While firearms designers have for many decades explored ways to prevent the accidental discharge of firearms that are dropped or that otherwise impact another object, such discharges continue to occur, sometimes resulting in injury or death. Although there are a multitude of mechanisms that can cause a drop fire, one particularly dangerous situation is that of a blow to an exposed hammer spur in a firearm with the hammer down on a chambered live round. In some firearms, a drop of only a few inches has been shown to result in a discharge. This paper will present examination and testing methods for the forensic engineer evaluating cases involving actual or alleged discharges of firearms due to a blow to the hammer spur. Examination of the firearm, mechanical function of safety mechanisms, and evaluation of primer indents will be discussed. Experimental results will be presented, along with an actual case study involving hammer spur discharges of the pre-1984 Winchester Model 94 rifle. Design factors that have been employed as a means of preventing such discharges will also be explored.
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