||This article's lead section may not adequately summarize key points of its contents. (June 2011)|
A trigger is a mechanism that actuates the firing sequence of a firearm or crossbow; a trigger may also start another mechanism such as a trap or a quick release. A small amount of energy applied to the trigger causes the release of much more energy. Triggers usually consist of levers or buttons actuated by the index finger. Some variations use the thumb or weak fingers to actuate the trigger, a feature found on mounted weapons such as the M2 Browning machine gun and the Springfield Armory M6 Scout.
- 1 Function
- 2 Actions (mechanisms)
- 3 Variable triggers
- 4 References
- 5 External links
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 action, where action refers to the mechanism (the trigger, hammer, and safeties considered as a unit) or to the logic of how it is built and how it is used. 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 (see progressive trigger), 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.
A single-action (SA) trigger performs the single action of releasing the hammer or striker to discharge the firearm each time the trigger is pulled. Almost all rifles and shotguns use this type of trigger. Single-action revolvers such as the Colt Dragoon Revolver require the hammer to be cocked by hand every time the weapon is fired. Single-action semi-automatic pistols, such as the M1911, Springfield Armory XD and the Browning Hi-Power, require that the hammer or striker be cocked before the first round can be fired, although most designs cock the hammer or striker as part of the loading process (e.g., the act of inserting the magazine and operating the slide mechanism to chamber the first round also cocks the hammer or striker into the ready-to-fire position). Once the first round is fired, the automatic movement (recoil) of the slide cocks the hammer or striker for each subsequent shot. The pistol, once cocked, can be fired by pulling the trigger once for each shot until the magazine is empty.
Double-action only 
A double-action, also known as double-action only (DAO) to prevent confusion with DA/SA designs, is similar to a DA revolver trigger mechanism. The trigger both cocks and releases the hammer or striker. However, there is no single-action function. A good example of this action is the SIG Sauer DAK trigger, or the DAO action of the Sig P250. For semi-automatic pistols with a traditional hammer (that employ only the double-action function of the trigger), the hammer will return to its decocked position after each shot. Subsequent shots require the double-action trigger firing sequence. 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, the Type 26 Revolver, and the Enfield No. 2 Mk I revolvers, in which there is no external hammer spur.
Traditional double-action or double-action/single-action 
A double-action/single-action (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 semi-automatic revolver that function on a DA/SA system. The Beretta 92 is a good example of a DA/SA semi-automatic pistol. On many DA/SA pistols (including the Beretta), 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.
A second distinct type is that used by the majority of double-action revolvers, where the weapon can be fired in either double-action mode by pulling the trigger, or single-action mode by cocking the hammer manually before firing. This is distinct from double-action only, since the weapon does not have to be fired in double-action mode, for example, the Colt Python.
A release trigger releases the hammer or striker when the trigger is released by the shooter, rather than when it is pulled. Release triggers are largely used on shotguns intended for trap and skeet shooting.
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 compared to having a conventional, very light trigger. There are two types: Single Set and Double Set.
Single set trigger
A Single Set Trigger is usually one trigger that may be fired with a conventional amount of trigger pull weight or may be 'set' – usually by pushing forward on the trigger, or by pushing forward on a small lever attached to the rear of the trigger. This takes up the creep in the trigger and allows for 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 set triggers can be further classified by phase. A double set, single phase trigger can only be operated by first pulling the set trigger, and then pulling the firing trigger. A double set, double phase trigger can be operated as a standard trigger if the set trigger is not pulled, or as a set trigger by first pulling the set trigger. Double set, double phase triggers offer the versatility of both a standard trigger and a set trigger.
Pre-set (striker or hammer)
Pre-set strikers and hammers 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. It differs from single-action in that if the striker or hammer were to release, it would generally not be capable of igniting the primer. Examples of pre-set strikers are the Glock, Smith & Wesson M&P, Springfield Armory XDS (only), Kahr Arms, FN FNS series and Ruger SR series pistols. This type of trigger mechanism is sometimes referred to as a Striker Fired Action or SFA. Examples of pre-set hammers are the Kel-Tec P-32 and Ruger LCP pistols.
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 double-action only (DAO) until the cartridge discharges or the malfunction is cleared. This allows the operator to attempt to fire a cartridge after a misfire malfunction. The Taurus PT 24/7 Pro pistol (not to be confused with the first-generation 24/7 which was a traditional pre-set) offered this feature starting in 2006. The Walther P99 Anti-Stress is another example.
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 single-action revolver, for which 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 made ready to fire by the process of chambering a round, and as a result an external safety is sometimes employed.
Double-action triggers provide the ability to fire the gun 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.
DA/SA pistols are versatile mechanisms. These firearms generally have a manual safety that additionally may serve to decock the hammer. Some have a facility (generally a lever or button) to safely lower the hammer. As a disadvantage, these controls are often intermingled with other controls such as slide releases, magazine releases, take-down levers, takedown lever lock buttons, loaded chamber indicators, barrel tip-up levers, etc. These variables become confusing and require more complicated manuals-of-arms. One other disadvantage is the difference between the first double-action pull and subsequent single-action pulls.
DAO firearms resolve some DA/SA shortcomings by making every shot a double-action shot. Because there is no difference in pull weights, training and practice are simplified. Additionally, negligent discharges are mitigated due to a heavier trigger pull. 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 drawback is that additional trigger pull weight and travel required for each shot reduce accuracy.
Pre-set triggers, only recently coming into vogue, offer a balance of pull weight, trigger travel, safety, and consistency. Glock popularized this trigger in modern pistols and many other manufacturers have released pre-set striker products of their own. The primary disadvantage 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. Clearing a malfunction also usually involves retracting the slide following the "tap rack bang" procedure. Many similar approaches are argued for generally accomplishing the same end.
A double-crescent trigger provides select fire capability without the need for a fire mode selector switch. Pressing the upper segment of the trigger produced semi-automatic fire, while holding the lower segment of the trigger produced fully automatic fire. Though considered innovative at the time, the feature was eliminated on most firearms due to its complexity. Examples include MG 34, Kg m/40 light machine gun, M1946 Sieg automatic rifle, and Star Model Z-70.
A progressive trigger allows different firing rates based on how far it is depressed. For example, when pulled lightly, the weapon will fire a single shot. When depressed further, the weapon fires at a fully automatic rate. Examples include Jatimatic, CZ Model 25, PM-63, and Steyr AUG.
A staged trigger is a trigger that, depending how far the trigger is pulled fires so many rounds, for instance the weapon fires single shots when pulled half way, the weapon fires full automatic when pulled completely back. Examples include BXP, F1 submachine gun, Steyr AUG, Vigneron submachine gun, and Wimmersperg Spz-kr.