AFTE Store - The Effect of Hand Grip Angle on the Measurement of Trigger Pull Forces

As defined by the Association of Firearms and Toolmark Examiners, trigger pull is the amount of force which must be applied to the trigger of a firearm to cause sear release. The frequency at which trigger pulls are featured in investigations of accidental discharge of a firearm has stimulated the development of trigger pull databases. These databases allow the firearms examiner to easily search and compare test values with those of other weapons of the same make, model, and caliber. Traditionally, trigger pulls have been conducted by applying the pulling force on the trigger parallel to the bore axis of the long gun. However, the design of some long guns results in the shooter assuming a natural grip and exerting a trigger pull at an upward angle of 20-30 degrees to the bore axis. The question of whether trigger pulls should be conducted parallel or at an angle to the bore axis was studied by examining the geometry of the pulling force applied on the trigger. It was hypothesized that the latter method would generate higher values due to the imposed angle on trigger movement. A series of trigger pulls were measured by pulling both parallel to the bore and at an angle for each long gun.The magnitude of the readings depended upon the trigger design and not the angle at which force is applied to the trigger. Specifically, the location of the applied force relative to the trigger+s axis of rotation and the direction of the trigger rotation were the factors which determined the magnitude of the trigger pull force. Since there is a potential for a significant difference in trigger pulls conducted parallel to the bore as opposed to at a natural angle, it is recommended that both trigger pull methods be used in casework.

Full Journal: AFTE Journal Vol 43 No 2 (2011)