Introduction: Knex Crossbow, With Real Arrows!

About: I am a student at The Ohio State University.

Please be a bit easy on me, this is my very first Instructable.

Here I am going to show you how to make a K'nex Crossbow that fires real arrows! This Crossbow is strong enough to shoot arrows 50 feet! You can hold it from your shoulder, or build a platform for it. I orginally designed it after the Roman ballista, but I call it a crossbow because you can hold it.

Disclaimer: the author of this Instructable is not responsible in any way for any damage or injury that might result from the creation of this Instructable. This Instructable, the "K'nex Crossbow, with real arrows!", has the potential to harm others, or damage property if used improperly. I am not responsible for anything that happens to you or your surroundings.


  1. Arrows: a standard kid-sized arrow (around 2-2.5 feet long) will work. You can buy arrows at Walmart, a local hunting store, or online.
  2. Duct tape: only used once in the Instructable, only need a small amount.
  3. Rubber bands: A minimum of 12 bands, but you will want more in case of breaks, extra strings, and more power if you want it.

Now for the K'nex (these are all close estimates, more or less may be needed.)

4. 4 yellow K'nex rods

5. 1 red (or sometimes green) rod

6. 53 yellow connectors

7. 6 red triangle connectors

8. 46 orange connectors

9. 6 white (full circle) connectors

10. 90 blue (full circle w/notch) connectors

11. 4 small(er) pulleys, around 1 inch in diameter.

12. 128 purple (grey) connectors w/ notch

13. lots of those little green (black) rods

14. 160+ white rods

15. 65+ blue rods

16. 3 green connectors

17. 27 blue spacers

Now we are ready to begin our Instructable!

Step 1: Build the Front Arm

Here we are going to make the "cross" part of the bow. First make the pulleys as defined in the first few pictures. After that, make a center box out of white rods the connectors with slots. This is what the rail and arrow will go through. Expand from that box two arms on either side, reinforce these. Then add another two arms that go on top of the previous two arms and reinforce those. Finally put the pulleys on the ends of the front arm. Please refer to the pictures for specifics.

Step 2: Making the Rail

This part is the part that the arrow will sit and travel on while it is in the bow. I do admit that the rail is not my own design but rather from an old K'nex gun from another instructable that I couldn't find. If you do find the instructable please comment and I will attribute the rail to that Instructable. Moving on. This part is probably the most frustration inducing part of the instructable, so be patient and ignore your sore thumbs and fingers. The rail is made of yellow D connectors, short green rods, and white rods. Pleae view the pictures for notes on specific building instructions.

Step 3: Building the Main Body

This is the part of the body that will reinforce and stiffen the the rail, and will connect the stock and front arm together. This step also includes connecting the rail to the main body. You will make the part that connects the rail to the main body. Then you will build the two halves of the main body, and combine them and the rail connector together. Next you will put the rail on the body. Finally you will put two reinforcers on the front of the body. Set it aside and move on to the next step.

Step 4: Building the Stock

Here you build the stock in two sections, the front and the back. The back part actually touches your shoulder, and is easy to modify. The front part extends the crossbow and will contain the action. The entire stock will use reinforcers like the ones we made earlier for the front arm. The stock is built similarly to the front arm, but with a different purpose. Follow the pictures for specifics.

Step 5: Building the Action (and the Release Handle)

This is the part that will be contained in the stock, attach to the rail, hold the rubber band in place, and bring the nock into contact with the rubber band. It is also probably the easiest part to build. It requires only a few simple parts. Attach the white (in the photo gray) bar with the spacers and green connector to that empty space we have left empty for so long, the red triangle connectors should be dangling down inside of the stock. The release handle is just a D connector, 1 red rod, 2 yellow rods in a T shape.

Step 6: Making and Attaching the Rubber Band to the Front Arm

This is a bit difficult to explain, but I will try to. You will need to make two strings out of rubber bands, out of around a dozen rubber bands total (I will let you do the math). String the rubber bands on their respective pulleys. Pull them back until they are taut, then at the middle point wrap a small amount of duct tape around both of the rubber bands. I recommend twisting the rubber bands somewhat before you duct tape them. The duct tape will be where you nock the arrow.

Step 7: Putting It All Together

Here is the fun part, now lets make it look like a crossbow! We will combine the three sections we build earlier into a functional crossbow. Here is how we do it... First (it really doesn't matter what order) we attach the stock to the main body by connecting the four blue rods of the main body to the four white connectors on the stock. It will be a little tight, but that is OK. Next the action and the rail, attach the green peg on the orange connector to the lower section of the rail. Finally we attach the main arm to the rest of the body. Bring the arm and the body together; thread the rail through the hole in the main arm (the part with out the white piece should be facing down and toward the main body). Observe the connections possible. The bottom connectors in that center square will line up with the reinforcer pegs and the white bars from the main body. Connect them. Now the rail will also connect to the white bar on the forward bottom part of the center square, connect them. Now make sure the rail is pretty flat, and that everything is decently sturdy. You should be looking at a complete crossbow! Move on to the next step for instructions on how to use it.

Step 8: Let's Play!

Allright, you have spent much time and endured much pain from this instructable, lets see the fruits of your labor act. Materials needed:


~Safety Glasses are recommended

How to use.

1. Position the D connectors on the action so that they are standing near verticle. The red triangle connectors should be visible through the slats in the stock. When you see the holes in the two triangle connectors, shove the long part of the T-shaped releaser handle through the stock, through both holes, and out the other side. This should now lock those D connectors in an upright position.

2. Pull the rubber band back and loop the band behind the D connectors. The duct tape should be resting across the slat between the two D connectors. Do the pulling back carefully, you should wear safety glasses in case the rubber bands snap. Also make sure the rubber bands stay on the pulley.

3. Put the arrow on the rail. The odd-colored fletching should be facing downward, in between the rail halves. The tip of the arrow should be through the hole in the front arm, and still be resting on the rail. Draw the arrow back along the rail until the nock is in between the two D connectors in the action. Adjust the rubber bands until they line up with the nock of the arrow. Slide the arrow back a little more until the nock grabs onto the duct tape.

4. Point the crossbow in the direction you want to shoot. Then grab the release handle and rapidly pull it to the side. This will pull the long part of the T-shaped release handle free of the crossbow and action. The two D connectors will fall down and the rubber bands will shoot the arrow out of the bow.

5. Repeat 1-4 if you wish to do it again.

Please read on to the last step

Step 9: Conclusion

First off I want to thank you for reading this Instructable. I would like to add that this is my first Instructable and am there for putting it into the First Time Author Challenge. Please vote for me in it!

Second I would like to add some trouble shooting tips.

1. If you have problems connecting certain parts, please review the pictures, and if that doesn't help, leave a comment and I will reply as soon as I can.

2. If the arrow fails to fire correctly, check the alignment of the rail. Also make sure that the rubber band is lined up correctly with the arrow. Make sure that the arrow tip is resting on the rail (if the tip sticks out past the gun, that is OK, but their still needs to be part of the front part of the arrow in contact with the rail.) Also make sure the fletching is oriented properly.

3. If the rubber band is too weak, add some more rubber bands. Too strong, knock some off.

4. For other problems please contact me.

Third, just some additional comments.

1. In this instructable I used normal long bow arrows. I have not experimented with others (i.e. actual crossbow bolts or DIY arrows)

2. The point where the blue rods connect the stock to the main body make a convenient take down point.

3. Feel free to make changes to this crossbow, but please attribute the crossbow to me.

4. You can make a stand for the crossbow if you want. It would mount on the forward part of the cross bow.

And finally, thank you for reading my very first Instructable! Please be safe.


FYI. My apologies for not including a video. I will make one sometime and upload it.

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