Mental chronometry, or the examination of the time it takes a human to perceive and react to a stimulus, has been studied by researchers for more than a century. The reaction time to commence and cease shooting a firearm varies to some extent by individual and is impacted by internal psychological and physiological aspects of the person and by external environmental factors. Public safety officers and others, such as combat-deployable military personnel, must be cognizant of the factors affecting high-stakes decision-making and reactionary threat response. This extends to the design of training programs, the development of comprehensive policy regarding force usage and defensive tactics, and the necessary knowledge for after action investigation of such events. While the psychological literature is robust on the issue of attention and mental chronometry, public safety trainers and the curriculums in law enforcement recruit academies have not adequately expanded topics to explain the relevance for law enforcement officers. Because of this gap within the criminal justice literature, the paper does not focus solely on previous research evidence.