I have an "IDEA" if someone knows GECK better then I do, or if gamesas wants to in a DLC or something can add this later on. Make a mod attachment M203 for Assault or better The All American, and in order to use the grenade launcher attachment you well have to switch ammo (Default Key 2), so when you change the ammo to grenade ammo it well launch Grenades. Well that's a Idea I just wanted to throw out, hope someone likes it and knows how to do it cause I don't.

I like this M203 attached to either the M16 or the Colt M4 (or what ever they call them!!!)

There's 2 guns, don't know if its allowed to link wiki website but these 2 would be perfect to add it to with out having to do a lot of work to it.

http://fallout.wikia.com/wiki/All-American

http://fallout.wikia.com/wiki/Marksman_Carbine