- Combat is inherently a demanding situation which may increase stress, heighten arousal, and increase anxiety. The Processing Efficiency Theory (PET) was specifically developed to account for how anxiety influences performance. Therefore, the PET provides a potential theory to explain the positive and negative changes in performance in a combat environment. This study is the first attempt to examine PET in the military domain. Using the Small Arms Simulator Testbed (SAST), we investigated the relationship between processing efficiency and performance, considering mental workload, stress, and anxiety effects. Shooting performance effectiveness measures were target acquisition and friend/foe discrimination. Changes in processing efficiency were manipulated by varying degrees of working memory demand and sustained information transfer. The results indicated that shooting performance and processing efficiency, as well as mental workload demands, decreased as the global demand of both tasks increased. Further analyses for anxiety and stress and future directions are discussed.