The US Army uses 40mm grenades in two weapons. One is the M-203, which is attached to the M-16 rifle, under the barrel. The other 40mm grenade comes linked like machine-gun bullets and is used on the M-19 automatic grenade launcher. Unfortunately, the two 40mm rounds are not interchangeable because the M-19 ones cannot be broken out from the linked belt. This design angle was intentional, as the M-19 round uses more propellant. If you do get an M-19 round into the M-203, the force of the additional propellant will most likely explode in the M-203's short barrel, killing or injuring whoever was trying to use the wrong 40mm round. At the moment there is a shortage of 40mm rounds for the M-203 and a surplus of ammo for the M-19, which has given rise to troops griping about "why can't we use the M-19 rounds." Unfortunately, about the only people who know the real story are some of the more experienced armorers (the guys who repair and maintain weapons.) So remember, troops, appearances can be deceiving.