Also keep in mind fallout has a few modern weapons including the freaking p90 (something I waiting for mods or DLC to add)
http://fallout.wikia.com/wiki/P90c
Also decided to wiki search grenade launchers
http://fallout.wikia.com/wiki/Grenade_launchers
By the looks of things the only time grenade launchers made it in was in New Vegas, Though there are some in fallout tactics but If I remember right only half of thats story is canon. Still makes me want grenade launchers more as they seem under used in the games.
also keep in mind the year of the great war 2077 with heavy tech, guess what a grenade launcher is really simple. Even before grenade launchers there where hand canons which acted the same way. a device to lob explosives has always existed just is strange that games like fallout ignore them.
Also any to bit crazy can make something like a crossbow designed to fling large things like grenades and rocks (I know some one who did it... thankfully dummy grenades)