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Forensic Firearms and Ballistics

Trigger Opperation Analysis

Why Analyze Trigger Opperation ?

In almost every civil and criminal case, the person who fired the gun claims that the weapon "just went off" (i.e. an accidental discharge). Most observers (lawyers, media, police, etc) dismiss these claims with a shrug and a smile; perhaps justifiably saying "well, he would say that, wouldn't he".

Traditionally, one of the first tests performed at the police crime lab has been the trigger pull test. The test usually indicates that the trigger mechanism is 'within normal operating limits'.

However, ARC believes that this is not the end of the story. The generally accepted view of trigger operation and actuation has been grossly oversimplified  - to the point where the routine test performed at the police lab is almost meaningless; and may even lead to cruel miscarriages of justice.

The results of a typical police lab trigger pull test are described below, and then compared to a more sophisticated (and much more meaningful) test, as performed by ARC.

Read on for enlightenment......

Firearm Trigger Operation Explained

Trigger operation analyses are performed on rifles, shotguns, and handguns. 

Trigger operation falls into two main categories; double action, and single action. 

A double action trigger both cycles and/or cocks the weapon, and fires it. Because pulling the trigger performs several functions, the pull is long and heavy. The most common examples are on handguns - pulling the trigger on a revolver raises the hammer, rotates the cylinder, and fires the weapon, for example.

A single action pull is one where the trigger is used only to fire the weapon. Rifles and shotguns have single action triggers, for example. Single action triggers require much less effort to discharge the weapon.

Most revolvers, and many semi-automatic pistols, have trigger mechanisms which can be used in either single action mode, or double action mode, depending upon the user's preference at any given time.

Figure 1 illustrates a revolver in the normal rested state. It may be fired 'double action' by pulling the trigger all the way to the rear and allowing the hammer to fall and discharge the weapon. Alternatively, the revolver may be 'cocked' by drawing back the hammer with the thumb (shown in Figure 2) and then fired 'single action' by pulling the trigger through a very short distance. 

Typically the double action pull will require a force of 12 pounds over a distance of 1/2", whilst the single action pull will require a force of 5 pounds over a distance of about 25/1,000" (or about the thickness of 5 sheets of copier paper)

Fig 1

Revolver ready to be fired in double action mode by pulling the trigger a relatively long distance

Revolver ready to be fired in single action mode by pulling the trigger a short distance

Typical Police Lab Test Results

The police lab technician will use either a spring gage or a 'dead weight' system to determine how much force is required to discharge the weapon. This will indicate the pounds of force required to discharge the weapon. This is often erroneously referred to as pounds of pressure. 

Figure 3 depicts the results of such a test when applied to the single action trigger of a  test gun (revolver), and 3 comparison guns (1 revolver, and 2 semi-automatics).

Note that by using this basic test procedure, the trigger of the test gun appears to require about the same effort to discharge the weapon as do those of the 3 comparison guns. Consequently, the trigger pull would be described as 'normal'.

A Better Way of Doing Things !

The method employed above, and illustrated in Figure 3, takes account of force (or weight) only. It takes no account of distance traveled.

An analogy might be that if you have two vehicles of the same weight, it is more difficult to push one 10 yards, than the other 50 yards !  Clearly, the effort required to push a vehicle 50 yards is much greater than that required to push it only 10 yards.

The ARC method of evaluating trigger pulls takes account of both weight and distance traveled, to provide a measure of the energy (or effort) required to discharge the firearm, not just the force required.

Figure 4 shows the trigger travel required to discharge the same four handguns as tested in Figure 3. Finally, Figure 5 illustrates just how much energy is required to discharge each of the handguns. Note that whilst the four triggers appeared to be almost identical in Figure 3, in Figure 5 the test gun is demonstrated to require far less energy to activate the trigger mechanism than that required to activate the triggers of the comparison guns. The test gun therefore has a far greater propensity for accidental discharge than do the others.

It is interesting to not that the test gun requires about 12 times less energy to activate than does comparison gun #3. Comparison gun #3 is used by many police departments, and has an established reputation for 'accidentally discharging'. 

If your client had discharged the test firearm he would have a very strong case to legitimately claim an accidental discharge. With the traditional method of analysis his case would be 12 times weaker !

Compare Figure 3 to Figure 5, and you can see the significant difference !

Copyright ã 2001 ARC