How to Make a Simple Knex Crossbow

Introduction: How to Make a Simple Knex Crossbow

About: Hi, I'm Bryce! I'm a Math + Computer Science major, graduating a bit early. This is my third year at ISU, and I might do graduate school after. I like being smart and solving fun, difficult problems, so I'm ...

This instructable shows how to create a very simple, yet well-designed crossbow out of Knex. The completed gun will shoot Knex bullets up to a distance of around 50 feet. The design was engineered to be constructed using a minimal amount of pieces, yet be extremely effective and efficient for simple tasks like reloading and accuracy. The design is also durable enough to withstand a bit of rough play, and can be used as a basis to develop more complicated Knex guns.

Step 1: Gather the Necessary Knex Pieces

Below is a complete list of the pieces you will need. As a key, the "Part Name"s are described in the following way:

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

Construct the front of the crossbow. Above is pictured exactly what this should look like.

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

Begin building the body of the crossbow. This is pictured above.

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

Add another section to the crossbow similar to the piece constructed in step 3.

Step 5: Construct the Bullet Bay

Construct the bullet bay exactly as pictured above. The gray connector is positioned opposite of the red connectors, so this will need to be snapped in simultaneously with the red pieces.

Step 6: Attach the Bullet Bay to a New Part of the Body

Construct another body section around the already constructed bullet bay and attach this to the rest of the design.

Step 7: Build the Back

Build the back of the crossbow exactly as pictured above.

Step 8: Construct the Trigger Mechanism

Construct the trigger mechanism as seen in the close-up above.

Step 9: Attach the Handle

Construct and attach the simple handle to the openings of the blue connectors pointed downwards. The building is now complete!

Step 10: Add Rubber Band 1

Add a rubber band to the front of the design, situated exactly as shown in the above image. This process is made easy by snapping the red rods out of the purple connectors, sliding them out above the red connectors, situating the rubber band, and reinserting the red connectors and attaching them back to the purple connectors.

Step 11: Add Rubber Band 2

Add the second rubber band as pictured above.

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, and the Knex library put together by Ryan Farrington ( The author would like to thank both of these sources for making this instructable possible.

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    5 Discussions

    Nice thank you i have been looking for the software for mlcad for some time now, and thank you for attaching a link


    Reply 7 years ago on Introduction

    thnk you for doing this it has put ny name back into the comunity thank you