Double-action Only

A trigger is a mechanism that actuates the function of a ranged weapon such as a firearm, airgun, crossbow, or speargun. The word may also be used to describe a switch that initiates the operation of other non-shooting devices such as a trap, a power tool or a quick release. A small amount of energy applied to the trigger leads to the release of much more energy.

Most triggers use a small flattened lever (called the ''trigger blade'') depressed by the index finger, 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 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, which allows for a spring-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 seated within the gun barrel chamber. 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, an intermediate mechanism to transmit the kinetic energy from the spring releasing, and a firing pin 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 like a mallet hitting a punch/chisel, which then relays the hammer impulse 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) that sparks to ignite the propellant powder within the cartridge case and thus discharges the projectile. The trapping interface between the trigger and the hammer/striker is typically 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.

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 trigger, Savage AccuTrigger) have an integral safety 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. 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.

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 inertially 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/ 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, 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 and repeating long arms (rifles, shotguns, submachine guns, machine guns, etc.) use this type of trigger.

The term "single-action" is a retronym 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 matchlocks, flintlocks, 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 relies on a rotating cylinder without using any reciprocating slide 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 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 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 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 on, or uncocked with an empty chamber (Colt M1911, 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 will reciprocate under recoil to automatically recock the hammer for a next shot, and is thus always cocked and ready unless the user manually ''