AFTE Store - Trigger Pull Variability and Its Uncertainty of Measurement

Measuring the trigger pull force of a firearm can provide useful information for shooting investigations. Accredited crime laboratories that report trigger pull must estimate uncertainty of measurement based on the influence of their equipment, operators, and environment. A trigger pull uncertainty of measurement study completed at the Bexar County Criminal Investigation Laboratory measured the trigger pull of nine firearms using a spring gauge. The measurements were taken from the central location of the trigger, where an index finger would typically rest. Because accidental shootings can occur in which the trigger of a firearm is not pulled at the central location, a second uncertainty of measurement study was completed that measured trigger pull force at the tip of each firearm trigger. Data revealed a significant difference in the mean trigger pull force at the two measured locations. The mean trigger pull force was found to be lower at the tip of the trigger than at the central location for most firearms tested. The expanded uncertainties between the central location and tip of trigger were compared. Measurement data revealed a high amount of variability associated with trigger pull measurements. Various data-driven approaches to measuring and reporting trigger pull are discussed.