Trigger (firearms)

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 Safe Action 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 with a force gauge in newtons in SI units, or alternatively kilograms or grams in metric units, and pounds and/or ounces in US customary units. 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, slow projectiles 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 ==