Tokyo Marui's amazing attempt on the M320A1. Iron Airsoft have also made one but Tokyo Marui have managed to make it a little different. Instead of charging the gas into the grenade itself, you can charge the gas into the pistol grip itself which is always very useful.
It comes with one grenade holding 18 BBs, 3 in each groove. Underneath, there is a whole where the gas can flow through to expel the BBs into the distance! Since it is a Tokyo Marui product, it isn't too powerful. If you use green gas, it'll reach distances of a little more than 25 meters with a spread of a couple of a meter or so. If there is a group of people you wish to overwhelm, this will overpower them all in one shot.
If you get rid of the extendable stock, it'll attach to a lower rail of any weapon system with rails underneath the rifle.