Introduction: A conventional spring-piston based air rifle is designed to propel metal pellets. By making certain alterations and modifications to the standard, legal weapon can transform into an illegal weapon and it can be used in different crimes. Aim: We report interesting firearm casework, related to a convertible weapon, particularly one which can be operated both as an air gun and as a firearm with some alterations. A .22" caliber break-barrel, spring powered air rifle was indigenously modified to an illegal firearm by the accused for hunting purpose. The air rifle was illegally converted by altering its chamber diameter to load .22" rimfire cartridge and by incorporating a movable metallic round plunger bolt as a firing pin. Methodology: The chemical analysis and work testing of submitted weapon was successfully carried out. This type of modified weapon allowed the successful test firing of .22" rimfire cartridges. Results: Forensic firearm identification revealed that the .22" rimfire cartridge cases recovered from the scene of crime had been fired from the submitted weapon. Conclusion: In this study, an interesting casework has been presented, wherein a .22" caliber air rifle was modified to chamber and fire .22" rimfire cartridges by altering chamber diameter. Laboratory examination and microscopic comparison revealed that the evidence cartridge cases had been fired from the modified air rifle-firearm.