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 *** (Handbook of Firearms and Ballistics by Brian J. Heard) - We need to cite this correctly ***

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

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.

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 their firearm had a “hair trigger” (a very imprecise term that examiners should avoid), causing an inadvertent or accidental discharge. In these cases, the firearms 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 firearms 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 *** Not Linked Yet ***.

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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