In 1928, the Italian army adopted a rifle-mounted grenade launcher. It was a potentially interesting weapon which wound up being fatally handicapped by the use of ineffective grenades. The basic idea was to mount a second rifle receiver to the side of a Model 91TS carbine, but with an integral grenade cup instead of a barrel. The trigger of the carbine was modified with an addition linkage to operate the sears on both receivers. When firing a grenade, the bolt would be removes from the carbine receiver and installed into the grenade launching receiver.
The most unique part of the design was in the grenade cup. Instead of using blank style grenade cartridges, the Tromboncino used standard ball ammunition to launch grenades. When the round was seater in the chamber, the front of the bullet was held firmly in place by a threaded plug in the front of the chamber. Upon firing, the bullet could not move forward, so the neck of the case would burst open, venting the propellent gas around the bullet and through vent holes into the base of the grenade cup, thereby throwing the grenade.
This system was quite clever, as it did not require any special ammunition types but also did not require designing a grenade which was capable of safely catching a fired projectile. However, the tactical use intended was for troops to use grenades in the immediate opening of a close assault, and so the grenades were designed to be concussion type more than fragmentation, to ensure that they could be used in relatively close quarters without injuring the firer. The result was a grenade design which was not effective enough on the enemy to be of much actual use, however, and most of the Modello 1928 carbines were decommissioned in the mid 1930s.