Mr.Suave said:the OICW already has a grenade launcher well technically its a high explosive round.
Thats true but its a 20mm HE round with a flat trajectory. There are only two or three variations whereas the M203 fires 40mm parabolic grenades of which there are thousands of variations e.g HE, Tear gas, smoke etc. I figured if a gun had both you would be less limited as to your choice of ammunition on the battlefield.