The Benelli CB-M2 was a submachine gun designed in the early 1980s around an experimental semi-caseless 9mm cartridge developed by Franchi. The gun itself looks fairly typical, with conventional controls, a bottom-mounted magazine, and polymer folding stock. The one external hint of its unusual nature is an open cutout located between the magazine and trigger guard, which is the ejection port for use when making the gun safe or clearing a dud round.
The 9mm AUPO ammunition (roughly equivalent to 9x19mm in ballistics) used in the CB-M2 is caseless insofar as it does not leave an empty case to be ejected after firing. Instead, the bullet jacket is extruded back as far as a typical 9mm cartridge, and the hollow interior behind the bullet is packed with powder. Instead of a normal primer, a ring of primer compound is built into the case just behind the bullet, much like a rimfire case. The hammer is located above the barrel, and strikes the cartridge through a hole bored in the top of the chamber. In order to affect a functional gas seal (normally done by the expanding brass cartridge case), the bolt runs a significant distance into the chamber. There is an extractor on the bolt which hooks onto the inside lip of the projectile. The extractor is only used for removing live or dud rounds, and is designed to slip off the round upon firing.
In theory, caseless ammunition offers a number of advantages over traditional ammunition: less weight, less firearm complexity, higher rate of fire. One of the major detriments is heat, as the traditional cartridge case acts as a heat sink and removes a significant amount of thermal energy form the gun when ejected. Benelli hoped to address that disadvantage with the 9mm AUPO ammunition, by providing a long and thin brass cylinder to absorb heat and then leave the gun with the bullet. However, adding that extra brass also pretty much negated the theoretical advantage of lightweight caseless ammo. In addition, maintaining a way to extract and eject live rounds left the gun no simpler than any other blowback subgun – more complex than many of them, in fact.
As a result of these rather self-defeating characteristics, it is not particularly surprising that the CB-M2 found brought significant contracts, and was shelved by Benelli in 1985.