This is my Knex Hammer-Gun originally it was meant to shoot rods but wound up being a short range stinger attack perfect for a sneak attack. Check Out The Video to see the damage it Inflicted On Our LEGO® Plane.
WARNING: Use of high power rubber bands such as those pictured will leave welts, bruises or could possibly pierce skin be cautious with this weapon.

Hammer Gun.wmv
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Step 1: Get All These Parts

2 Red/Pale Green Rods
2 Black/Green Rods
2 Yellow/Dark Grey Rods
2 White/Silver (Not Pictured)
2 Full Moon Connectors (White/Black)
9 Red/Dark Grey Connectors
2 Green Connectors
3 Orange/Brown Connectors
5 Silver spacers
1 Reinforced Grey rod (Black) (This is a must or it will probably break)
2 rubber bands 1 stronger than the other
masking tape

Step 2: Building a Barrel

Place the 2 Full Moon and 9 quarter moons on the 2 Red/Green rods with the spacers in there proper locations (See Images)

Step 3: Trigger

Build the Trigger with 1 of the Yellow/Dark Grey rods, 2 Green Connectors, 2 green Rods and 2 Orange/Brown Connectors

Step 4: Trigger Attachment

Place the Trigger mechanism to Step 1 with the 2 White/Silver Rods.

Step 5: Taping

Tape the spacer and Orange/Brown Connector to the Reinforced Black Rod. Keep in mind to keep a tiny bit of space between the spacer and the connector. This acts as a Shock Absorber so the Orange/Brown Connector won't fly of when fired.

Step 6: Rubber Bands

Attach the rubber bands as shown in the pictures. Your gun is now ready.

Step 7: How to Use

Pull the Ram Rod back until it locks (The Trigger Should Block it) Hold it next to your unfortunate victim, push the trigger down, and wham! You might even give your victim a Bruise :D