Double action, hammer trigger mechanism for a firearm by Moore, Wildey J.

Trigger actions (not to be confused with the operating mechanism which loads and unloads the firearm) are classes that categorise triggers according to how many functions they perform when actuated. They are different from firearm actions which describe the methods by which an ammunition cartridge is loaded into and ejected from a gun’s chamber (known as “cycling”).

To understand this, one should look at the fundamental principle behind how repeating firearms work. In any repeating firearm (manual- or automatic / self-loading), the round of ammunition is held in the chamber by a breechblock; rotary cylinder in a revolver, or a bolt in an auto-loading firearm. The bolt is the part of the bolt carrier group which manipulates the cartridge; it strips a round from the feed device and feeds it into the chamber, then secures the cartridge in place as the hammer strikes the firing pin, which then hits the primer located at the base of the cartridge case.

The primer in a centre-fire cartridge is a small metallic cup, usually 4.5-8mm in diameter, containing an impact-sensitive chemical compound (potassium chlorate in contemporary times, preceded by the less reliable mercury fulminate). Upon impact, the reactive compound in the primer produces sparks within the loaded case, igniting the heat-sensitive propellant (gunpowder or its modern successor, smokeless powder). This in turn produces rapidly expanding, high-pressure gases (mainly carbon dioxide and nitrogen) that propel the projectile down the barrel, out the muzzle, and towards its target at high velocity (usually supersonic at the muzzle; above 343m/s in air at 20°C).

The force of the exiting projectile(s), along with the propellant gases, creates an equal and opposing force which is transmitted from the cartridge case in the firing chamber, to the reciprocating bolt and the firearm itself with respect to Newton’s Third Law of Motion, which is dampened as the energy travels through these parts, and finally being felt by the shooter as recoil. In a manually-operated bolt-action firearm, the bolt is turned by hand to unlock, then pulled to the rear to extract the fired cartridge as the ejector in the bolt kicks the empty case out of the breech. Whereas in auto-loading firearms, this action is performed via recoil impulse or by harnessing the expansion of propellant gases. With this operating principle in mind, one can then look at triggers and how they fit into the process.

In most long arms (rifle, shotgun, submachine gun etc..), the firing pin is held within the bolt as part of the bolt carrier group, and is responsible for setting off the combustion and propelling sequence with the aid of a spring-loaded hammer. In auto-loading or semi-automatic handguns (commonly referred to as “pistols”), the firing pin is held in the slide, which is a reciprocating breech-block acting as the bolt and charging mechanism. In revolvers, the firing pin is positioned in the frame directly behind the chamber that is aligned with the barrel.

There are two distinct types of firing mechanisms present in repeating firearms: Hammer and Striker. Pistols can either be hammer- or striker-fired; revolvers are typically hammer-fired only, while some designs have an internal hammer within the frame itself. Most long arms are hammer-fired, with the exception of bolt-action rifles and a few auto-loading rifles like the SKS semi-automatic carbine (for which the 7.62x39mm cartridge was designed). Within the hammer-fired category of handguns (which make up the majority), there are three different classes of triggers: Single-Action, Double-Action Only, and Double-Action/Single-Action.

1) Single-Action (SA):
This is the earliest and simplest trigger type, found in the first repeating firearms, like the Colt Single-Action Army M1873 .45-calibre Long Colt (11.5x33mmR) revolver for example. SA triggers execute the sole function of releasing spring tension in the hammer to hit the firing pin. The hammer must be cocked (drawn back and held under tension) before this can happen. Cocking can be done physically by pulling the hammer back with a thumb, as on a revolver, or mechanically via the bolt or slide driving it back into cocking position when the auto-loading firearm is charged or subsequently cycled. SA triggers have the lowest resistance, travel (including take-up / slack and over-travel) and reset lengths. Apart from SA handguns, long arms use this type of trigger as the hammer is always re-cocked after cycling when the sear re-engages. In a revolver, cocking the hammer engages a gear which simultaneously rotates the cylinder clockwise by one chamber.
Examples of Single-Action trigger handguns include: Browning Hi-Power (P35 in Belgian military service) 9x19mm-calibre pistol, Colt M1911 .45ACP-calibre (11.43x23mm) pistol and derivatives, and the FN Five-seveN 5.7x28mm-calibre pistol.

2) Double-Action Only (DAO):
DAO triggers perform the dual job of cocking and releasing the hammer. Unlike SA handguns, the hammer remains at rest and can only be cocked by depressing the trigger. As a result, DAO pistols do not feature a decocking lever (or simply “decocker”), which serves to safely lower the hammer into resting position without indenting the firing pin. DAO triggers have the longest trigger travel lengths, and also have the highest resistance. DAO triggers enable the hammer to perform what is known as a “double strike”, re-engaging the sear to re-cock and release the hammer on an unsuccessfully fired round. This allows the shooter to reinitiate a firing sequence without having to eject the chambered cartridge by racking the slide. DAO triggers also act as a safety precaution against negligent discharges (ND) or unintentional firing of the handgun, due to the longer and heavier trigger pull necessitating the application of deliberate pressure. These characteristics make DAO handguns well-suited for close range self-defence or backup weapons, but do not lend themselves well for accurate fire at longer ranges as compared to their SA counterparts.
Examples of Double-Action Only trigger handguns include: Smith & Wesson M-37 .38Spl-calibre (9x29mmR) revolver, Beretta Px4 Model D pistol, and SIG Sauer Double-Action Kellerman (DAK) P2XX series of pistols.

3) Double-Action/Single-Action (DA/SA):
Exclusive to auto-loading pistols and some contemporary revolvers, DA/SA triggers are a combination of the two types mentioned above. As the name suggests, DA/SA triggers feature a long and heavy trigger pull for the initial shot, followed by a short and light trigger pull after each cycle. This type of trigger functions by locking the hammer in the cocked position as the slide recoils backwards, readying the hammer to fall when the slide feeds a fresh round into battery (fully forward against the firearm’s chamber). Due to such a configuration, the long and heavy DA trigger mode can be forgone by manually cocking the hammer with a thumb or the pistol’s slide. DA/SA pistols feature a decocker, usually integrated into the safety lever (such as the one found on a Heckler & Koch USP where the shooter can push the safety lever downwards beyond the “Safe” position to decock the hammer).
Examples of Double-Action/Single-Action trigger handguns include: Beretta 92 (M9 in U.S. military service) 9x19mm-calibre pistol, CZ-75 9x19mm-calibre pistol, and the Smith & Wesson M-1006 10x25mm-calibre pistol.

Striker-fired pistols feature a spring-loaded firing pin in place of a hammer, which is held in a half- or partial-cocked position (50% or 75-90% cocked) within the slide. These triggers are similar in function to DAO triggers, in that they fully cock and release the firing mechanism. Striker-fired pistols differ from DAO pistols by having shorter and lighter trigger pulls, as well as internal safety mechanisms in lieu of external safeties. One such internal safety is a firing pin block (also called a “drop safety”), which prevents unintended contact between the firing pin and the cartridge primer unless the trigger is depressed, especially when the firearm is dropped on a hard surface. Upon actuating the trigger (which typically also incorporates a safety to be simultaneously depressed by the finger), the firing pin block moves out of the striker’s path for it to fully cock and strike the primer.
Examples of Striker-fired pistols: Glock, Smith & Wesson Military & Police (M&P), and Steyr M series of pistols.

General characteristics for each type of trigger action:

– Trigger Pull Weight/Resistance: 3.5-6.5lb
– Trigger Travel: 8-14mm
– Reset Length: 4-8mm
– External Safety: Yes for all handguns
– Internal Safety: Varies by handgun
– Decocker: Varies by handgun

– Trigger Pull Weight/Resistance: 5.5-13lb
– Trigger Travel: 12-25mm
– Reset Length: 6-12mm
– External Safety: Yes for semi-automatic handguns
– Internal Safety: Yes for all handguns
– Decocker: No

– Trigger Pull Weight/Resistance: 5.5-13lb DA, 5-6.5lb SA
– Trigger Travel: 8-25mm
– Reset Length: 4-10mm
– External Safety: Yes for semi-automatic handguns
– Internal Safety: Varies by handgun
– Decocker: Yes

– Trigger Pull Weight/Resistance: 5-6.5lb
– Trigger Travel: 8-20mm
– Reset Length: 4-10mm
– External Safety: No
– Internal Safety: Yes
– Decocker: No