A locking mechanism for securely attaching an accessory mount to a firearm having a Picatinny/Weaver mounting rail. The locking mechanism includes a resiliently biased slider jam that is received within a transverse groove of the Picatinny/Weaver rail. A clip is provided to clamp the accessory mount to the rail by a locking screw. A locking screw spring may urge the clip away from a tight engagement with the rail when the locking screw is in a loosened condition. The locking mechanism is adaptable to all Picatinny/Weaver rails and provides a rock solid firearm accessory mounting system.