Abstract: A firearm having a smooth-bore barrel for firing fin-stabilized projectiles includes a muzzle assembly at the end of the barrel for stabilizing the projectile as it leaves the barrel upon being fired. The muzzle assembly includes a radial opening portion having apertures along the periphery of the assembly to exhaust firing gases therethrough. A gas guiding or diverting structure is mounted along the outside of the assembly and includes a deflection surface adjacent the apertures in the radial opening portion to divert any exhausted firing gases radially from the barrel and thereby prevent the exhausted firing gases from affecting the trajectory of the finned projectile. The aperture may be formed as slots in the barrel adjacent its end or provided between the end of the barrel and the gas guiding structure which is displaced laterally from the muzzle opening of the barrel along the longitudinal axis thereof.