Trigger ( firearms ) A trigger is a mechanism that actuates the firing sequence of a firearm . Triggers almost universally consist of lever s or buttons actuated by the index finger . Some variations use the thumb or weak fingers to actuate the trigger , a feature typically found on mounted weapons . Examples are the M2 Browning machine gun and the Springfield Armory M6 Scout . Function . 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 . Actions ( mechanisms ) . There are numerous types of action , which is the logic of how the mechanism is built and how it is used ( where the mechanism is the trigger , hammer , and safeties considered as a unit of interconnected parts ) . 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 . Single - action . 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 HS2000 ( Springfield XD and XDm ) , require that the hammer be cocked before the first round can be fired , although most designs cock the hammer 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 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 and Browning Hi - Power are single - action pistols that function in this manner . Double - action . 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 . 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 . Glock and Kahr semi - automatic pistols are not DA ( or DAO ) pistols because the striker is " cocked " to an intermediate position by the operation of the slide and they cannot be re - activated by pulling the trigger a second time . 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 functions 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 . Release trigger . 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 . 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 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 and Wesson M & P , Springfield Armory XDS ( only ) , Kahr Arms , and Ruger SR series pistols . Examples of pre - set hammers are the Kel - Tec P - 32 and Ruger LCP pistols . 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 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 . 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 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 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 . 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 exponentially 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 discharge s 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 generally accomplishing the same end .