To show how various factors influence trigger pull quality, I thought it might be useful to follow the process in reverse — we’ll start with the firing pin. The primer must be struck both fast and hard. The firing pin can be pinned directly to the hammer, it can be mounted in the frame or slide or it can be powered directly by the mainspring with no hammer involved, in which case it’s called a striker. How hard the primer is struck depends on the weight of the hammer/striker (H/S henceforth) and the strength of the mainspring. In any firearm, the mainspring is the spring that powers the H/S. Mainsprings can be leaf springs or more commonly, coil springs.
The H/S must first be drawn back to compress the mainspring. In single-action fire, either with revolver or semi-auto, the H/S is drawn back until a component called the sear holds it in fully cocked position. The sear can be integral with the trigger itself or it can be a separate component, as with the 1911. In either case, the sear holds the H/S back until pressure on the trigger moves it out of the way.
If we want to fire another shot, obviously the trigger has to move forward. In most semi-autos, the trigger is returned to its original position by a spring called, aptly enough, the trigger return spring. S&W revolvers have a component called the rebound slide, powered by a coil spring, to move the trigger forward.
This is the interior of a S&W 686 revolver. I’ve removed the hand so the hammer/sear/trigger
components can be seen. Here the revolver is “at rest,” ready to be fired by one long pull
on the trigger or by manually cocking the hammer and then pressing the trigger.
With a revolver, the ammunition supply is housed in a cylinder, allowing the sear and hammer to engage directly with no additional components to add play to the system.
Semi-auto pistols introduce new challenges. The ammunition supply is housed in a magazine in the grip frame, with the trigger in front. The trigger is in the frame, the firing pin in the slide. Some sort of linkage is needed to connect trigger and sear, adding complexity to the system.
To add to the challenge, the linkage between trigger and sear has to disconnect each time the slide cycles. This allows the sear to reset and once again hold the H/S at full cock, while at the same time, the trigger return spring moves the trigger forward. Moreover, if the H/S engagement with the sear is insufficient, the impact of the slide against the frame as it cycles could jar it off.
For reliable ignition we want a powerful mainspring and for safety we want substantial sear engagement. On the other hand, if the engagement is under heavy spring load it takes more pressure on the trigger to release it. If the engagement surface between sear and H/S has a relatively large surface area, we’re going to feel it move under load. That movement is called creep. Even tiny imperfections in the surfaces will be felt as a series of steps. Creep can be tolerated if it’s smooth and consistent, but steps, or gritty creep, are less forgivable. With S&W revolvers, the sear/hammer surfaces are polished very smooth and then surface hardened for durability, to help combat grittiness.
A Springfield Armory 1911A1 pistol with the lockwork assembled outside the
frame to show the components. Semi-autos typically need additional components
to connect the trigger and the hammer or striker. In this design the trigger bow goes
around the magazine; when the trigger is pressed the bow presses on the disconnector/sear,
releasing the hammer to strike the slide-mounted firing pin.
Simple But Complicated
The single-action pull on a quality DA revolver is about as good as it gets if we’re talking unaltered factory handguns. The shooter applies pressure on the trigger until, with virtually no perceptible trigger movement, the sear/hammer engagement suddenly releases — called a clean break.
Trigger movement that occurs after the sear releases is called overtravel. It’s possible to use a trigger stop to eliminate all overtravel. The risk is the stop might move, or a bit of dirt might keep the sear from releasing at all. Overtravel is something we tolerate — because the alternative is worse.
A semi-auto needs a bit of free play to ensure reliability. It would be possible to tune a semi-auto trigger so it moves forward just barely enough to let the sear reset. Again, the downside is a speck of dirt, or even metal expansion from heat, might result in the sear not resetting and not allowing the gun to fire. To ensure reliability, it’s better to let the trigger move a bit further forward than absolutely necessary. Semi-autos typically have from 1/16″ to 3/8″ of free play (slack, trigger slack or takeup) before getting to the actual sear release.
We’ll delve further into these mysteries soon and tie it all together, learning more about triggers.
By Dave Anderson