Empirical evaluation of spring powered air rifle storage and modifications on forensic practice and casework

Air weapons are commonly used by civilian populations across the world, particularly by those under 18, and discharges often result in desecration, criminal damage and animal abuse. Online forums and websites provide an accessible resource for civilians to access airgun modification methods proposing to increase muzzle velocity. However, there is limited published research that empirically evaluates the impact of air weapon modification and the potential to influence casework interpretation. Therefore, this paper aims to initiate such research by quantifying the effect of storage conditions (mainspring compression and oil travel/dieseling) and two modifications (reduction of barrel length and preloading through addition of washers) encountered in casework on recorded muzzle velocities using a small number of break barrel, spring powered air rifles. Storing airguns vertically and/or cocked statistically effected the consistency of air pellet discharge and recorded muzzle velocities. Modifications typically resulted in significant variation in air rifle muzzle velocities, often with unfavourable side effects and/or to the detriment of the airgun. Deliberately reducing barrel length or incorporating preload demonstrated the greatest impact on muzzle velocity; however, the direction of muzzle velocity change could not be predicted by air rifle calibre, brand or model. This preliminary study reinforces the requirement for practitioners to undertake timely weapon examinations and interpret casework on a case-by-case basis, especially for modified airguns. In addition, this research strongly recommends the re-evaluation of current air weapon storage and/or testing procedures to ensure accurate and reliable interpretations are obtained for legal classification and casework.