An interbar assembly (34) for reversibly attaching a supplemental device, e.g., grenade launcher (32), to a host weapon, e.g., rifle (10). A preferred interbar assembly (34) attaches at its forward end to the rifle barrel (16), and at its rearward end to a "brace" (37) located in the breech area of the rifle (10). In a preferred embodiment, such connection can be made without the need for special tools after simply removing the host weapon lower hand guard (26b), which is readily removable, but without any other alterations.