Caseless ammo is a small arms ammunition that attempts to reduce the cost and weight of ammo and simplify its operation by eliminating the cartridge case that usually holds the propellant, primer, and projectile together. The lack of metal casing decreases the weight, allows for smaller cartridges and higher cyclic rates. However, the sealing of the chamber and heat sensitivity over sustained gunfire is an issue of concern. Here is our in depth article about caseless ammo.