I've been browsing through Nexus for a grenade launcher and I found a couple, but they all had one thing in common: they weren't real. All of them fire ammo that the modder created specifically for the mod and the grenades found in game. I wanted to make a real grenade launcher, so I did as a part of my http://www.gamesas.com/index.php?/topic/1083073-conceptwipuwfup/ mod, but I'm also going to release it independently. Here is a video:

http://www.youtube.com/watch?v=YbVMiyZQkIE

What do you think? This one actually [b]launches] the grenades (instead of lobbing them) and actually uses the grenades. Also, it is compatible with all vanilla game grenades. It is based off the RockIt Launcher (obviously).