Developed in the late 1920, the Tromboncino M28 works on the same mechanical principles as a typical "cup" launcher (a discharged cartridge supplies propellant gas to a projectile resting in a chamber), but with a twist; instead of simply attaching a grenade chamber to the muzzle of a rifle, the M28 uses a dedicated blank-only receiver with mounted grenade chamber, which is then affixed to the right side of a complete infantry rifle. This combination was only in use from about 1928 to 1934, where it was retired in favor of a dedicated light infantry mortar; most were scrapped with the base firearms returned to standard configuration. The trigger mechanism is interconnected to both the rifle and the launcher, and in order to save weight and prevent inadvertent discharge both share a single bolt. For sighting, the rear sight is notched and marked for both the rifle and launcher, with the rifle front sight in the conventional location and the launcher front sight mounted to the left side of the stock (akin to some types of volley sight), "1930" marked on the chamber and "MADE IN ITALY" on the barrel. The launcher receiver is marked "MBT-1930", and numbered to match the rifle. Fitted with a smooth straight wrist stock, which is properly cut to accommodate the launcher components, with a Terni stamp and matching serial number on the left side.