The Benelli CB-M2 was a unique submachine gun of Italian conception resulting from a joint venture between Benelli and Fiocchi Munizioni, the biggest ammunition manufacturer of Italy. The weapon itself was unremarkable, and operated by a simple blowback system, which is very common for submachine guns. The unusual aspect of the weapon was the round itself.
The weapon was chambered for the semi-caseless 9mm AUPO round. The AUPO was a much simpler system than the expensive and complicated advanced caseless round for the Heckler & Koch G11. The AUPO bullet had an elongated, hollow base that acted as the case. The propellant filled the hollow space, and was sealed into the bullet with a fulminate plug. What made the AUPO round "semi-caseless" was that the hollow base of the bullet detached from the bullet itself after firing. However, it was not necessary to eject it from the side of the weapon as in a normal cartridge, because the base followed the bullet down the barrel and exited at the muzzle, in a similar fashion to a sabot. This simplified the action of the weapon, allowing it to move back and draw another round from the magazine, since there was no case to eject. With the omission of the ejector mechanism and the ejection step in the action, there were fewer moving parts to jam and cause unreliable functioning of the weapon.
The CB-M2 never found any customers and the project was shelved.