ThIs gun is finished...but the firing mechanism sucks....I would really appreciate it if someone could build this model, and modify the firing mechanism...I am willing to collaborate. other than that, this gun really looks promising, the grenade design is perfectly designed to fit in the barrel. I will show you how to make each individual piece afteri get them modification, or when I have time. as you can see, this is a really long instructable...but each step goes straight from the last, and the pictures are really clear.
Step 1: the Barrel
you need 4 red connector, 4 yellow connectors, and 8 little green rods.make a square using the connectors, yellow ones facing out of the circle and put two green rods on each yellow as shown.