Grenade launchers are operated with 40mm grenades which are filled with gas or CO2 and BBs. As the trigger is pulled the gas is released and the BBs are blown out of the barrel. The higher the capacity of the shell the greater the surface that can be covered with a shot. With an increasing number of BBs the range decreases.
Short, modular grenade launchers can be mounted on the rail systems of rifles. Some models require no rails but have to be installed directly on to the weapon. The article details give more information about it. These launchers are often used for hitting areas which are not directly accessible by shooting in a curve.
Grenade launchers, as well as grenades are delivered without gas which must be purchased separately. In the category Gas & CO2 it is described which product should be used for the weapon and the product details shed light on whether the grenade is compatible with CO2.