Accessory for the installation, positioning and fixing of a telescopic sight or any other aiming accessory on an air rifle or firearm. The referred rifle or firearm comprises a series of longitudinal grooves (11) on its upper part, facing said accessory (10) and characterized in that said accessory comprises: first longitudinal support (14), with at least one orifice (17) perpendicular to the air rifle (12) or firearm, through which positioning means (16) pass that are housed on the rifle (12) or firearm, defining a firm supporting position between chamber (26) of said firearm and said first longitudinal support (14).