I have seen all sorts of k'nex grenades. and i think at least one launcher but they aren't really universal.
What makes my launcher unique is that it fires almost any knex grenade.

Step 1: The Base

To build the base you will need:

-5 blue rods
-1 blue+grey double clip.
-2 yellow rods
-1 grey rod
-1 black end clip
-2 grey clips

assemble it by looking at the picture bellow. It's very simple.

Step 2: The Ram

This step shows how to build the ram.
you will need:
-1 white connector
-3 white rods
-one (or 2) elastics NOTE: if using a spring don't put elastics on!
-2 grey connectors

Follow the pictures to put it together.

Step 3: Put Together

You will need:

-completed base
-completed ram

follow the pictures

Step 4: Testing Phase.

Now for the fun part... testing!

just thread on a grenade with a central hole through it or you can just launch the ram.

The pictures just show some ammo on it.