An assembly for a modular, compact firearm includes a universal action and an interchangeable, non-reciprocating barrel. The barrel may directly mount various optical sighting systems and provide integral muzzle compensation and sound suppression capabilities. The firearm utilizes a universal action comprised of a slide assembly, breechblock assembly, recoil assembly, and receiver which mounts a variety of barrel assemblies and assembles into a variety of different grip frames. The action of the firearm utilizes a partially-locked breechblock which amplifies the inertia of a slide.