: Single Set and Double Set. A Single Set Trigger is usually one trigger that may be fired with a conventional amount of trigger pull weight or may be 'set' by usually pushing forward on the trigger. This takes up the creep in the trigger and allows the shooter to enjoy a much lighter trigger pull.
 Double set trigger
As above, a double set trigger accomplishes the same thing, but uses two triggers: one sets the trigger and the other fires the weapon. Set triggers are most likely to be seen on customized weapons and competition rifles where a light trigger pull is beneficial to accuracy.
 Double action (DA)
Invented by Robert Adams, a double-action trigger performs the two functions of cocking and then releasing the hammer or striker. When this term is applied to revolvers, the trigger also rotates the cylinder. Though this is technically a third action, it is correct to refer to the mechanism as double-action. More confusingly, revolvers with a double-action trigger mechanism almost always retain the single action functionality. The hammer may be cocked and the trigger pulled. A typical DA revolver is the Smith and Wesson model 19 revolver.
 Double action only
A double action only of DAO is similar to a DA revolver trigger mechanism however there is no single action function. For semi-automatic pistols with a traditional hammer, the hammer will return to its decocked position after each shot. For striker-fired pistols such as the Taurus 24/7, the striker will remain in the rest position through the entire reloading cycle. This term applies mostly to semi-automatic handguns; however, the term can also apply to some revolvers such as the Smith & Wesson Centennial revolver.
 Double action/Single action
A double action/single action or DA/SA firearm combines the features of both mechanisms. Often called traditional double action, these terms apply almost exclusively to semi-automatic handguns. The function of this trigger mechanism is identical to a DA revolver. However, the firing mechanism automatically cocks the hammer or striker after the gun is fired. This mechanism will cock and release the hammer when the hammer is in the down position but on each subsequent shot, the trigger will function as a single action. The Mateba Autorevolver is a hybrid revolver that functions on a DA/SA system. However, it is different in function than either a conventional revolver or semi-automatic pistol. The H&K USP is a good example of a DA/SA semi-automatic pistol. On many DA/SA pistols (including the USP) there is the option to cock the hammer before the first shot is fired. This removes the heavy pull of the double-action. Also, there is often a de-cocker to return the pistol to double-action.
Pre-set hammers and strikers apply only to semi-automatic handguns. Upon firing a cartridge or loading the chamber, the hammer or striker will rest in a partially cocked position. The trigger serves the function of completing the cocking cycle and then releasing the striker or hammer. While technically two actions, it differs from a double-action trigger in that the trigger is not capable of fully cocking the striker or hammer. Glock pistols use a pre-set striker mechanism. An example of a pre-set hammer is the Kel-Tec P-32 pistol.
 Pre-set hybrid
Pre-set hybrid triggers are similar to a DA/SA trigger in reverse. The first pull of the trigger is pre-set. If the striker or hammer fail to discharge the cartridge, the trigger may be pulled again and will operate as a DAO until a malfunction is cleared or the cartridge discharges. This allows the operator to attempt to fire a cartridge after a misfire malfunction. The Taurus 24/7 Pro pistol (not to be confused with the first-generation 24/7 which was a traditional pre-set) offers this feature as of 2006.
 Relative merits
Each trigger mechanism has its own merits. Historically, the first type of trigger was the single action. This is the simplest mechanism and generally the shortest, lightest, and smoothest pull available. The pull is also consistent from shot to shot so no adjustments in technique are needed for proper accuracy. On a revolver, the hammer must be manually cocked prior to firing, an added level of safety is present. On a semi-automatic, the hammer will be cocked and ready to fire by the process of chambering a round, and as a result an external safety is required.
Double action triggers provide the ability to fire the gun no matter whether the hammer is cocked or uncocked. This feature is desirable for military, police, or self-defense pistols. The primary disadvantage of any double-action trigger is the extra length the trigger must be pulled and the extra weight required to overcome the spring tension of the hammer or striker.
DAO firearms attempt to solve the problems with DA/SA mechanisms by making each and every shot a double-action shot. Because there is no difference in pull weights, training and practice are simplified. Additionally, accidental discharges are reduced because of the heavier trigger pulls. This is a particular advantage for a police pistol. These weapons also generally lack any type of external safety. DAO is common among police agencies and for small, personal protection firearms. The primary deficiency is that accurate fire is difficult due to the additional trigger weight and travel required for each shot.
DA/SA pistols are versatile mechanisms. These complex firearms generally have a manual safety that may or may not serve to decock the hammer. Some have a decocking lever and a manual safety as well. As a disadvantage, these levers are often intermingled with other levers such as slide releases and takedown levers with variables that become confusing. Training will largely overcome this weakness. One other disadvantage is the difference between the first double-action pull and subsequent single-action pulls (if the hammer is not set before the first shot by the user).
Pre-set triggers, only recently coming into vogue, offer what some would consider an optimum balance of pull weight, trigger travel, safety, and consistency. Glock pioneered this trigger and many other manufacturers have followed suit. The primary disadvantage of the pre-set trigger is that pulling the trigger a second time after a failure to fire will not re-strike the primer. In normal handling of the firearm, this is not an issue; loading the gun requires that the slide be retracted, pre-setting the striker.
There is some debate over the merits of a lightweight single action or pre-set trigger in defensive firearms. Recently, Glock has been sued with varying success for having triggers that are allegedly too easy to manipulate. To their credit, Glock offers concerned law enforcement agencies a so-called New York Trigger with a pull weight similar to a DAO pistol.
- Trigger Analysis Ballistics-Experts.com
- Trigger Options of the Semi-Automatic Service Pistol Chuckhawks.com
- Firearms - Handgun Action Types Outdoors.net