![Thompson submachine gun Firecontrols](https://upload.wikimedia.org/wikipedia/commons/5/5c/Thompson_submachine_gun_Firecontrols.jpg)
A trigger is a
mechanism
Mechanism may refer to:
* Mechanism (engineering), rigid bodies connected by joints in order to accomplish a desired force and/or motion transmission
*Mechanism (biology), explaining how a feature is created
*Mechanism (philosophy), a theory that ...
that actuates the function of a
ranged weapon such as a
firearm,
airgun
An air gun or airgun is a gun that fires projectiles pneumatically with compressed air or other gases that are mechanically pressurized ''without'' involving any chemical reactions, in contrast to a firearm, which pressurizes gases ''chem ...
,
crossbow
A crossbow is a ranged weapon using an elastic launching device consisting of a bow-like assembly called a ''prod'', mounted horizontally on a main frame called a ''tiller'', which is hand-held in a similar fashion to the stock of a long fire ...
, or
speargun
A speargun is a ranged underwater fishing device designed to launch a tethered spear or harpoon to impale fish or other marine animals and targets. Spearguns are used in sport fishing and underwater target shooting. The two basic types are ''pn ...
. The word may also be used to describe a
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
that initiates the operation of other non-shooting devices such as a
trap
A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research.
Trap or TRAP may also refer to:
Art and entertainment Films and television
* ''Trap'' (2015 film), Fil ...
, a
power tool
A power tool is a tool that is actuated by an additional power source and mechanism other than the solely manual labor used with hand tools. The most common types of power tools use electric motors. Internal combustion engines and compressed ...
or a
quick release
A quick release skewer is a mechanism for attaching a wheel to a bicycle. It consists of a rod threaded on one end and with a lever operated cam assembly on the other. The rod is inserted into the hollow axle of the wheel, a special nut is t ...
. A small amount of energy applied to the trigger leads to the release of much more energy.
Most triggers use a small flattened
lever
A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is d ...
(called the ''trigger blade'') depressed by the
index finger
The index finger (also referred to as forefinger, first finger, second finger, pointer finger, trigger finger, digitus secundus, digitus II, and many other terms) is the second digit of a human hand. It is located between the thumb and the mid ...
, but some weapons such as the
M2 Browning machine gun or the Iron Horse TOR ("thumb-operated receiver") use a
push-button-like thumb-actuated trigger design, and others like the
Springfield Armory M6 Scout
The Springfield Armory M6 Scout is a .22 Hornet over .410 bore combination gun that is virtually identical to the original M6 Aircrew Survival Weapon. It is also made in .22 Long Rifle over .410 bore and .22 Magnum over .410 bore. The Scouts are ...
use a squeeze-bar trigger similar to the "ticklers" on
medieval European crossbows. Although the word "trigger" technically implies the entire mechanism (known as the ''trigger group''), colloquially it is usually used to refer specifically to the trigger blade.
Most firearm triggers are "single-action", meaning that the trigger is designed only for the single function of disengaging the
sear
The Sahar Elevated Access Road, abbreviated to SEAR, is a dedicated, elevated, express access road in Mumbai that connects the Western Express Highway (WEH) near Hanuman Nagar junction in Vile Parle, with the forecourts of Terminal T2 of the C ...
, which allows for a
spring
Spring(s) may refer to:
Common uses
* Spring (season)
Spring, also known as springtime, is one of the four temperate seasons, succeeding winter and preceding summer. There are various technical definitions of spring, but local usage of ...
-tensioned
hammer/
striker to be released. In "double-action" firearm designs, the trigger also performs the additional function of
cocking the hammer – and there are many designs where the trigger is used for a range of other functions.
Function
Firearms use triggers to initiate the firing of a
cartridge
Cartridge may refer to:
Objects
* Cartridge (firearms), a type of modern ammunition
* ROM cartridge, a removable component in an electronic device
* Cartridge (respirator), a type of filter used in respirators
Other uses
* Cartridge (surname), a ...
seated within the
gun barrel
A gun barrel is a crucial part of gun-type weapons such as small firearms, artillery pieces, and air guns. It is the straight shooting tube, usually made of rigid high-strength metal, through which a contained rapid expansion of high-pressu ...
chamber
Chamber or the chamber may refer to:
In government and organizations
* Chamber of commerce, an organization of business owners to promote commercial interests
*Legislative chamber, in politics
* Debate chamber, the space or room that houses delib ...
. This is accomplished by actuating a striking device through a combination of
mainspring (which stores
elastic energy), a trap mechanism that can hold the spring under
tension
Tension may refer to:
Science
* Psychological stress
* Tension (physics), a force related to the stretching of an object (the opposite of compression)
* Tension (geology), a stress which stretches rocks in two opposite directions
* Voltage or el ...
, an intermediate mechanism to transmit the
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
from the spring releasing, and a
firing pin
A firing pin or striker is a part of the firing mechanism of a firearm that impacts the primer in the base of a cartridge and causes it to fire. In firearms terminology, a striker is a particular type of firing pin where a compressed spring ...
to eventually strike and ignite the primer. There are two primary types of striking mechanisms – ''
hammer'' and ''
striker''. A ''hammer'' is a pivoting metallic component subjected to spring tension so when released will swing forward to strike a
firing pin
A firing pin or striker is a part of the firing mechanism of a firearm that impacts the primer in the base of a cartridge and causes it to fire. In firearms terminology, a striker is a particular type of firing pin where a compressed spring ...
like a
mallet hitting a
punch
Punch commonly refers to:
* Punch (combat), a strike made using the hand closed into a fist
* Punch (drink), a wide assortment of drinks, non-alcoholic or alcoholic, generally containing fruit or fruit juice
Punch may also refer to:
Places
* Pun ...
/
chisel, which then relays the hammer
impulse
Impulse or Impulsive may refer to:
Science
* Impulse (physics), in mechanics, the change of momentum of an object; the integral of a force with respect to time
* Impulse noise (disambiguation)
* Specific impulse, the change in momentum per uni ...
by moving forward rapidly along its longitudinal axis. A ''striker'' is essentially a firing pin directly loaded to a spring, eliminating the need to be struck by a separate hammer. The firing pin/striker then collides into the cartridge
primer positioned ahead of it, which contains
shock-sensitive compounds (e.g.,
lead styphnate
Lead styphnate (lead 2,4,6- trinitroresorcinate, C6HN3O8Pb ), whose name is derived from styphnic acid, is an explosive used as a component in primer and detonator mixtures for less sensitive secondary explosives. Lead styphnate is only slightly ...
) that
spark
Spark commonly refers to:
* Spark (fire), a small glowing particle or ember
* Electric spark, a form of electrical discharge
Spark may also refer to:
Places
* Spark Point, a rocky point in the South Shetland Islands
People
* Spark (surname)
* ...
s to
ignite the
propellant powder
A powder is a dry, bulk solid composed of many very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms ''powder'' and '' granular'' are sometimes used to distin ...
within the
cartridge case
A cartridge or a round is a type of pre-assembled firearm ammunition packaging a projectile ( bullet, shot, or slug), a propellant substance (usually either smokeless powder or black powder) and an ignition device ( primer) within a metal ...
and thus discharges the
projectile. The trapping interface between the trigger and the hammer/striker is typically referred to as the ''
sear
The Sahar Elevated Access Road, abbreviated to SEAR, is a dedicated, elevated, express access road in Mumbai that connects the Western Express Highway (WEH) near Hanuman Nagar junction in Vile Parle, with the forecourts of Terminal T2 of the C ...
'' surface. Variable mechanisms will have this surface directly on the trigger and hammer or have separate sears or other connecting parts.
Stages of a trigger pull
The trigger pull can be divided into three mechanical stages:
# Takeup or pretravel: The movement of the trigger ''before'' the sear moves.
# Break: The movement of the trigger ''during'' the sear's movement up to the point of release, where the felt resistance suddenly decreases.
# Overtravel: The movement of the trigger ''after'' the sear has already released
Takeup
When considering the practical accuracy of a firearm, the trigger ''takeup'' is often considered the least critical stage of the trigger pull. Often triggers are classified as either single-stage or two-stage based on the takeup.
* Single-stage triggers have no discernible resistance during the entire takeup, and only encounter resistance at the very end of takeup (often described as encountering "the wall") when actually actuating the trigger break.
* Two-stage triggers have a noticeable but relatively light resistance during the takeup, followed by a distinctly increased resistance of the actual trigger break. This allows the shooter to gradually "ease into" the trigger pull instead of hitting the trigger blade with a hasty hard squeeze.
A single-stage trigger is often called ''direct trigger'', and is popular on hunting rifles. A two-stage trigger is often called ''pressure trigger'', and is popular on competition rifles.
Some fully adjustable triggers can be adjusted to function as either a single-stage or two-stage trigger by adjusting the takeup. Setting the takeup travel (also known as the first stage) to near zero essentially makes the trigger a single-stage trigger. Some single-stage triggers (e.g.
Glock
Glock is a brand of polymer-Receiver (firearms), framed, Recoil operation#Short recoil operation, short recoil-operated, locked-breech semi-automatic pistols designed and produced by Austrian manufacturer Glock Ges.m.b.H., Glock Ges.m.b.H. The ...
trigger,
Savage AccuTrigger) have an integral
safety
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
with a noticeable spring resistance that can functionally mimic a two-stage trigger.
Break
The trigger ''break'' is named for the sudden loss of resistance when the sear reaches the point of release, which is described as resembling the breaking of rigid materials when the strength fails under
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
. The actuation force required to overcome the sear resistance during the break is known as the ''trigger weight'', which is usually measured in
pounds with a
force gauge
A force gauge (also called a force meter) is a measuring instrument used to measure forces. Applications exist in research and development, laboratory, quality, production and field environment. There are two kinds of force gauges today: mechanical ...
.
The break is often considered the most critical stage of the trigger pull for achieving good practical accuracy, since it happens just prior to the shot being discharged and can cause some unwanted shakes from the shooter's hand at the instant of firing. Shooter preferences vary; some prefer a soft break with a smooth but discernible amount of trigger travel during firing, while others prefer a crisp break with a heavier weight and little or no discernible movement. A perceivably slow trigger break is often referred to as a "creep", and frequently described as an unfavorable feature.
Overtravel
The trigger ''overtravel'' happens immediately after the break and is typically a short distance, and can be considered an
inertia
Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
lly accelerated motion caused by the residual push of the finger coupled with the sudden decrease in resistance after the trigger break. It can be a very critical factor for accuracy because shaking movements during this phase may precede the
projectile leaving the
barrel, and is especially important with firearms with long barrels and heavy trigger weights, where the more significant resistance drop can make the trigger finger overshoot and shake in an uncontrolled fashion. Having some overtravel provides a "buffer zone" that prevents the shooter from "jerking the trigger", allowing the remnant pressing force from the finger to be dampened via a "follow-through" motion. Although a perceivable overtravel can be felt as adding to the "creep" of the trigger break, it is not always considered a bad thing by some shooters. An ''overtravel stop'' will arrest the motion of the trigger blade, and prevent excessive movement.
Reset
When user releases the trigger and it travels to its resting position. On semiauto firearms the movement eventually passes by reset position where trigger-disconnector mechanism resets itself to its resting state, in which pulling the trigger releases the sear. The reset event does not occur in double action firearms and in full auto firearms.
Types
There are numerous types of trigger designs, typically categorized according to which functions the trigger is tasked to perform,
a.k.a. the trigger action (not to be confused with the
action of the whole firearm, which refers to ''all'' the components that help handle the cartridge, including the
magazine,
bolt,
hammer and
firing pin
A firing pin or striker is a part of the firing mechanism of a firearm that impacts the primer in the base of a cartridge and causes it to fire. In firearms terminology, a striker is a particular type of firing pin where a compressed spring ...
/
striker,
extractor and ejector in addition to the trigger). While a trigger is primarily designed to set off a shot by releasing the hammer/striker, it may also perform additional functions such as cocking (loading against a spring) the hammer/striker, rotating a
revolver's
cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
, deactivating
internal safeties,
transitioning between different firing modes (see
progressive trigger), or reducing the pull weight (see
set trigger).
Single-action
A single-action (SA) trigger is the earliest and mechanically simplest of trigger types. It is called "single-action" because it performs the ''single'' function of releasing the hammer/striker (and nothing else), while the hammer/striker must be cocked by separate means.
Almost all
single-shot
Single-shot firearms are firearms that hold only a single round of ammunition, and must be reloaded manually after every shot. The history of firearms began with single-shot designs, then multi-barreled designs appeared, and eventually many cent ...
and
repeating long arms (rifles,
shotguns,
submachine guns, machine guns, etc.) use this type of trigger.
[
The term "single-action" is a ]retronym
A retronym is a newer name for an existing thing that helps differentiate the original form/version from a more recent one. It is thus a word or phrase created to avoid confusion between older and newer types, whereas previously (before there were ...
that was not in use until weapons with double-action triggers were invented in the mid-19th century; before that, all triggers were single-action (for example, all matchlock
A matchlock or firelock is a historical type of firearm wherein the gunpowder is ignited by a burning piece of rope that is touched to the gunpowder by a mechanism that the musketeer activates by pulling a lever or trigger with his finger. Befor ...
s, flintlock
Flintlock is a general term for any firearm that uses a flint-striking ignition mechanism, the first of which appeared in Western Europe in the early 16th century. The term may also apply to a particular form of the mechanism itself, also know ...
s, muskets, etc.). While originally all hammers required a separate hand motion to cock manually, with the birth of repeating rifles such as the Henry rifle, it was found to be easy to design the cocking phase of the hammer into the cycling of the bolt. Although these weapons do not require the user to physically manipulate the hammer in addition to squeezing the trigger, they are still single-actions because the cocking is not performed by the trigger mechanism, which is only in charge of releasing the sear.
In modern usage, the terms "single-action" and "double-action" almost always refer to handguns, as few (if any) long guns feature double-action triggers. In handguns, genuinely single-action triggers with external hammers now are only seen on revolvers, while mostly all semi-automatic pistols that support single-action firing are actually DA/SA hybrids, which is technically a two-stage double-action trigger with the cocking (first) stage being separately operable (hence can be "skipped" by the trigger). The longevity of purely single-action triggers on revolvers is mainly due to the gun's design layout – the revolver action
Action may refer to:
* Action (narrative), a literary mode
* Action fiction, a type of genre fiction
* Action game, a genre of video game
Film
* Action film, a genre of film
* ''Action'' (1921 film), a film by John Ford
* ''Action'' (1980 fil ...
relies on a rotating cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
without using any reciprocating slide
Slide or Slides may refer to:
Places
* Slide, California, former name of Fortuna, California
Arts, entertainment, and media Music Albums
* ''Slide'' (Lisa Germano album), 1998
* ''Slide'' (George Clanton album), 2018
*''Slide'', by Patrick Glees ...
and bolt that can prohibit the shooter placing the fingers near the hammer for easy access, and it is not uncommon to see revolver shooters " fanning" the hammer to perform rapid single-action firing. While a "single-action" revolver or semi-automatic pistol must always be cocked prior to firing (either manually or by the operation of the weapon), most "double-action" handguns are capable of firing in both single- and double-action modes. Only "double-action only" weapons are incapable of firing from a cocked hammer.
The "classic" single-action revolver of the mid-to-late 19th century includes black powder caplock
The percussion cap or percussion primer, introduced in the early 1820s, is a type of single-use percussion ignition device for muzzle loader firearm locks enabling them to fire reliably in any weather condition. This crucial invention gave rise ...
muzzleloaders such as the Colt 1860 "Army" Model, and Colt 1851 "Navy" Model, and European models like the LeMat, as well as early metallic cartridge
A cartridge or a round is a type of pre-assembled firearm ammunition packaging a projectile (bullet, shot, or slug), a propellant substance (usually either smokeless powder or black powder) and an ignition device (primer) within a metallic, p ...
revolvers such as the Colt Model 1873 "Single Action Army" (named for its trigger mechanism) and Smith & Wesson Model 3, all of which required a thumb to cock the hammer before firing. Single-action triggers with manually cocked external hammers lasted a while longer in some break-action shotguns and in dangerous game rifles, where the hunter did not want to rely on an unnecessarily complex or fragile weapon. While single-action revolvers never lost favor in the US right up until the birth of the semi-automatic pistol, double-action revolvers such as the Beaumont–Adams were designed in Europe before the American Civil War
The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States. It was fought between the Union ("the North") and the Confederacy ("the South"), the latter formed by states ...
broke out, and saw great popularity all through the latter half of the 19th century, with certain numbers being sold in the US as well.
While many European and some American revolvers were designed as double-action models throughout the late 19th century, for the first half of the 20th century, all semi-automatic pistols were single-action weapons, requiring the weapon to be carried cocked and loaded with the safety
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
on, or uncocked with an empty chamber
Chamber or the chamber may refer to:
In government and organizations
* Chamber of commerce, an organization of business owners to promote commercial interests
*Legislative chamber, in politics
* Debate chamber, the space or room that houses delib ...
(Colt M1911
The M1911 (Colt 1911 or Colt Government) is a single-action, recoil-operated, semi-automatic pistol chambered for the .45 ACP cartridge. The pistol's formal U.S. military designation as of 1940 was ''Automatic Pistol, Caliber .45, M1911'' for th ...
, Mauser C96, Luger P.08, Tokarev TT, Browning Hi-Power). The difference between these weapons and single-action revolvers is that while a single-action revolver requires the user to manually cock the hammer before each firing, a single-action semi-automatic pistol only requires manual cocking for the first shot, after which the slide
Slide or Slides may refer to:
Places
* Slide, California, former name of Fortuna, California
Arts, entertainment, and media Music Albums
* ''Slide'' (Lisa Germano album), 1998
* ''Slide'' (George Clanton album), 2018
*''Slide'', by Patrick Glees ...
will reciprocate under recoil
Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force r ...
to automatically recock the hammer for a next shot, and is thus always cocked and ready unless the user manually '' decocks'' the hammer, encounters a misfiring cartridge, or pulls the trigger on an empty chamber (for older weapons lacking "last round bolt hold open" feature).
In the late 1930s and early 1940s, Walther
Walther is a masculine given name and a surname. It is a German form of Walter, which is derived from the Old High German ''Walthari'', containing the elements ''wald'' -"power", "brightness" or "forest" and ''hari'' -"warrior".
The name was fir ...
introduced the first "double-action" (actually DA/SA hybrid) semi-automatic pistols, the PPK and P.38 models, which featured a revolver-style double-action trigger, allowing the weapon to be carried with a round chambered and the hammer lowered. After the first shot, they would fire subsequent shots like a single-action pistol. These pistols rapidly gained popularity, and the traditional single-action-only pistols rapidly lost favor, although they still retain a dedicated following among enthusiasts. Today, a typical revolver or semi-automatic pistol is a DA/SA one, carried in double-action mode but firing most of its shots in single-action mode.
Double-action only
A double-action trigger, also known as double-action only (DAO) to prevent confusion with the more common hybrid DA/SA designs, is a trigger that must perform the ''double'' function of ''both'' cocking and releasing the hammer/striker. Such trigger design either has no internal sear mechanism capable of holding the hammer/striker in a still position (so cocking and releasing have to happen in one uninterrupted sequence), or has the whole hammer shrouded and/or with the thumb spur machined off, preventing the user from manipulating it separately.
This design requires a trigger pull to both cock and trip the hammer/striker for every single shot, unlike a DA/SA, which only requires a double-action trigger pull for the first shot (or a typical DA/SA revolver, which can fire single action any time the user wishes, but uses double-action as a default). This means that there is no single-action function for any shot, and the hammer or striker always rests in the down position until the trigger pull begins. With semi-automatics, this means that unlike DA/SA weapons, the hammer does not remain cocked after the first round is fired, and every shot is in double-action mode. With revolvers, this means that one does not have the option of cocking the gun before shooting, and must always shoot it in double action mode.
Although there have been revolvers that were designed with trigger mechanisms totally lacking a single-action mechanism altogether, more commonly DAO revolvers are modifications of existing DA/SA models, with identical internals, only with access to the hammer prevented, either by covering it with a shroud or by removing the thumb spur. In both cases, the goal is to prevent the possible snagging of the hammer spur on clothing or holster. Due to the imposed limitation in accuracy, the majority of DAO revolvers have been short-barrel, close-range "snub" weapons, where rapidity of draw is essential and limited accuracy is already an acceptable compromise.
The purpose of a DAO action in a semi-automatic is mostly to avoid the change in trigger pull between the first and subsequent shots that one experiences in a DA/SA pistol, while avoiding the perceived danger of carrying a cocked single-action handgun, although it also avoids having to carry a cocked and loaded pistol, or having to lower the hammer on a loaded chamber, if one only fires a partial magazine. A good example of this action in a semi-automatic is the SIG Sauer DAK trigger, or the DAO action of the Sig P250. 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 most often to semi-automatic handguns; however, the term can also apply to some revolvers such as the Smith & Wesson Centennial
S&W Centennial is a family of revolvers made by Smith & Wesson on the "J-Frame". Depending upon caliber, the cylinder holds either 5 (9x19mm Parabellum, .38 Special +P, and .357 Magnum), 6 (.32 H&R Magnum), 7 (.22 Magnum), or 8 (.22 LR) cartridge ...
, the Type 26 Revolver
was the first modern revolver adopted by the Imperial Japanese Army. It was developed at the Koishikawa Arsenal and is named for its year of adoption in the Japanese dating system (the 26th year of the Meiji era, i.e., 1893). The revolver saw a ...
, and the Enfield No. 2 Mk I revolvers, in which there is no external hammer spur, or which simply lack the internal sear mechanism capable of holding the hammer in the cocked position.
Double-action/single-action
A double-action/single-action (DA/SA) trigger is a hybrid design combining the features of both single- and double-action mechanisms. It is also known as traditional double-action (TDA), as the vast majority of modern "double-action" handguns (both revolvers and semi-automatic pistols) use this type of triggers instead of "double-action only" (DAO).
In simple terms, "double-action" refers to a trigger mechanism that both cocks the hammer and then releases the sear, thus performing two "acts", although it is supposed to describe doing both strictly with one trigger pull only. However, in practice most double-action guns feature the optional ability to cock the hammer separately, reducing the trigger to perform just one action. This is opposed to "double-action only" firearms, which completely lack the capability to fire in single-action mode.
In a DA/SA trigger, the mechanism is designed with an internal sear that allows the trigger to both cock and release the hammer/striker when fully pulled, or to merely lock the hammer/striker in the cocked position when it is pulled to the rear and the trigger is not depressed. In a revolver, this means that simply squeezing the trigger when the hammer is lowered will both cock and release it. If the user uses their thumb to pull the hammer to the back, but does not press the trigger, the mechanism will lock the hammer in the cocked position until the trigger is pressed, just like a single action. Firing in double-action mode allows a quicker initiation of fire, but compromised by having a longer, heavier trigger pull, which can affect accuracy compared to the lighter, shorter trigger pull of a single-action fire.
In a DA/SA semi-automatic pistol, the trigger mechanism functions identically to that of a DA revolver. However, this is combined with the ability of the pistol slide
The slide on the majority of fully/semi-automatic pistols is the upper part that reciprocates ("slides") with recoil during the gun's operating cycle. It serves as the bolt carrier group (BCG) and partly as the receiver, and generally houses ...
to automatically cock the hammer when firing. Thus, the weapon can be carried with the hammer down on a loaded chamber, reducing perceived danger of carrying a single-action semi-automatic. When the user is ready to fire, simply pulling the trigger will cock and release the hammer in double action mode. When the weapon fires, the cycling slide automatically cocks the hammer to the rear, meaning that the rest of the shots fired will be in single-action mode, unless the hammer is manually lowered again. This gives the positive aspects of a single-action trigger without the need to carry "cocked and locked" (with a loaded chamber and cocked hammer), or with an empty chamber, which requires the user to chamber a round before firing.
A potential drawback of a DA/SA weapon is that the shooter must be comfortable dealing with two different trigger pulls: the longer, heavier DA first pull and the shorter, lighter subsequent SA pulls. The difference between these trigger pulls can affect the accuracy of the crucial first few shots in an emergency situation. Although there is little need for a safety on a DA/SA handgun when carrying it loaded with the hammer down, after the first shots are fired, the hammer will be cocked and the chamber loaded. Thus, most DA/SA guns either feature a conventional safety
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
that prevents the hammer from accidentally dropping, or a "decocker
Close-up shot of a safety of an M16A2 rifle
In firearms, a safety or safety catch is a mechanism used to help prevent the accidental discharge of a firearm, helping to ensure safer handling.
Safeties can generally be divided into subtypes suc ...
" – a lever that safely and gently drops the hammer (i.e. ''decocks the gun'') without fear of the gun firing. The latter is the more popular because, without a decocker, the user is forced to lower the hammer by hand onto a loaded chamber, with all of the attendant safety risks that involves, to return the gun to double-action mode. Revolvers almost never feature safeties, since they are traditionally carried un-cocked, and the hammer requires the user to physically cock it prior to every shot; unlike a DA/SA gun, which cocks itself every time the slide is cycled.
There are many examples of DA/SA semi-automatics, the Little Tom Pistol being the first, followed up by the Walther PPK
The Walther PP (german: Polizeipistole, or police pistol) series pistols are blowback-operated semi-automatic pistols, developed by the German arms manufacturer Carl Walther GmbH Sportwaffen.
It features an exposed hammer, a traditional double-a ...
and Walther P38
The Walther P38 (originally written Walther P.38) is a 9 mm semi-automatic pistol that was developed by Carl Walther GmbH as the service pistol of the Wehrmacht at the beginning of World War II. It was intended to replace the costly Luger P08 ...
. Modern examples include weapons such as the Beretta 92
The Beretta 92 (also Beretta 96 and Beretta 98) is a series of semi-automatic pistols designed and manufactured by Beretta of Italy. The Beretta 92 was designed in 1975, and production began in 1976. Many variants in several different calibers c ...
, among others. Almost all revolvers that are not specified as single-action models are capable of firing in both double- and single-action mode, for example, the Smith & Wesson Model 27
The Smith & Wesson Model 27 is a six-shot, double-action revolver chambered for the .357 Magnum cartridge and manufactured by the United States company Smith & Wesson. It was first produced in 1935, and many versions of it are still in productio ...
, S&W Model 60, the Colt Police Positive
The Colt Police Positive is a small- frame, double-action revolver featuring a six- round cylinder, chambered for either .32 or .38 caliber. A .22 caliber model was also offered. Designed primarily for sale to federal, state, or local law enforce ...
, Colt Python
The Colt Python is a .357 Magnum caliber revolver manufactured by Colt's Manufacturing Company of Hartford, Connecticut.Dougherty, Martin ''Small Arms: From the Civil War to the Present Day'', New York City: Fall River Press, 2005, page 48. It w ...
, etc. Early double-action revolvers included the Beaumont–Adams and Tranter black-powder muzzleloaders. There are some revolvers that can only be fired in double-action mode (DAO), but that is almost always due to existing double-action/single-action models being modified so that the hammer cannot be cocked manually, rather than from weapons designed that way from the factory.
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.
Binary trigger ("pull and release")
A binary trigger is a trigger for a semiautomatic firearm that drops the hammer both when the trigger is pulled, and also when it is released. Examples include the AR-15
An AR-15-style rifle is any lightweight semi-automatic rifle based on the Colt AR-15 design. The original ArmaLite AR-15 is a scaled-down derivative of Eugene Stoner's ArmaLite AR-10 design. The then Fairchild Engine and Airplane Corporatio ...
series of rifles, produced by Franklin Armory, Fostech Outdoors, and Liberty Gun Works. The AR-15 trigger as produced by Liberty Gun Works only functions in pull and release mode, and does not have a way to catch the hammer on release; while the other two have three-position safety selectors and a way to capture the hammer on release. In these triggers, the third position activates the pull and release mode, while the center selector position causes the trigger to only drop the hammer when pulled.
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. Set triggers are most likely to be seen on customized weapons and competition rifles where a light trigger pull is beneficial to accuracy.
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 trigger slack (or "take-up") in the trigger and allows for a much lighter trigger pull. This is colloquially known as a hair trigger.
Double set trigger
A double set trigger achieves the same result, but uses two triggers: one sets the trigger and the other fires the weapon. 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
Glock is a brand of polymer-Receiver (firearms), framed, Recoil operation#Short recoil operation, short recoil-operated, locked-breech semi-automatic pistols designed and produced by Austrian manufacturer Glock Ges.m.b.H., Glock Ges.m.b.H. The ...
, Smith & Wesson M&P
The Smith & Wesson M&P (Military and Police) is a polymer- framed, short recoil operated, locked breech semi-automatic pistol introduced in the summer of 2005 by the American company Smith & Wesson. It uses a Browning-type locking system. Whil ...
, Springfield Armory XD
The Springfield Armory XD is a series of semi-automatic pistols sold by Springfield Armory, Inc., in the United States along with follow-on variants: XD-M, XD-S, and XD-E. Polymer-framed and predominantly striker-fired, the series is manufact ...
-S variant (only), Kahr Arms, FN FNS
The FN FNS pistol is a series of striker-fired semi-automatic, polymer- framed pistols manufactured in Fredericksburg, Virginia, by FN America, a division of Fabrique Nationale Herstal. The pistol is chambered for the 9×19mm Parabellum and .40 ...
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
The Ruger LCP (Lightweight Compact Pistol) is a subcompact .380 ACP pocket pistol manufactured by Sturm, Ruger & Co. It was introduced at the 2008 SHOT Show.
Overview
With an overall length of and weighing only , this is Ruger's first entry int ...
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
A malfunction is a state in which something functions incorrectly or is obstructed from functioning at all.
Some types of malfunctions are:
* Malfunction (parachuting), malfunction of a parachute
* Sexual malfunction, also called "sexual dysfunc ...
is cleared. This allows the operator to attempt a second time to fire a cartridge after a misfire malfunction, as opposed to a single-action, in which the only thing to do if a round fails to fire is to rack the slide, clearing the round and recocking the hammer. While this can be advantageous in that many rounds will fire on being struck a second time, and it is faster to pull the trigger a second time than to cycle the action, if the round fails to fire on the second strike, the user will be forced to clear the round anyway, thus using up even more time than if they had simply done so in the first place. The Taurus PT 24/7
The Taurus PT 24/7 is a semi-automatic pistol using the short type of recoil operation, and available in various models with double-action-only (DAO) and double action/single-action-type trigger actions. Using a hammerless, striker-fired des ...
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
The Walther P99 () is a semi-automatic pistol developed by the German company Carl Walther GmbH Sportwaffen of Ulm for law enforcement, security forces and the civilian shooting market as a replacement for the Walther P5 and the P88. The P99 an ...
Anti-Stress is another example.
Variable triggers
Double-crescent trigger
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
The MG 34 (shortened from German: ''Maschinengewehr 34'', or "machine gun 34") is a German recoil-operated air-cooled general-purpose machine gun, first tested in 1929, introduced in 1934, and issued to units in 1936. It introduced an entirely n ...
, Kg m/40 light machine gun, M1946 Sieg automatic rifle, and Star Model Z-70.
Progressive/staged trigger
A progressive, or staged 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 FN P90
The FN P90 is a compact 5.7×28mm personal defense weapon designed and manufactured by FN Herstal in Belgium. It can also be considered a submachine gun. Created in response to NATO requests for a replacement for 9×19mm Parabellum firearms, th ...
, Jatimatic
The Jatimatic is a Finnish 9×19mm Parabellum submachine gun developed in the late 1970s and early 1980s by Jali Timari. The submachine gun made its debut in 1983. The Jatimatic was manufactured in very limited numbers (approx. 400) initially by ...
, CZ Model 25, PM-63, BXP, F1 submachine gun, Vigneron submachine gun
The Vigneron is a submachine gun manufactured in Belgium during the 1950s. It used the 9×19mm NATO cartridge and was used by the Belgian Army until the 1980s.
The Vigneron is a selective-fire weapon for short-range street and brush fighting. It ...
, Wimmersperg Spz-kr
The Wimmersperg Spz ("Sp" stands for STEN-pistole, "z" for zweiteilig. English: Sten pistol, made of two parts) was a family of German assault rifles that was in the planning stage during the latter days of Nazi Germany.
Overview
The overall w ...
, and Steyr AUG.
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
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
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, the heavier trigger pull can help to prevent a negligent discharge
An unintentional discharge is the event of a firearm discharging (firing) at a time not intended by the user. An unintended discharge may be produced by an incompatibility between firearm design and usage, such as the phenomenon of cooking off a ...
under stress. 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 the additional trigger pull weight and travel required for each shot reduces accuracy.
Pre-set triggers offer a balance of pull weight, trigger travel, safety, and consistency. Glock
Glock is a brand of polymer-Receiver (firearms), framed, Recoil operation#Short recoil operation, short recoil-operated, locked-breech semi-automatic pistols designed and produced by Austrian manufacturer Glock Ges.m.b.H., Glock Ges.m.b.H. The ...
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.
References
{{Firearms
Firearm components
Mechanisms (engineering)