Firearm triggers are critical to the function of a firearm.
In the bad old days of matchlocks, the energy needed to light the priming powder was supplied by a burning cord, called a match. The match was simply dipped in the priming powder by a lever system.
When flint on steel ignition came along the energy had to be supplied by a spring. The spring pivoted the hammer, which held the flint, against the steel frizzen creating sparks which fell in the priming pan full of powder. For this to work the hammer had to be released all of a sudden, without disturbing the aim of the shooter.
This is the back side of a flint lock. (Or would that be a flintlock lock?)
This one is about as simple as they get. The hammer and the tumbler are attached to each other through the lockplate. They pivot on the same axis which can be seen as the dark spot on the tumbler.
The main spring pushes down on the nose of the tumbler (arrow), causing it to rotate counter clockwise.
In this picture the top of the tumbler is against its stop.
In the next photo the hammer has been pulled back and the sear has engaged the half cock notch.
The main spring has been lifted a little, and the tumbler is away from its stop.
At this point the sear can not be disengaged because the pawl (the end of the sear in the notch) is trapped. The sear pivots around the screw that holds it to the lockplate. To move it, the hammer must be pulled back until the pawl is clear of the notch.
Note that if the pawl or the outside of the notch were to break, the hammer would fall. This is an example of what I am talking about elsewhere on this site when I say "mechanisms can fail."
Here we see the hammer all the way back, the main spring fully compressed, and the sear engaging the full cock notch.
The sear spring keeps the sear in contact with the tumbler.
When the lock is installed on the gun, the trigger simply lifts up on the right hand end of the sear until the pawl slips out of the notch allowing the hammer to fall.
The same principles apply to modern triggers, although the mechanisms might be more complicated.
Triggers often perform more than one job. For instance, in a double action, the trigger cocks the hammer. Triggers may also set or disable various safety devices. However, the primary function of a trigger is to release the firing mechanism, causing the gun to fire.
How well the trigger does this job relates to safety and to accuracy. For the sake of safety a trigger should be resistant to accidental firing and predictable when it comes to intentional firing. The same applies to accuracy.
There are some individuals with the skill and discipline to shoot well with horrible triggers. The rest of us will benefit from certain desirable trigger characteristics.
These graphics show a simple sear arrangement.
In the top graphic the hammer is shown in black and the trigger/sear is in green. The main spring (not shown) will cause the hammer to try to rotate counter clockwise, but the sear is engaging the notch in the hammer preventing this.
If the trigger is pulled, the sear disengages, and the hammer rotates and fires the gun.
Graphic 1 is a close up of the sear and notch showing them mating at an obtuse angle. This may cause the sear to slip from the notch under pressure from the main spring causing an unintended discharge.
Graphic 2 is a close up of the sear and notch showing them mating at an acute angle. In this case the hammer will have to rotate clockwise, against the main spring, before the sear will clear the notch. This will make the trigger pull heavy, and probably inconsistent.
Graphic 3 shows an excessively deep notch. This will cause trigger creep as the sear travels across the notch.
Graphic 4 shows a shallow notch. This is unsafe due to the lack of engagement of the notch and sear.
These graphics are two dimensional. Real triggers are three dimensional.
In addition to the sear geometry discussed above there is a nearly endless list of other geometric considerations. The relative positions of pivot points, the ratios of levers, and spring forces, to name a few. The search for the perfect trigger is far from over.
When the trigger is pulled the sear disengages and the gun fires. Then the slide or bolt comes back, recocks the action and chambers a new round.
The problem is that this happens before the trigger is released. In a simple sear arrangement the hammer would follow the slide or bolt back home, possibly firing another round.
Enter the disconnector. This part disconnects the trigger from the sear when the action starts its cycle, leaving the sear free to catch the hammer. When the trigger is released the disconnector resets and is ready to fire the next round.
Another way this is done is with a secondary sear which is not connected to the trigger. When the action cycles the secondary sear catches the hammer. When the trigger is released first the primary sear engages then the secondary sear releases and you are ready to fire again.
Set triggers overcome the danger of a very light trigger pull. There are several variations.
In some variations the setting device is essentially an automatic safety which must be deactivated to fire the gun.
Another variety causes the trigger to have a normal pull if not set, and a light pull if it is set.
The differences between a set trigger and other kinds of safety devices are the automatic nature of set triggers and the fact that they consist of manipulations of the trigger, secondary levers attached to the trigger, or additional triggers. In other words they can be worked with the trigger finger while on target.
Most military and some sporting arms have two stage triggers. The first stage is usually just a spring and some distance which must be overcome before you get to the "real" trigger. This is done for safety reasons.
Lock time is the time between when the sear releases and the main powder charge is ignited.
A flintlock has a heavy hammer with a rock (flint) clamped in its jaws. The hammer swings through a long arc and drags across the steel frizzen. This causes sparks to fall in the priming pan full of powder. If the weather is right and the planets are aligned the sparks light the powder on fire. The burning powder eventually burns its way through the touch hole and lights the main powder charge. By this time you may as well make dinner reservations because your quarry is back in its den enjoying a good night's sleep.
The point is that reduced lock time is a real asset. In addition to not giving the target a chance to move it reduces the chances that the shooter will move.
Quick lock times depend on low mass ignition parts, traveling short distances, driven by strong springs, striking high quality, properly seated primers.