The original is here: http://www.instructables.com/id/Knex_gun_29/
Step 1: Gather the parts
2 single shot K'nex guns Here.
2 white rods
2 grey connectors
Step 2: Assemble
2. Attach a white rod to the bottom of the yellow connectors
3. Attach a grey connector to the white rod connected to the green connectors. This needs to face up, so it is obscured by the green connectors.
4. Attach a grey connector to the white rod in the front of the gun, facing forward (as opposed to up).
Step 3: Fire it
2. Place a bullet(s) in front of the orange connector(s), in between the two sides of the rubber band(s)
3. Slide your finger out of the way quickly