I think we are on the wrong track here. We can make the grenade attachments for M1903, M1 Garand and for the Mauser. BUT! If we are to modify the weapon for that we will have to skin the entire weapon for the sake of having a launcher added to the end of the barrel.
Why not just make the actual grenade for the US and the Germans (I guess its added as a state or something) and make the grenade model including the launcher attachment. So its properly there for visuals and when it is fired away it spawns anyway.
Rifle grenades should be available for all factions I agree but there is no Lee Enfield model with an attachment in game so I guess it is safe to conclude that we dont need one either. We would just have to model the grenade itself and stick it on the end of the rifle. The business end that is.