Trigger Analysis

Why Analyze Trigger Operation ?

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". Testing is performed to determine if a given firearm is too easy to discharge, thereby rendering it unsafe.

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".

The generally accepted view of trigger operation and actuation has been grossly oversimplified and misunderstood - consequently, the routine test performed at the police lab is, at best, almost meaningless; and in many instances completely misleading. The commonly employed 'dead weight' and 'spring' testing techniques are not scientifically valid techniques for assessing the ease with which a firearm may be discharged.

So, Why do AFTE, & Police Laboratories Around the Country, Continue to Employ Trigger Testing Techniques That are Clearly Scientifically Invalid and Misleading ?

These tests are typically performed by Firearm & Toolmark Examiners, working to strict laboratory protocols. Some of these individuals may be members of the Association of Firearm & Toolmark Examiners (AFTE). Those examiners who may have realized that the prescribed tests are not scientifically valid are unable to do anything about it, because they have little autonomy, and MUST adhere to laboratory protocols.

AFTE support this position by encouraging the use of the incorrect and misleading test procedures. There has been much speculation as to why AFTE, and crime laboratory management, would support testing procedures which are so obviously invalid. Possibilities raised include:

* AFTE is a police / law enforcement based organization, and the current tests tend to support prosecutorial arguments in court. Even if they have realized that the commonly employed tests are misleading, they have no motivation to revise procedures, and possibly reduce their 'customers' ability to successfully prosecute suspects.

* Police laboratories have no inherent interest in justice, but do have a strong interest in assisting prosecutors to secure convictions. Prosecutors are employed and funded by Government. Police labs are funded by Government, staffed by government employees (often police). The police are funded and employed by Government. From a defense perspective, there is clearly a conflict of interest here.

* AFTE is comprised of generalists who have little knowledge of mechanical systems.

* Scientifically valid test equipment is prohibitively expensive, and if AFTE recommended its use then laboratories around the country would be unduly burdened by the expense.

You, the reader, can decide which is the more likely, but it should be pointed out that good trigger test equipment can be purchased for under $5,000 - a relatively small sum, even for small rural police departments; and the level of technical knowledge required to understand the principals described below is rudimentary (high school).

The results of a typical police lab trigger pull test are described below, and then compared to a more sophisticated (and scientifically valid) 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)

Typical Police Lab Test Results

The police lab technician / firearms examiner will use either a spring gauge or a 'dead weight' system to determine how much force is required to discharge the weapon. This will indicate the pounds of force that need to be applied to the trigger 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', and approximately equal to the other 3 guns.

A Better Way of Doing Things !

The method employed above, and illustrated in Figure 3, takes account of peak 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 peak 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 !