Designing Triggers part 1: Simple Trigger Theory

The trigger is an integral part to any firearms design (various improvised ‘zip guns’ aside).  In its simplest form the trigger holds the firing pin, hammer or striker, hereafter referred to as the cocking piece, to the rear and releases it upon actuation.  It then resets to capture the cocking piece.  The trigger is sometimes also called into duty to perform auxiliary tasks such as cocking a hammer or striker, indexing a cylinder, and deactivating safety appliances such as firing pin blocks.

It is probably impossible to completely enumerate characteristics desirable in a trigger, and those characteristics may vary depending on the purpose of the firearm, but speaking in generalities, the following characteristics are desirable.

  1. Safety. The trigger mechanism should not be susceptible to accidental release by jarring, component fatigue, or other design flaw.
  2. Length of pull.  The trigger should have a comfortable length of pull that a normal human finger can traverse without strain.
  3. Weight of pull.  A trigger which requires a low force to activate is easier to use and is easier to activate without disturbing the alignment of the sights, resulting in greater functional accuracy.
  4. Crispness.  The ideal trigger, as the old saw goes, “breaks like a glass rod”.  This phrase is meant to convey that the ideal trigger accepts force, without apparent motion, up until the time when it gives way (releases) completely and without tactile feedback.  This aides the shooter by not telegraphing the moment the shot will break.
  5. Retraction is the ability of a trigger to fully reset after force has been applied, but the shot has not yet fired.  It would be dangerous to have a trigger that could remain ‘partially pulled’, resulting in a much lighter pull than expected.
  6. Consistency.  It is imperitive to accurate shooting that a trigger have the same weight and length of pull each time. Some firearms may have multiple ‘modes’, such as the first shot on a trigger-cocking firearm, or the single versus double action triggers on a revolver, but in a given ‘mode’ the trigger characteristics should be consistent.
  7. Ease of installation and removal, as well as maintainability
  8. Resistance to malfunctioning from introduction of debris or temperature variations.
  9. Price and manufacturability.  Modern manufacturing methods make light work of what was once difficult, but some features will increase the expense of a trigger beyond its value in some cases.
  10. Low parts count.  More parts means added complexity, as well as additional logistical support for manufacturers.  An additional piece may cost only a few cents more, but the tooling required to make it at such a price can be many thousands of dollars of overhead.

There are also factors which desirability may vary with purpose or user.

  1. Pre-travel.  Pre-travel is the amount of ‘slack’ that must be taken up before the full weight of the trigger begins.  Some pre-travel may give the user the ability to feel the trigger prior to discharge, it also increases the length of pull, which may add some safety margin.
  2. Overtravel.  Overtravel is the amount that the trigger is free to move after the point at which it activates.  In most applications, minimal overtravel is consdiered advantageous as it prevents any jarring caused by the trigger hitting a sudden stop after release.  With self-loading firearms, overtravel considered detrimental because it increases the reset distance.
  3. Reset.  Reset is the distance the trigger must travel forward (as pressure is released) before the trigger is ready to be fired again.  Reset is not a concern in single-shot firearms, but in self-loaders where a fast follow-up shot may be desirable, a short reset is preferred.
  4. Adjustability is frequently desirable in target triggers, but rarely so in military or hunting triggers, as adjustments introduce another potential failure point.

Finally there are features that are considered part of the trigger group, or trigger assembly, that may or may not be required depending on the firearm for which the trigger is intended:

  1. Disconnectors and secondary sears.  The term ‘disconnector’ in common use can mean any one of several things.  As used here, it is a device which disconnects the trigger from firing until the breech is closed.  In self-loading firearms this is necessary to prevent discharging a round while out of battery or allowing the hammer or striker to ‘follow’, resulting in an ineffectual strike (not firing when desired).  In a semi-automatic, a secondary sear remains engaged until the trigger has been released to the point of reset, before returning control to the trigger. In a fully-automatic, closed-bolt, firearm it then activates the hammer, or striker as soon as the breach is closed.  In other words, after firing is initiated by the trigger, it takes over the job of the trigger, timing each release to occur only after the breech is closed.
  2. Deactivating safety appliances.  The trigger may perform a secondary function of moving a firing pin block (for example) or other safety appliance.
  3. Indexing cylinders.  In a revolver, the trigger mechanism may be responsible for rotating the cylinder to the next chamber as a precursor to firing.
  4. Cocking the piece.  The trigger may be used to compress the spring that drives the hammer, striker,  or firing pin.

In this installment we will examine simple triggers: those which act directly to fire. We shall ignore for the moment the auxiliary functions as well as methods used to refine the quality of the trigger pull, and focus on the bare essentials. The triggers discussed in this part are suitable only for single shot or open-bolt firearms, but the principles involved apply to all triggers.

The basic trigger

A basic trigger is shown below.  It’s function should be immediately obvious upon inspection.  The striker at ‘A’ is held to the rear by the trigger ‘B’.  The trigger is held into it’s set position by trigger return spring ‘C’, which also returns the trigger to position after release.  The bevel at ‘D’ allows the sear on the striker to pass back over the trigger hook in order to re-engage the sear upon cocking.

basic trigger

Parts of a basic trigger.

The trigger shown is representative only, and not properly designed.  The primary impropriety being the angle of the engagement surfaces.  The angle of engagement surfaces for the primary release should almost always be positive.  A neutral engagement surface, as shown in the previous figure, would be susceptible to release from jarring and is less likely to have good retraction.  Neutral engagement may be acceptable in very special circumstances, such as Benchrest competition, where the rifle is only loaded while aiming at a target and is not subject to handling while cocked. A negative engagement angle could slip off due to spring pressure and should never be used.   A slightly positive engagement is necessary for any general purpose trigger.

engagement angles

Engagement angles. From left to right: Positive, Neutral, and Negative.

Here, the trigger is shown with appropriate angles and this trigger could be used, with a modicum of safety, in a real firearm.

good engagement.

Basic trigger with “good” engagement.

The width of the trigger piece, where the finger makes contact, is important in that it changes the user’s perception of the weight of pull.  A wider contact surface makes the trigger feel a bit lighter to the user and is often more comfortable.  Conversely, on triggers made for precision shooting, a narrow trigger may be used when the trigger pull is very low to increase the users feel of the trigger.  On a standard trigger (3.5 lbs or more) the perceived improvement by adding trigger width stops at a width of about 3/8″ and there is little point in making the trigger wider.

The trigger guard should always be at least as wide as the trigger if it is to be effective.  “Trigger shoes” are an accessory used to add width to a trigger.  These have fortunately fallen out of favor as the ones that were wider than the trigger guard created a dangerous condition.  A trigger wide enough to protrude beyond the sides of the trigger guard is susceptible to catching the edge of a holster or brushing against an object and discharging.

over wide trigger

A trigger that is wider than the trigger guard poses a risk of accidental discharge.

Triggers are basically composed of two mechanical systems: engagement surfaces and levers.  Engagement surfaces are theoretically simple and such refinements as are applicable will be discussed in a later article on practical triggers.  Levers on the other hand form the complexity of most trigger systems so it is worth understanding how they work.

There are three broad categories of levers, designated by the relative positions of the applied effort (E), the resisting force (R) and the pivoting point, or fulcrum (F).

Class A, or First Class levers have the fulcrum between the applied effort and the resisting force (See below).

class a lever

A class A lever. The fulcrum is between the applied force (the red arrow) and the resistance (the weight).

They reverse the direction of the applied effort and offer the possibility of decreasing the amount of effort required to overcome a force or, conversely, to decrease the amount of motion required to move a force through a larger distance, at the expense of requiring greater effort.  The simple triggers shown thus far are all class A levers.

mechanical advantage greater than one

A class A lever with the fulcrum closer to the resistance allows a force that is smaller than the resistance to work.

 

fractional mechanical advantage

A class A lever with the fulcrum closer to the applied force requires greater force than the resistance to work, but amplifies the distance moved by the resisting arm.

Class B, or Second Class levers have the fulcrum at one extreme, the effort at the other extreme, and the resistance somewhere in between.

class B lever

A class B lever always reduces the force required overcome the resistance. It also moves the resistance in the same direction as the applied force.

Class B levers move the resistance in the same direction as the applied effort, but always a shorter distance than the applied effort.  A Class B lever is therefore always used to overcome a greater force with a smaller effort.
Class C levers are rarely found in triggers.  A Class C lever places the fulcrum at one extreme and the resistance at the other, with the effort being applied in between.  Class C levers require a greater effort than the resisting force, but act to accelerate the movement of the force end of the lever relative to the effort applied.
In any lever, the ratio of the distances between the fulcrum and the applied effort and the fulcrum and the resistance is called “mechanical advantage”.  A mechanical advantage greater than one indicates that less effort is required to move the resistance than the force of the resistance itself.  A mechanical advantage of less than one indicates that the resistance will move a greater distance than the effort, but greater force will be required.

Applying this theory, the length of the trigger may be increased to provide mechanical advantage, resulting in a reduced trigger weight.

long trigger

Elongating the effort arm of this class A lever reduces the amount of force required to disengage the striker, but increases the length of pull.

This is one of the techniques used in the Winchester Model 70 trigger

In a simple trigger, When moving the fulcrum (making the ‘leg’ on one side of the pivot pin longer) the designer is always trading pull weight for movement.  A small movement will require a harder pull, and a lighter pull will require a longer motion.  When designing with a significantly offset fulcrum point, it’s important to consider the weight distribution of the trigger.  A long ‘leg’ that has significantly more mass than its short ‘leg’ may result in a trigger pull that varies with the angle at which the firearm is held (normally only perceptible in light triggers) or be more susceptible to accidental discharge from jarring.  Heavier return springs can offset this problem, but normally defeat the purpose of having the offset fulcrum in the first place.

In the next installment we’ll discuss some of the aspects of prototyping simple triggers and in later installments we’ll look at multi-lever triggers, secondary sears and disconnectors and other more complex arrangements.

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