Trigger Pull Measurements

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Trigger pull is a measurement that can assist in determining the level of difficulty related to discharging a particular firearm. Various types of firearms will have a range of typical trigger pulls.

Range of Typical Trigger Pulls

Typical Trigger Pulls
Firearm	Trigger Pull Range
.22 caliber rimfire rifles	3 lb to 5 lb
Military rifles	4 lb to 7 lb
Single-action revolvers	4 lb to 6 lb
Double-action revolvers	10 lb to 15 lb
Semiautomatic pistols	4 lb to 5 lb

These ranges are typical, but there may be exceptions. If a trigger pull is measured by an examiner and it falls outside of known factory specifications or the ranges shown above, then it would be advisable for the examiner to determine the cause. Trigger pull values can point to other root causes of functional and operational problems.

Click here to read some sample results for the trigger pull section of a lab report.

There are a number of reaons for unusual trigger pull values; they may include

inherent design characteristics,
poor quality factory assembly work,
customization of a firearm,
flawed lock mechanism,
flawed safety feature.

Measuring the trigger pull range in single- and/or double-action modes is the first step in identifying many of these problems.

Note:

Occasionally a suspect in a case will indicate that the firearm had a “hair trigger” (a very imprecise term that examiners should avoid), causing an inadvertent or accidental discharge. In these cases, the firearm examiner should measure the trigger pull. However, if asked in court, it would be wise to strictly avoid using this information as a basis for an opinion regarding the shooter’s state of mind. This is scientifically unknowable and lies outside the expertise of a firearm examiner. This line of questioning can be defused during a comprehensive pretrial conference.

There are several approaches to measuring trigger pull, including the use of

standard trigger weights,
a spring gauge,
mechanical trigger pull testers,
digital trigger pull testers,
the TriggerScan™ system.

Each technique requires different equipment and protocols, which will be addressed in more detail in Module 8, Examination of Firearms.

It should be noted that there are common conceptual threads in each of the techniques:

A range of trigger pull values must be established.
Multiple tests should be performed to establish reproducibility within the range of values.
Contemporaneous documentation must be kept for each test result.
Results can be expressed as a range of weights:
- From a low end at which the trigger never releases the sear.
- To an upper end at which the trigger always releases the sear.

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