Police officers, who are professional risk takers, have a duty to make effective risk decisions in order to protect the public, both directly and indirectly, and any modifications in these risk behaviours could place the officer or another in danger. This is even more pronounced in police firearms officers who operate in high-threat, stressful conditions. This research sought to establish whether the risk behaviour of a sample of firearms officers, from a UK police force specialising in the protection of nuclear sites, modified following exposure to an acute stressor in the form of a realistic terrorist exercise scenario. The training exercise was carefully and meticulously planned so that it resembled what might be experienced in a naturalistic setting. The experiment was conducted on a sample of twenty-five experienced firearms officers and twenty-five recruits. Stress levels were investigated through the measure of heart rate (for physiological stress) and self-reporting surveys (for psychological stress), and risk behaviour was measured through the administration of a widely used computerised assessment of risk-taking propensity. The study found that experienced officers evidenced risk-seeking behaviours following the intervention, and the recruits demonstrated risk-averse behaviours. These findings add to the growing array of research into how the cognitive and behavioural functions of police firearms officers are affected by an acute stressful experience.