Trigger Function and Terminology

B. E. Spencer

The sights settle in on the target, you tighten your finger ever so gently on the trigger, and the gun fires. A solid hit! If the gun is set up properly, you were probably totally unaware of the functioning of the trigger, but if it hadn't worked well, that hit may have been a miss. What exactly happens when you pull that trigger? As with so many other all depends.

One of the basic things it depends upon is the type of trigger in the gun. The least complicated type is the 'simple' trigger shown at the right. This amounts to nothing more than a right-angle shaped lever which transmits the pressure from your finger to the sear bar of the lock, pushes it up until the sear pawl disengages from the full-cock notch and allows the tumbler and hammer to rotate forward, firing the gun. This trigger is frequently mounted into the gun so that it pivots on a pin set into the wood of the stock, running through the pivot hole (A) in the trigger, and so that the trigger bar bears on the sear bar at point (B). A simple trigger may also be a self-contained unit, with wings of metal cast into the trigger plate so that the pin only fastens the trigger to this unit, not to the stock. The entire unit is then inlet into the stock in the proper position. Although a simple trigger is a simple arrangement, it has certain advantages. It can, for instance, be used with a lock which has no fly in the tumbler. (See the Basic Lock Function FAQ for an explanation of the function of a fly in the tumbler.) It will also allow cocking the gun in silence, by first pulling the trigger and holding it back, then pulling the hammer to full-cock, releasing the trigger, and then easing the hammer down onto the stop. If this trigger is properly installed, it can function very well, be firm but not stiff, and have a light, crisp let-off.

In an effort to achieve a lighter trigger for fine accuracy work, the 'set trigger' was developed. It is a favorite of target shooters, but finds application in hunting situations, also. There are many varieties of set trigger, only a few of which will be included in this discussion.

Some definition of terms is in order. Why is it called a set trigger? Because it is a self-contained mechanism, which functions the same out of the gun as in it, and because it is 'set' and fired, not just pulled.

A 'single set trigger' has one trigger, only, and is usually set by pushing it forward, then fired by pulling it back.

A 'double set trigger' has two triggers, one for setting the mechanism, one for firing the trigger.

A 'double set, single phase' trigger has a setting trigger and a firing trigger, but the gun can only be fired by setting the trigger, it won't fire in the un-set condition.

A 'double set, double phase' trigger has two triggers, one for setting the mechanism, one for firing it, but the gun can be fired without setting the trigger first.

For our demonstration trigger in this discussion, we will use a double set, double phase trigger, as shown here.

Double Set, Double Phase Trigger

The labels are: A=trigger bar or plate, B=set trigger spring screw, C=set trigger blade, D= set trigger bar, E=firing trigger blade, F=firing trigger bar, G=set trigger spring, I=set trigger pivot pin, J=firing trigger sensitivity adjustment screw, K=firing trigger pivot pin, L=pawl of set trigger on the nose of the set trigger bar, and M=ratchet stud on the firing trigger.

And the same trigger as seen from the top, from inside the gun.

Same Trigger, Top View

The labels are the same as in the previous view, with the addition of H=the firing trigger spring. This spring bears on the firing trigger in such a way as to rotate it counterclockwise, moving the blade forward, the bar down. The main purpose of this view is to show the overlap of the set trigger bar and the firing trigger bar.

Alright, we have pictures and terminology, how does the thing work?

What many people fail to realize is that a set trigger is a free standing assembly, and works out of the gun the same as in. The curved trigger on the left is the set trigger, the straight one on the right is the firing trigger. You can see that the strong set trigger spring (the one with the fat screw) is bearing down on the set trigger in such a way as to constantly be forcing the curved part forward, toward the muzzle, which would make the horizontal part, the arm, pivot upward. The firing trigger also has an arm, but it extends backward, toward the butt. There is a notch or ratchet stud on the firing trigger, and a pawl on the nose of the set trigger. When you set the trigger, the arm of the set trigger is forced down, and the pawl on it catches in the notch of the firing trigger. Just like the sear nose in the full-cock notch in a lock, it hangs there. When the front, straight, firing trigger is pulled, it moves the notch a fraction of an inch forward, releasing the pawl on the arm of the set trigger. Under strong spring pressure to rotate counter-clockwise, the arm of that trigger snaps up with considerable force. Since it is positioned in the gun to be almost touching the arm or bar of the sear itself, it smacks the sear arm, knocking the sear nose out of the full-cock notch on the tumbler, allowing the tumbler to rotate forward under pressure from the main lock spring, firing the gun.

Here's what the trigger mechanism looks like after it has fired. You can see (1) that the set trigger bar has raised up to contact the sear bar of the lock, which would be right above it. This also provides a good view of the set trigger pawl (2) and the ratchet stud (3) on the firing trigger.

Double Set, Double Phase Trigger, Fired

Close inspection of these graphics will show you that even with the trigger not set, you can pull the front, firing trigger, which will rotate it clockwise around its pivot pin, moving its arm upward. Of course, that arm is also positioned directly under the sear arm, so the sear arm is forced upward under direct pressure, and the gun fires without being set. It's important to realize that the arm of the set trigger is what strikes the sear arm and fires the gun, when the trigger is set. However, when the gun is fired unset, the set trigger arm never touches the sear arm, and all the work is done by the arm of the other, firing, trigger.

In a simple trigger, or in firing the gun with the firing trigger alone, unset, the firing trigger arm pushes up on the sear arm and moves the sear nose out of the full-cock notch on the tumbler and HOLDS IT THERE. The tumbler will rotate fully forward, the gun will fire, all before you can let go of that trigger, so the sear nose is held away from the tumbler all throughout the sequence.

Think about the situation, though, in which the gun is fired by setting the trigger, then firing the trigger, which fires the gun. The arm on the set trigger slaps the sear arm for only an instant, then bounces away, releasing it. When that happens, the sear spring, which is constantly pushing the sear nose into the tumbler, does just that, and the sear nose rubs on the circumference of the tumbler as it rotates. When the half-cock notch rotates by, the sear nose will drop into it, either preventing the gun firing, or, more likely, breaking either the sear nose or the half-cock notch. In order to prevent this, a little gadget called the "fly" or the "detent" is introduced. Refer to the Lock function FAQ for further explanation of this.

So, we see that the set trigger, in any of its various forms, is a mechanism in its own right. It can be set and fired out of the gun as well as in. In the normal sequence of things, you set the trigger, then fire the trigger, which fires the gun.

The set trigger is an adjustable trigger. In the first graphic, you see the adjustment screw (J). This is threaded through the trigger plate so that the tip of the screw bears on the bottom of the horizontal bar of the firing trigger (F), limiting its downward travel. Screwing the screw in pushes the firing trigger bar up a little, which moves the ratchet stud just a hair, making it easier to release the pawl of the set trigger bar (L) from the notch (M). This makes the trigger lighter. That's a good thing, but you must always be aware that the adjustment can be carried too far, so that the gun will fire when being bumped, or even so that the set trigger will not catch the notch, and the trigger cannot be set. Backing off on the screw will solve all those problems.

The reason that the 'double set, single phase' trigger cannot fire the gun without being set is that on this trigger, the firing trigger has no bar. Pulling it causes no contact with the sear bar. Only by setting the trigger, then letting the set trigger bar fly up and hit the sear bar can the gun be fired.

Set triggers come in many shapes and varieties, but the basic function of all is the same. The principles outlined here apply to most all of them.

Copyright ©1997 B. E. Spencer

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