Step 1: Gather the Necessary Knex Pieces
K'nex [connector or rod] # color
All Knex pieces used in this design are either connectors or rods. This is fairly self-explanatory. If the piece is a connector, # represents the number of rods that piece can connect. If it is a rod, # represents the number of perpendicular connectors the rod can hold.
Finally, color is just the color of that piece. Every Knex piece has a distinct color, so hopefully this is familiar to you.
As an example, look at the piece labelled "1" in the image above. This is a "K'nex connector 1 gray". It can connect to one rod, hence the 1, and is gray.
As another example, the piece labelled "2" in the image is a "K'nex rod 3 white". It can be seen from "3" that 3 connectors can connect to it perpendicularly, and obviously it is the color white.
Finally, "No." is the number of pieces of that type you will need.
No. Part Name
6 K'nex connector 1 gray
1 K'nex connector 1 tan
13 K'nex connector 3 red
42 K'nex connector 4 purple
2 K'nex connector 7 blue
33 K'nex rod 0 green
12 K'nex rod 11 yellow
4 K'nex rod 18 red
15 K'nex rod 3 white
2 K'nex rod 6 blue
Step 2: Construct the Front
Note: to maintain maximum strength in the design, it is important that for this step the purple connector pieces are constructed exactly as pictured. The red rods should be positioned in such a way so that they can only snap in and out of the connectors by moving sideways, rather than forward/ backward.
Step 3: Begin Construction of the Body
Note: in the body, it is not as important in what way the purple connectors are laid out. As long as they connect the design together, this is okay.
Step 4: Finish Body Construction
Step 5: Construct the Bullet Bay
Step 6: Attach the Bullet Bay to a New Part of the Body
Step 7: Build the Back
Step 8: Construct the Trigger Mechanism
Step 9: Attach the Handle
Step 10: Add Rubber Band 1
Step 11: Add Rubber Band 2
Note: the second rubber band should be thinner and less large than the first rubber band. Its only purpose is to keep the trigger in the release position, rather than to propel the actual bullet.
Step 12: Load and Fire the Gun!
Load the gun as pictured above. Take a bullet exactly as shown, with a tan connector, green rod, and gray connector, hook the rubber band with the tan tip, and slide the gray hole on top of the movable yellow rod on the bullet bay.
Pulling the trigger will move this yellow rod downwards, therefore releasing the bullet.
Note: The handle is more of a means to steady your shot. The handle cannot be used to hold the gun in entirety, rather, you should hold the base of the body with your left and the trigger with your index and middle finger. Try moving your thumb to the back of the trigger to provide more support as you pull the trigger.
Additional Comments: The design can be modified to your liking. One common modification is to add another section into the body. This weakens the gun slightly, but provides more time for the bullet to accelerate as it is being pulled by the rubber band.
Credits: The pictures were drawn using the MLCad program (availble from http://www.ldraw.org/), and the Knex library put together by Ryan Farrington (http://www.ncgraphicarts.com/ryan/construction.htm#architecture). The author would like to thank both of these sources for making this instructable possible.