Abstract: A quick-attachment/quick-release firearm accessory mounting assembly releasably and rotatably attached to a firearm rail, chassis, or handgun slide to receive any of a number of different accessories, such as, a telescopic sight, reflector sight, laser sight, night vision sight, thermal scope, range finder, and other secondary instruments. The mounting assembly is in the form of an adaptor plate having attachment structures for receiving the accessory on a top surface, and securing to the firearm structure on a bottom surface. The firearm is modified to receive the adaptor plate, the firearm frame having a recess portion with a center protrusion in rotatable communication with a center aperture in the adaptor plate, and having grooves or slots formed at each end of the recess portion to receive flanges at each end of the adaptor plate upon rotation.