Abstract: An alignment plug for mounting a laser beam projector assembly on the barrel of a weapon, comprising an elongate body (10) of substantially cylindrical cross-section of which a first body portion serves for location within the muzzle end of the barrel and a second body portion serves for attachment of the projector assembly. A window (17) in said plug serves for entry of light emitted on detonation of a blank round, the light being transmitted to a light detector (14) by a light guide (16) to initiate projection of a coded laser beam. Aperture means (8, 9) extending axially of said plug serves to relieve the pressure of the detonation to the exterior of the barrel.