A firearm has a frame, a barrel assembly and a slide assembly which holds a recoil assembly. The slide and recoil assemblies are located above the barrel so that the barrel is situated between the topmost part of the frame and the bottom of the slide assembly. The barrel assembly includes a housing which is covered by the slide assembly and secures one side of the recoil assembly. The barrel assembly can be removably connected to the frame through front and rear mounts that may be secured using removable pins. The barrel assembly can modularly connect with the frame and slide assembly so that different caliber barrels can be interchanged with each other using the same frame and slide assembly. The firearm is operated by the firing system which includes the trigger assembly and linear striker assembly.